Section 19. The student should now compare the figures on [Sheet 17]. In the male, tubular connections are established between the testes and the middle part of the primitive kidney (mesonephros). These connections are the vasa efferentia (v.e.), and the mesonephros is now equivalent to the epididymis of the rabbit. The Wolffian duct is the urogenital duct of the adult, and the Mullerian duct is entirely absorbed, or remains, more or less, in exceptional cases.
In the female, the Mullerian duct increases greatly in length-- so that at sexual maturity its white coils appear thicker and longer than the intestine-- and becomes the oviduct; the Wolffian duct is the ureter, and the mesonephros is not perverted in function from its primary renal duty.
Section 20. Tabulating these facts--
In the adult male:
Pronephros disappears.
In the adult female:
The Mullerian duct (? = pronephric duct) disappears.
Mesonephros = Epididymis; its duct, the urogenital.
Metanephros and duct, not clearly marked off from
Mesonephros.
(Compare Dog-fish, [Section 19].)Pronephros disappears.
The Mullerian duct, the oviduct.
Mesonephros and Metanephros, the kidney, and their unseparated ducts, the ureters.
Section 21. Hermaphrodism (i.e., cases of common sex) is occasionally found among frogs; the testis produces ova in places, and the Mullerian duct is retained and functional. The ciliated nephrostomata remain open to a late stage of development in the frog, and in many amphibia throughout life. Their connection with the renal tubuli is, however, lost.
Section 22. The alimentary canal is, at first, a straight tube. Its disproportionate increase in length throws it into a spiral in the tadpole (int. [Figure 11]), and accounts for its coiling in the frog. The liver and other digestive glands are first formed, like the lungs, as hollow outgrowths, and their lining is therefore hypoblastic. The greatest relative length of intestine is found in the tadpole, which, being a purely vegetable feeder, must needs effect the maximum amount of preparatory change in its food.
2. _The Development of the Fowl_
Section 23. The frog has an ovum with a moderate allowance of yolk, but the quantity is only sufficient to start the little animal a part of its way towards the adult state. The fowl, on the contrary, has an enormous ovum, gorged excessively, with yolk, and as a consequence the chick is almost perfected when it is hatched. The so-called yolk, the yellow of an egg, is the ovum proper; around that is a coating of white albumen, in a shell membrane and a shell. At either end of the yolk ([Figure 1], y.) twisted strands of albuminous matter, the chalazae (ch.) keep the yolk in place. The animal pole is a small grey protoplasmic area, the germinal area (g.a.), on the yolk.