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Cet article est un compte-rendu du livre : La Henriade de Voltaire : poésie, histoire, mémoire, sous la direction de Daniel Maira & Jean-Marie Roulin, Paris : Honoré Champion, coll. « Les Dix-huitièmes siècles », 2019, 306 p., EAN 9782745351609.

Cet article est un compte-rendu du livre : François Rosset, L’Enclos des Lumières. Essai sur la culture littéraire en Suisse romande au XVIIIesiècle, Genève : Georg, 2017, 256 p., EAN 9782825710531.

S’il est une tragédie qui s’inscrit bien, en son lieu et en son temps, comme une tragédie politique, c’est l’Hector de Luce de Lancival. Adulée par Napoléon, vivement critiquée par le journaliste Geoffroy, elle représente, tant sur le plan du langage que de son emploi spécifique du « matériel » tragique, un cas particulier. Non qu’elle valorise à outrance, par le biais d’une application directe, les faits d’armes de l’Empereur : mais elle parvient à cristalliser, en dépit de son manque d’intensité dramatique, l’état d’esprit qui est celui de 1809.

Since its beginning, at the turn of the 19th century, biology has been mostly – if not exclusively – analytical. Reductionism has progressively unveiled a series of structures buried into one another like Russian dolls. The study of the genome, the deepest structure of organisms, represents the triumph of reductionism. With the deciphering of the genome and the birth of what is called the ‘proteome’ – i.e. the study of the proteins and of their interactions –, a new stage appears. To the disorganisation that characterised two centuries of biology, a phase of reconstruction of living organisms is substituted. This is concerned first with interactions of proteins and of cells. In addition, one of the most remarkable tools for this latter research has been provided by embryonic stem cells.

Tetrachloroethylene (PCE) is a chlorinated aliphatic compound essentially used as a degreasing and dry-cleaning solvent. A methanogenic and sulfate-reducing consortium was obtained from anaerobic digested sludge from a waste water treatment plant (Bourg-en-Bresse, France). An ascending fixed-bed reactor inoculated with this mixed culture was fed semi-continuously with various PCE loading. The dechlorination of PCE was efficient and complete for high concentrations: 40–215 μ M of PCE. Indeed, 215 μ M PCE was degraded rapidly at the rate 3 μ mol l −1 h −1 and with 98% PCE removal. No chlorinated products were accumulated in the effluent. The tracer experiments with 13 C-labeled PCE indicated that the PCE was completely degraded to carbon in the biomass and CO 2 . The inhibitor of methanogenesis stopped the PCE dechlorination. Methanogens had a deciding role in the dechlorination process after the enrichment period with high concentration of PCE. To our knowledge, this is the first methanogenic consortium which dechlorinated with this capability. This fixed-bed reactor is an attractive method with high dechlorination rates, and the absence of chlorinated products, for removal of PCE in bioremediation processes.

With the death of Andre Lwoff, one of the greatest French representatives of science has disappeared. He began his career in 1921, at the age of 19. He lived therefore throughout the century during which biology became a science instead of a collection of heterogeneous disciplines. Andre Lwoff made a major contribution to this unification and became one of the fathers of what has been called molecular biology. His father, a psychiatrist, and his mother, a painter and sculptor, were both of Russian origin. His taste for research probably came from his father who, like most of the Russian intelligentsia, had had to escape from the oppression of the tsarist regime. Like many of the progressives of this generation, his father cherished an unshakeable faith in science, especially in Darwin's evolutionary biology, which was considered to demonstrate the existence of a natural law of progress.

Tetrachloroethylene (PCE) is a toxic compound essentially used as a degreasing and dry-cleaning solvent. A methanogenic and sulfate-reducing consortium that dechlorinates and mineralizes high concentrations of PCE was derived from anaerobically digested sludge obtained from a waste water treatment plant (Bourg-en-Bresse, France). A methanogenic bacterium, strain FR, was isolated from this acclimated consortium. On the basis of morphological and physiological characteristics, strain FR was classified in the genus of Methanosarcina. Phylogeny analysis with the 16S rRNA gene sequence revealed that strain FR is highly related to Methanosarcina mazei and Methanosarcina frisia (99.6 and 99.5% identity, respectively). High concentrations (50-87 μM) of PCE were completely dechlorinated by strain FR cultures at the rate of 76 nM·mg protein -1 ·day -1 . PCE dechlorination produced a nonidentified compound. The tracer experiments with ( 13 C)PCE revealed that the product was nonchlorinated. Dechlorination of PCE to trichloroethylene was still active in the presence of boiled cell extract of the strain FR. However, no further dechlorination was observed. This result suggests that a cofactor rather than an enzymatic system is responsible for the first dechlorination of PCE. Dechlorination-active fractions purified from cell extracts on a XAD-4 column revealed the presence of F420 ,F 430, and cobamides cofactors. This is the first report of the isolation of a methanogenic bacterium with the ability to dechlorinate high concentrations of PCE to a nonchlorinated product.

