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If the somitic mesoderm of a 2-day chick embryo is destroyed by X-irradiation, the adjacent limb develops with a normal pattern of connective tissues, but is devoid of muscle. The innervation of muscleless wings produced in this way was examined in silver-stained whole mounts, fixed 3 to 8 days later. The main nerve trunks and their cutaneous branches developed normally; but the nerve branches which in a normal limb would lead to individual muscles were generally absent. In almost all those exceptional cases where muscle nerve branches were present, muscle was found to be present also, despite the X-irradiation. Where there was no muscle, the muscle nerve branches apparently did not even begin to form. As a control for side effects of the X-irradiation, wing buds were grafted from normal to irradiated embryos and vice-versa, and again analysed for their innervation. The results confirmed that the absence of muscle nerve branches was due to the absence of muscle cells in the limb. Thus (1) the routes taken through a limb by the main mixed nerve trunks and by their cutaneous branches are determined by the connective tissues, and not by any mechanisms requiring muscle cells; but (2) muscle cells are necessary to provoke the formation of the side branches leading to the sites of individual muscles.

The developmental origin of the sternal component of the ribs and the conditions for the development of the rib cage have been elucidated in bird embryos.Experiments consisted in homo-or heterospecific transplantations of 2-day-old quail or chick embryo somitic mesoderm into chick hosts of the same age in ortho-or heterotopic position along the cephalo-caudal axis.Results show that vertebral halves, ribs (not only their vertebral segment, but also their sternal component when they possess one), the trunk and intercostal muscles, as well as at least part of the scapula originate exclusively from the somitic material, while the sternum, ventral muscles and the other parts of the pectoral girdle are derived from the somatopleural mesoderm.The development of the rib basket is subjected to following rules:—only the somitic mesoderm of the prospective thoracic region (somites 19–26) is able to give rise to ribs.—only the somitic mesoderm of the posterior thoracic region (somites 22–26) is able to develop ribs with a sternal component.—the vertebral component of ribs can develop outside the thoracic region.—contrariwise, the sternal component can form only in the vicinity of the sternal anlage, i.e. within about three somites in front and rear of the normal limits of the thoracic region.It is concluded that the somitic mesoderm is already regionalized at a stage slightly pre-ceding its metamerization and that the somatopleural territory of the sternum plays a morphogenetic role in the development of the sternal component of ribs, although it does not make a cellular contribution to their construction.

The aim of this study is to test the ability of the intrinsic wing musculature to develop in the absence of somitic mesoderm. The experiments were performed on 2- to 2.5-day chick embryos either by replacing the somitic mesoderm adjacent to the wing field with a piece of 9-day chick embryonic midgut or by destroying, through local X-irradiation, not only the somitic mesoderm of the wing level, but also at least three somites (or presumptive somites) anterior and/or three presumptive somites posterior to the wing level. The replacement of somitic tissue scarcely affected the organogenesis of the forearm musculature, at least when both bones were present. In the other experiments, radio-destruction severely impaired the development of the forearm muscles, which were seldom all present and in most cases were entirely missing. The absence of a given muscle involves the simultaneous absence of the corresponding tendons. The possible origins of the muscles that formed despite the removal of the somitic mesoderm are discussed.

The regional embryonic origin of trunk and limb musculature was determined through heterospeciflc homotopic or heterotopic transplantations of quail somitic or somiticsomatopleural mesoderm into chick hosts, and through localized X-irradiation of the somitic mesoderm. Experiments were performed on 2-day embryos. Results show that the myoblastic component of all truncal and appendicular muscles is of somitic origin. Intra- and perimuscular connective tissue as well as tendons are of somatopleuralorigin. X-ray destruction of the somitic mesoderm at and beyond the wing level resulted in the complete or almost complete absence of musculature in the wing and corresponding truncal region. The mapping of the cephalocaudal origin of the various muscles was found to be asfollows: Intrinsic and extrinsic muscles of the wing and scapular girdle Somites 12—20 Giand pectoral muscle Somites 12—22 Intercostal muscles Somites 19—26 Abdominal muscles Somites 27—29 Intrinsic and extrinsic muscles of the leg and pelvic girdle Somites 26—32 Dorsal and intervertebral muscles Metameric level, specific origin In the heterotopic transplantations, the grafted somitic mesoderm gave rise to site-specific morphogenesis, irrespective of the cephalocaudal level of its origin. This result demonstrates that, at 2 days of incubation, the myogenic somitic cells are not regionalized.

The aim of this study performed in the embryonic chick wing is to test the ability of the tendons to form and develop in the absence of the muscle bellies. The experiments were performed on 2-day chick embryos by destroying a portion of the somitic mesoderm by local X-irradiation. The irradiated part included the wing somite level 15—20 and extended three somites (or presumptive somites) in front and two to six presumptive somites in the rear of the wing somite level. The wings of the operated side were examined histologically 3—8 days after the X-irradiation. The radio-destruction of the somitic mesoderm totally inhibited or severely impaired the development of the forearm muscles. But, despite the absence of the flexor and extensor muscles the differentiation of the distal manus tendons could be observed. This differentiationoccurred at the same time and in the same position as in controls. However, these tendons were transient structures. They disappeared within three days after their individuation. Two mechanisms that progressed in proximo-distal direction were involved in their resorption: cellular dislocation and cell death. We conclude that tendons start to develop autonomously from the muscle bulks, but for their maintenance and further development they require connexion to a muscle belly.

Quail-to-chick grafting experiments performed during the third day of incubation demonstrate that somites can contribute to limb development. In orthotopic recombinations, migrating cells originating from the grafted unsegmented or segmented somitic mesoderm adjacent to the wing or leg field end up in the musculature respectively of the wing or the leg, where they express exclusively myogenic properties. Thus, in these heterospecific recombinations, the anatomical muscle has a double origin: muscle bulk of somitic origin; tendons and connective tissues of somatopleural origin. Similar features are observed in heterotopic recombinations with (segmented or unsegmented) somitic mesoderm located cranially or caudally to the limb levels. In the reverse chick-to-quail grafting experiments, the somitic participation to the limb mesoderm can also be observed. But it is less regular than that obtained in the quail-to-chick recombinations, and the muscle bulk is made up in various proportions of graft-originated somitic cells and of host somatopleural cells. The possible existence of juxtaposed and interdigitated myogenic and tendinogenic compartments is discussed in view of the dissimilarity between the results of the two kinds of heterospecific recombinations.

The capacity of the neural crest cells to differentiate into medullary cells of the adrenal gland has been tested for different levels of the cephalo-caudal axis of the chick embryo and at different stages.The procedure consisted in associating a fragment of neural crest cells containing spinal cord with a section of an embryo containing the prospective cortical territory. The latter was previously deprived in ovo of presumptive medullar adrenal cells by localized X-irradiation of its spinal cord.The portion of the spinal cord which has the capacity to give off medulloblasts is located not only within the prospective adrenal area (somite levels 18–22), but also within a region extending over a length of six somites in front and seven somites behind the presumptive zone.This capacity, which is restricted in space, is also limited in time. It is possessed by the portion of spinal cord defined above from 6 h before the migration of the first neural crest cells and lasts roughly for 24 h after the beginning of migration. Furthermore, the experiments indicate that the differentiation of neural crest cells into medullocytes requires the inductive influence of the cortical