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By means of purified rabbit anti-adult chicken kidney antibodies two types of antigens have been identified in the mesonephros: one, localised in the cells of the proximal segment of the secretory tubules, the other characteristic of the collecting segments derived from the Wolffian duct. During the transformation of certain mesonephric tubules into ductuli efferentes and conjugentes of the epididymis the collecting tubule antigen disappears whereas the proximal secretory tubule antigen not only persists but can also be detected in the parietal layer of Bowman's capsules as they transform before connecting with the rete testis.

The presence of a vitellogenin stimulating ovarian hormone (VSOH) in Orchestia gammarella has been demonstrated by gonadectomy and ovary grafting. Vitellogenin synthesis is not affected by molting hormone injection; it decreases after Y-organ (molting gland) cauterization. Studies in progress seem to indicate that the destruction of the median area of the protocerebrum is followed by a decrease of this synthesis.

Gonadal and mesonephric protein patterns from 19 day old normal chick embryos were investigated by two-dimensional polyacrylamide gel electrophoresis. Under these conditions, several sex-specific polypeptides were detected. As concerns gonadal extracts, four sex-specific polypeptides, all restricted to the cytosol, were present in the testis, whereas three sex-specific polypeptides, two localized in the cytosol, the other being membrane-bound, were identified in the ovary. Among the ovary-specific polypeptides two proved to be estrogen-dependent. They appeared in the left testis of embryos after early estradiol benzoate treatment and their expression was reduced in the ovary after early exposure to the antiestrogen, tamoxifen. Mesonephros extracts of both sexes also differed in their protein composition since three additional polypeptides (one in both the cytosolic and membrane fractions, the others in the cytosol) not found in females were found to be present in males. None appeared to be affected after either estradiol or tamoxifen treatment.

After absorption with adult Chicken liver homogenate, antisera produced in Rabbits against the DOC-soluble fractions of the sediments obtained after centrifugation of adult Chicken kidney homogenates, react with at least two types of antigens. One, found to be kidney-specific, is localised at the apical part of the cells of the proximal segment of the secretory tubules. The other, also present in the digestive tract, is detectable in the ureter and its derivatives, the collecting tubules by which it seems to be excreted. Despite the lack of organ specificity the latter represents an interesting marker for studying the early steps of the development of the renal collecting system.

In the chick embryo the nephrogenic blastema cells are determined before contact with the natural inductor (Wolffian duct or ureter) is established. In response to heterogeneous inductors, such as spinal cord or embryonic endoderm, the blastema cells differentiate into complete secretory tubules as shown by immunohistology.

The cytotoxic properties of purified antibodies reacting specifically with an antigen of the proximal secretory tubule cells have been studied. Intravenous injections of these antibody preparations together with complement (Guinea pig serum) into 2-3,5 day old or 12-13 day old chick embryos do not interfere with the development of either the meso-or metanephros. Furthermore, when suspensions of live metanephric cells treated first with anti-proximal segment antibodies and second with complement are cultured under organotypic conditions, one observes the reconstitution of all the characteristic segments (proximal, intermediate, distal and collecting) of the urinary tubule. It appears therefore that, in vivo as well as in vitro, these antibody preparations do not exert any cytolytic activity although it can be demonstrated that, at least under in vitro conditions, they bind to the surface of the proximal secretory cells and that the antibody-sensitised cells fix complement.

By means of liver-absorbed rabbit anti-adult rat lens sera, and by using the indirect immunoperoxidase technique, lens-specific antigens are detectable in a few isolated cells, or in small groups of cells, of the placode shortly after the onset of invagination. In the deeply invaginated lens rudiment, all the cells, exept those in the area adjacent to the ectoderm (presumptive lens epithelial cells) display a positive reaction. Once the lens vesicle has formed, all the cells contain detectable amounts of lens-specific antigens.

Among the cellular antigens, demonstrated by their reaction with Rabbit anti-embryonic Chick liver serum, two are present in the supernatant fraction obtained after ultracentrifugation of liver extracts from 6-day-old Chick embryos, and in the sedimentable fraction from the 9th day onward. A third antigen, detectable from the 9th day of incubation, is found only in the sedimentable fraction. The three antigens exist, but at much lower concentrations, in adult Chick kidney. Rabbit anti-embryonic Chick mesonephros serum permits the demonstration of at least one kidney-specific antigen. The latter, localized at the apical part of the cells of the proximal secretory tubules, is present in mesonephros and embryonic or adult metanephros. Although suggested by several observations, the presence of antibodies reacting with mesonephros-specific antigens could not be definitely established.