All recent results in the fields of development and evolution point to a role of regulatory circuits as a major cause of evolutionary changes.

François Jacob : Philidor, a European artist. If François André Danican Philidor, whose bicentenary we have just celebrated, allowed the Comic Opera to expand as it did, it was perhaps thanks to behaviour, or more generally an ethics, which consisted primarily of openness to others. He was open to the influences of his own dynasty which had provided illustrious royal musicians for nearly 150 years, but he was also open to the concert of nations ; his frequent journeys meant that, perhaps despite himself, he made a vast synthesis of it. But Philidor, whose natural tendency was to introspection, as his passion for chess shows, did not manage to impose his genius. Unlucky in friendship and in circumstances, he could only express his musical originality on the stage of the Comic Opera, and left to posterity only one important instrumental work and the finally tragic picture of a lonely, little-known and misunderstood man., Jacob François. Philidor, une éthique de l'écoute. In: Dix-huitième Siècle, n°28, 1996. L'Orient. pp. 523-540.

The conversion of crude sweet whey to single cell protein by a mixed culture of the yeastsCandida kefyr LY496 andCandida valida LY497 in non aseptic conditions was investigated. In comparison with single “pure culture” ofCandida kefyr LY496, ethanol accumulation during batch was prevented using the mixed culture. The efficiency of whey conversion to biomass was thus increased by as much as 20%.

While the expression of fetal characters by malignant cells is now well-documented, the mechanism involved and the nature of the cells that are the target of malignant transformation remain controversial. It has often been assumed that carcinogenesis results in 'dedifferentiation' of specialized cells. The alternative hypothesis is that neoplasia results from a disorder of some normal stem cells. This view is discussed in relation to several examples, in particular teratocarcinomas.

Tetrachloroethylene (perchloroethylene, PCE) is widely used in many industries and particularly as a degreasing and dry-cleaning solvent. It is commonly found as a groundwater contaminant and because of its carcinogenic properties is considered a pollutant, which must be eliminated by proper treatment. This research examines the role of a mixed culture in PCE dechlorination at high concentration from an ecological point of view. The respective role of sulfate-reducing and methaogenic bacteria in tetrachloroethylene cechlorination is studied. 19 refs., 5 figs., 2 tabs.

