A table for the normal stages of development of a hynobiid salamander, Hynobius nigrescens Stejneger, is presented. The materials used in the present study were collected from a breeding pond in Iwamuro-mura, Niigata Prefecture. The eggs were ca. 2.6mm in diameter. The larvae were fed on Tubifex. Under a constant larvae metamorphosed at the age of 80 days. and the total
The process of gonad development in the frog Rana nigromaculata was observed using the electron microscope. The gonadal medulla was formed by the proliferation and displacement of the epithelial cells within the primordial gonad, and a distinct continuity was observed between the cortical and medullary cells. Sex differentiation of the gonad occurred directly from the sexually indifferent primordial gonads. In the rudimentary testes, the continuity between the cortical and medullary regions increased closer, and the intermingling of cortical and medullary cells was evident. The inner region of the cortex developed into a cord-like structure and subsequently differentiated into rudimentary seminiferous tubules. The medulla differentiated into the testicular rete and efferent duct. In the rudimentary ovaries, the cortex and medulla were separated and the ovarian cavity was formed in the medullary region. In the cortex, the cortical cells surrounding oocytes which had reached the diplotene stage, differentiated into follicular cells. The intrusion of mesenchymal or blastemal cells derived from extragonadal regions into the cortex or medulla was never observed. These findings do not support Witschi's cortico-medullary antagonistic theory of sex differentiation.
Gonadal development in Rhacophorus arboreus, a sexually semidifferentiated type of tree frog, was observed by means of the electron microscope, and cell proliferation kinetics were examined autoradiographically. The genital ridge consisted of coelomic epithelial cells and primordial germ cells. The gonadal medulla was formed by the segregation of epithelial cells within the primordial gonad. Thereafter, the medullary cell mass was well developed and oogenesis began in the gonadal cortex, irrespective of genetic sex. During metamorphosis, the ovarian cavity was formed in the medullary mass. This ovarian structure developed further in females. In males, on the other hand, a layer of medullary cells comprising the epithelium of the ovarian cavity proliferated rapidly and reformed a large cell mass. The degeneration of ovarian follicles and the formation of cell cords (rudimentary seminiferous tubules) were seen in the cortex. These cell cords were separated from the superficial epithelium and continued to the medullary mass (rudimentary testicular rete). These results clearly indicate that both the cortical and medullary cells are derived from the coelomic epithelium and that the development of the cortex and medulla is not always antagonistic in the course of sexual differentiation.
A table for the normal stages of the development of the Japanese lungless salamander, Onychodactylus japonicus, is presented. Adult males and females, collected in Hinoemata, Fukushima Prefecture, in the breeding season, were injected with frog pituitaries, and oviposition was induced. The number of eggs laid by one female was 11 on the average.
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