[Figure][1] CREDIT: INSTITUT PASTEUR What is the operon, whose 50th anniversary is being celebrated this week? The word heralded the discovery of how genes are turned on and off, and it launched the now-immense field of gene regulation. The idea was born in Andre Lwoff's laboratory at the Institut Pasteur. At one end of a long corridor in the loft of a building devoted to research on bacteria were Lwoff, Elie Wollman, and myself. At the other end were Jacques Monod and his group. Lwoff studied lysogenic Eschericia coli bacteria capable of producing bacteriophage without infection. Monod was analyzing the properties of the β-galactosidase enzyme in the same bacterium: an enzyme required for the metabolism of lactose that was produced only when the culture medium contained galactosides. To all and sundry the two systems appeared mechanistically miles apart. But their juxtaposition would produce a critical breakthrough for our understanding of life, demonstrating that we cannot presume to know how new ideas will arise and where scientific research will lead. ![Figure][1] CREDIT: ISTOCKPHOTO Toward the end of the 1940s, after 2 years of hospital care for my wounds from the war and a rather slapdash end to my medical studies, I wandered aimlessly in Paris. Unable to realize my dream of becoming a surgeon, I was persuaded by a cousin to launch out into the newly hatching science of biology, and I decided to join a laboratory. After several unsuccessful attempts, no doubt due to my notorious incompetence, I was kindly received by Professor Trefouel, the director of the Institut Pasteur. He quizzed me on my wartime escapades; explained the importance of sulphamides, of which he was one of the fathers; described the wartime horrors at the Institut; and finally offered me a research bursary. I spent the first year taking the “Grand Cours,” learning bacteriology, immunology, and virology, and then sought a lab in which to use my newfound talents. There were two exceptional labs in Paris: that of Boris Ephrussi and that of Andre Lwoff. After several fruitless visits, I returned to see Lwoff again. His eyes seemed bluer, the turn of his head more dignified, and his manner warmer. Without giving me time to either display ignorance or express my wishes, he said “We've just discovered how to induce the prophage.” I didn't know what a prophage was, let alone what it meant to induce it. Nevertheless, I retorted immediately “That's exactly what I'd like to work on.” And Lwoff agreed. Much later came a day in 1958 when, my mind wandering on a lazy July evening, I sensed in a flash that there were important analogies between the systems studied at the two ends of our corridor: In both cases, the expression of a cluster of structural genes directed the synthesis of several proteins, and this expression was modulated by a “repressor” encoded by an adjacent regulatory gene. Monod and I baptized this structural gene–regulatory gene ensemble an “operon” (from “to operate”), and we quickly recognized that the operon-repressor system could be combined ad infinitum to produce circuits of increasing complexity, adapting to the demands of the cell. Thus did we discover a “mechanism fundamental to all living beings from their very beginnings, and that would persist as long as they exist … More than ever, research seemed to be identified with human nature … It was by far the best means found by man to face the chaos of the universe.”[*][2] Our breakthrough was the result of “night science”: a stumbling, wandering exploration of the natural world that relies on intuition as much as it does on the cold, orderly logic of “day science.”[**][3] In today's vastly expanded scientific enterprise, obsessed with impact factors and competition, we will need much more night science to unveil the many mysteries that remain about the workings of organisms. [1]: pending:yes [2]: #fn-1 [3]: #fn-2

Genetics, the study of the gene, developed all through the twentieth century. It began with a search for mutations in a variety of organisms and the mapping of these mutations on chromosomes. Since it applied to humans, genetics became of interest to some of the ideologies that flourished during this century. In particular, eugenic ideas were used as an argument by the Nazis for the atrocities they committed in concentration camps. The gene long remained a ‘being of reason’, until the middle of the century, when it turned out to be made of DNA. In 1953, the discovery of the structure of DNA gave a completely new turn to genetics.

We, the members of the Executive Committee of the International Human Rights Network of Academies and Scholarly Societies (Network), oppose renewed initiatives that support an academic boycott of Israeli academic institutions. We also oppose Israeli restrictions on Palestinian students that prevent them from studying at institutions of higher education in Israel, the West Bank, and abroad. We call on national academies affiliated with our Network to do the same. We reiterate our belief in "the free exchange of ideas and opinions among scientists and scholars in all countries," which thereby stimulates "the development of collaborative educational, research and human-rights endeavors within academies and the institutions with which they are affiliated." Boycotts "deny our colleagues their rights to freedom of opinion and expression; interfere with their ability to exercise their bona fide academic freedoms; inhibit the free circulation of scientists and scientific ideas; impose unjust punishment," and impede "the instrumental role played by scientists and scholars in the promotion of peace and human rights" ([1][1]). We also oppose Israeli restrictions on Palestinian students such as the ban imposed in 2000 that prevents all Palestinian students in Gaza from traveling to the West Bank to study, and a statement earlier this month by the Israeli military that it will continue to prevent Gaza students from studying in Israel. Additionally, a recommendation by the Israeli Supreme Court that the Ministry of Defense submit criteria for allowing Palestinian students from the West Bank to study in Israel has repeatedly been delayed to the point that West Bank residents, still banned from studying in Israel, now risk missing Israeli university application deadlines for the coming academic year. We reiterate the hope expressed in our 6 November 2006 statement to the Israeli authorities ([2][2]) that their "policy of academic exclusion will be promptly reversed." In that same statement, we joined the Israel Academy of Sciences and Humanities in opposing "any measures, by any government, restricting or impairing the ability of scientists and students to carry out their scientific work and to discharge their scientific or academic responsibilities." We also agree with four Israeli university presidents and a number of prominent intellectuals who recently wrote that "[b]locking access to higher education for Palestinian students from Gaza who choose to study in the West Bank casts a dark shadow over Israel's image as a state which respects and supports the principle of academic freedom and the right to education" ([3][3]). Lastly, we recall and continue to support the joint statement of cooperation, signed at our Network's May 2005 meeting by Sari Nusseibeh and Menachem Magidor, presidents of Al-Quds University and Hebrew University, respectively, that said, "Our disaffection with, and condemnation of acts of academic boycotts and discrimination against scholars and institutions, is predicated on the principles of academic freedom, human rights, and equality between nations and among individuals. We therefore call upon academics here and worldwide to act in support of our mission, as one which might allow for ending our shared tragedy rather than prolonging it ([4][4]). 1. 1.[↵][5] For full text of 13 June 2002, statement, see [www7.nationalacademies.org/humanrights/In\_Support\_of\_Scientific\_Exchange.html][6]. 2. 2.[↵][7] See [www7.nationalacademies.org/humanrights/Network\_Statement\_Access\_to\_Education\_Nov\_2006.html][8]. 3. 3.[↵][9] The presidents of Ben-Gurion University (Rivka Carmi), the Hebrew University (Menachem Megidor), Haifa University (Aharon Ben-Zeev), the Technion (Yitzhak Apeloig), and a group of Israeli authors, including Amos Oz, A. B. Yehoshua, David Grossman, Nathan Zach, Ariel Hirschfeld, Agi Mishol, and Yitzhak Laor (see [www.gisha.org/index.php?intLanguage=2&intItemId=426&intSiteSN=113][10]). 4. 4.[↵][11] The full text can be found in the Proceedings of the meeting: [www.nap.edu/catalog/11740.html][12]. [1]: #ref-1 [2]: #ref-2 [3]: #ref-3 [4]: #ref-4 [5]: #xref-ref-1-1 "View reference 1. in text" [6]: http://www7.nationalacademies.org/humanrights/In_Support_of_Scientific_Exchange.html [7]: #xref-ref-2-1 "View reference 2. in text" [8]: http://www7.nationalacademies.org/humanrights/Network_Statement_Access_to_Education_Nov_2006.html [9]: #xref-ref-3-1 "View reference 3. in text" [10]: http://www.gisha.org/index.php?intLanguage=2&intItemId=426&intSiteSN=113 [11]: #xref-ref-4-1 "View reference 4. in text" [12]: http://www.nap.edu/catalog/11740.html

On the occasion of the November 2006 annual meeting of the Israeli-Palestinian Science Organization (IPSO), we, the members of IPSO's International Scientific Council, noted with considerable satisfaction the receipt of 71 proposals for joint scientific research between Palestinian and Israeli

My Life in Scienceby Sydney Brenner BioMed Central Limited, 2001. $22.10 pbk (191 pages) ISBN 0 9540278 0 9I first met Sydney Brenner at a Symposium in the USA at the beginning of the 1950s. Short and broad of back, this character seldom went unnoticed. His squarish head, his blue eyes beneath blond brows, enormous, hirsute, shaggy, he ressembled certain Dutch portraits – a real Franz Hals! But behind his slightly sarcastic manner and even devilish aspect, his smile revealed a child's face. Born in South Africa, he had settled in Cambridge University's Laboratory of Molecular Biology, already home of Fred Sanger, Max Perutz, John Kendrew and Francis Crick. A beautiful string of prima donnas into which Sydney fitted perfectly.As soon as he appeared on the scene, Sydney took part in almost every advance in molecular biology. He was involved in the establishment of colinearity between gene and protein, in the demonstrations of the triplet nature of the genetic code, in the discovery of messenger RNA. And when he decided to switch from bacteria to metazoa, he ‘invented’ the nematode, a small worm of which he analysed both genetics and anatomy. As quick with his mind as with his hands, he was interested in everything. In addition, he had a good sense of humour, which could sometimes turn to nasty wittiness.Scientists’ autobiographies are a special kind of literature, for scientists can rarely bring themselves to discuss personal matters. They describe an orderly train of concepts and experiments that they have carefully purified from all affective and irrational dross. They get rid of any personal scent, any human smell. Very few scientists – Jim Watson, Max Perutz – dare to write a piece on life that is not restricted to work. Scientists spend most of their life in puzzlement and doubt. Yet they describe their work as a straightforward achievement, a victorious march from darkness to light. In reality, they hesitate, stumble, feel their way, question themselves constantly. They go from hope to disappointment, from exaltation to melancholy, always wondering whether they will emerge from the dark. Scientists write mainly for their fellow scientists and, above all, for historians of science. Historians of science, however, mistrust scientists’ autobiographies. They know that two scientists telling the same piece of history will not tell exactly the same story.Like many of his collegues, Sydney Brenner has written his ‘scientific life’. He dissects and recalls with great care his major achievements and the steps he took to get to them. What I find most revealing in his account is the less scientific part; that is, the beginning of his life and the way he came to science. It is the story of a young Jewish boy, the son of Lithuanian emigrants who lived in a small town of South Africa. It is of the hard time he endured at school, where his small size attracted a lot of teasing and rough play. He said of himself, ‘I grew up to be a professional coward. I would agree to anything not to get bullied’. But the exceptional talents of this small boy were soon recognized. He decided to become a scientist, and the beginning of the book describes how this remarkable boy became one of the most successful biologists of the century. The book is clearly and simply written. It is difficult to resist the fantastic drive and the intelligence of its leading figure.

Fucose-labeled glycopeptides from four human teratoma cell lines of independent origin show similar elution profiles on Sephadex G-50 column chromatography. The fucosyl glycopeptides elute in two major regions: one near the void volume, the other in fractions corresponding to a molecular weight of 2500-3000. These elution profiles are very different from those obtained with the other human cell lines examined which included 3 lymphomas, 2 colon carcinomas, and HeLa. The elution profiles of the human teratomas, however, show remarkable similarities to those obtained with murine embryonal carcinoma cell culture and early mouse embryos. These results suggest that the excluded G-50 fraction may well contain glycopeptides playing a role in mammalian embryogenesis.

Hexamethylene bisacetamide (HMBA) induces in vitro the cytodifferentiation of PCC3/A/1 mouse embryonal carcinoma (EC) cells. In EC cells, actin is associated with surface structures but microfilament bundles are not seen. After 2 days of HMBA treatment, rounded EC cells are converted to flat adhesive ones with a developed cytoskeleton containing actin and tropomyosin. The ratio of actin to total proteins is constant in EC cells and their HMBA derivatives; but a striking difference is observed for one of the newly synthesized proteins (Mr 34,000) identified as tropomyosin. Synthesis of tropomyosin is followed by its association with actin microfilament bundles, as revealed by indirect immunofluorescence microscopy with specific antibodies.

Two cultured lines of murine embryonal carcinoma, F9 and PCC3, have been grafted to a variety of allogeneic hosts. The host strains have been classified by their resistance or sensitivity to these carcinomas. Resistance seems to be immunological in nature.Allograft rejection does not correlate withH-2 haplotype, and seems to be controlled by a limited number of recessive factors, presumably histocompatibility genes. We infer that these factors have limited polymorphism in the mouse species. Recombinational analysis of strain A/He has revealed the presence of a recessive factor linked to theH-2 locus. Tumor resistance of strains C57BL/6 and AKR appears to result from the interaction of dominant or semi-dominant factors in theH-2 region with other recessive elements in the genetic background.Though F(1) hybrids between resistant mouse strains and the syngeneic strain 129 are largely tumor-sensitive, a low level of hybrid resistance to F9 has been observed and shown to be eliminated by X-irradiation.

THE F9 antigen, defined by antisera raised in syngeneic mice against pluripotent embryonal carcinoma cells, is present on early mouse embryos, spermatozoa and male germinal cells, but not on adult somatic tissues1,2. This antigen, thought to be coded by gene(s) located at, or linked to, the developmentally important T/t complex, may play a part in early embryogenesis3–5. This idea is supported by the fact that the antigen seems to have been conserved during mammalian evolution. Anti-F9 activity is absorbed by sperm of several species including man, rat, rabbit and bull5–7 and there is evidence for its presence on the morulae of rabbit, rat and cow, and on human foetal testicular cells2,5. The cross reaction of anti-F9 with human sperm suggested that undifferentiated stem cells in human teratocarcinomas might also carry F9 on their surface, and we report here the isolation of human teratocarcinoma cell line which expresses F9 antigen.

The ETn (for "early transposon") family of long repeated sequences in abundantly transcribed in early mouse embryos from retroviral-like long terminal repeats. Nucleotide sequencing of two elements does not reveal any long open reading frame nor significant homology to retroviral proteins. The genetic polymorphism, monitored by Southern blotting within and across mouse species, reflects a concerted mode of evolution for the ETn sequences.

Six-day to 12-day mouse embryos were dissected and radiolabelled by culture in the presence of [3H]fucose. The radiolabelled embryos were extensively digested with Pronase. The resulting glycopeptides were analysed by Sephadex G-50 column chromatography. Glycopeptides from 6-day-old embryos were separated into two main peaks: one eluted near the excluded volume, the other in a well-retarded position. This elution profile was similar to that observed with glycopeptides prepared from embryonal carcinoma cells. The relative amount of the high-molecular-weight glycopeptides decreased during embryonic development and particularly around day 9. Glycopeptide elution profiles from 10-day embryos, or isolated organs of 12-day embryos, were indistinguishable from those obtained from differentiated teratocarcinoma-derived or adult cells. At least 30% of the large molecular weight glycopeptides appear to be located at the cell surface.

Surface galactosyl residues of alive embryonal carcinoma cells were tritium-labeled by the galactose oxidase NaB(3H)4 method. The labeled glycopeptides isolated from Pronase digests of the cells, were found to be similar to those prepared from endogeneously labeled embryonal carcinoma cells. They consisted mostly of high molecular weight material, and contained receptors for peanut agglutinin. It is concluded that at least a fraction of the high molecular weight glycopeptides characteristic of early embryonic cells, is displayed on the cell suface.

Embryonal carcinoma (EC) cells and differentiated derivatives grown in tissue culture have rather similar amounts of actin and tubulin. Indirect immunofluorescent microscopy with antibodies to actin shows striking differences in the actin organization in the different teratocarcinoma derivatives. In the EC cells, actin is found predominantly in ruffles and in surface protrusions, as well as in the cytoplasm, but microfilament bundles are not seen. Some of the differentiated clones contain strongly stained microfilament bundles; others contain actin arrangements which appear to be characteristic of the particular cell type. Indirect immunofluorescence microscopy with antibody to tubulin suggests that cytoplasmic microtubules are present both in the EC cells and in the various differentiated states studied. However, the ease with which microtubules can be documented is dependent on how cells are spread on the substratum. During in vitro differentiation of EC cells, changing patterns of actin distribution appear. Cells at the edge of the colony show the characteristic changes in microfilament and microtubular organization before those in the center.

Peanut agglutinin (PNA), a lectin specific for terminal d -galactosyl residues, was found to react with embryonal carcinoma cells, but not with their differentiated derivatives. Receptors for PNA were detectable at the surface of all cells of the quasinullipotent F9 line and on only 50% of the multipotent PCC3/A/1 line. The fraction of the PCC3/A/1 population which expresses the F9 antigen was found to be included in the subpopulation carrying the PNA receptors. PNA + and PNA − subpopulations of PCC3/A/1 were separated by a PNA-mediated reversible agglutination of PNA + cells with rabbit erythrocytes. These subpopulations were essentially F9 + and F9 − , respectively.

SUMMARYRecombination between two mouset–haplotypes,tw12tfandtLub-1, was investigated by screening the tailless progeny of the cross ♀tw12tf/tLub-1+ × ♂Ttf/+tffor the segregation of tufted phenotype,tw12andtLub-1lethal factors, and metacentric chromosome (sincetLub-lhaplotype is associated with a Robertsonian fusion involving chromosomes 4 and 17). The results give a 17% estimate of the recombination frequency between centromere andtf, withtLub-llethal factor mapping about two-thirds of the distance from centromere totfand thetw12lethal factor behaving as if closely linked totf. This further extends the findings of Silver & Artzt (1981) and of Artzt, McCormick & Bennett (1982), and shows that twot–haplotypes with quite independent laboratory histories recombine at a normal level, supporting the notion that allt–haplotypes basically share the same structure.

A muscle cell line was obtained as a clonal isolate from a contractile zone of cultured cells derived from a mouse teratocarcinoma. The cell line (C17-S1-D-T984) differentiates in vitro when the cells become confluent, with the formation of multinucleated myotubes. The muscle fibres contract spontaneously and electron microscopy demonstrates the presence of organized sarcomeric structures. In addition to the presence of the contractile proteins, there is also an accumulation of acetylcholine receptors and the appearance of the M form of creatine phosphokinase, all characteristic markers of differential skeletal muscle.

Another family of long moderately repetitive and dispersed sequences has been identified in the mouse genome. These sequences have a transposon-like structure. A 6-kilobase RNA transcript is detected in undifferentiated embryonal carcinoma cell lines but not in any of the differentiated cell types tested. By R-loop formation, the RNA is colinear with a DNA fragment from a randomly selected genomic clone.

Biochemical and immunological investigations of the early mammalian embryonic development are hampered by the scarcity and the heterogeneity of the material. It is possible to get round some of these difficulties by the use of mouse teratocarcinoma as a model system for studying certain aspects of cellular differentiation.