ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 453 



The allantois is the receptacle for the urine formed within the body 

 of the embryo. It is present as a reservoir only in those animals which 

 have an embryonic excretion. Its size varies with the size of the 

 Wolffian bodies and with their duration. The urethral opening, though 

 present, is- not normally used for the passage of foetal urine. 



In tliose animals without the possibility of a continuous urinary 

 ■excretion within the embryo, i.e. with an early degeneration of the 

 Wolffian body, the placenta is provided with an apparatus similar to 

 that found in the glomeruli of the Wolffian body or the kidney, thin 

 plates of epithelium overlying the foetal capillaries. These appear in 

 the placenta at about the time when the Wolffian body begins to 

 degenerate, or, in the case of the rat, which never develops mesonephric 

 glomeruli, at about the time of the normal development of the glomeruli 

 in other embryos. These plates continue and increase in number till 

 birth. They are apparently of greater extent in animals whose embryos 

 are provided with large Wolffian bodies. In the placenta of animals 

 with a continuous embryonic urinary excretion, similar plates are not 

 found. It appears that embryonic and foetal urinary excretion takes 

 place wholly through the placenta in the rat ; that it takes place first 

 through the Wolffian body, and later through the placenta in the rabbit, 

 guinea-pig, and man ; and that it never takes place through the placenta 

 in the pig, sheep, or cat. 



Yolk-Sac of Pig- Embryo.*— H. E. Jordan gives an account of the 

 minute structure of the yolk-sac, with its endoderm, mesothelium, 

 mesenchyine, endothelium and blood-cells. The latter include (1) the 

 hajmoblasts or IJood mother-cells ; (2) the erythroblasts ; (8) thenormo- 

 l)lasts, which along with (2) form the erythrocytes; and (i)' the 

 giant cells, both megakaryocytes and polykaryocytes. 



The yolk-sac endoderm is of course continuous with the epithelial 

 lining of the gut, and the cells show numerous basal filaments, like 

 mitochondria, which probably have secretory significance. In ancestors 

 with yolk-laden eggs the endodermal cells undoubtedly had the function 

 primarily of elaborating a digestive fluid for the liquefaction and 

 assimilation of the yolk. In yolkless umbilical vesicles, the endoderm 

 apparently still develops and differentiates in accord with an " ancestral 

 memory," though it can perform no true digestive function. The 

 umbihcal vesicle has, however, taken on a secondary function as an 

 early, perhaps original, centre of haemopoiesis. 



The angioblast or original primordium of the vascular tissue in the 

 yolk-sac has its derivation from the mesenchyme. The mesothelium 

 of the yolk-sac of pig embryos between .5 and 12 mm. does not produce 

 hnemoblasts. Nor is there any satisfactory evidence that the meso- 

 thelium of the body-stalk and chorion function to this end. The 

 mesenchyme may differentiate directly into endothelium or into htemo- 

 blasts. Hffimoblasts arise extensively at the 10 mm. stage from the 

 endothelium of the yolk-sac blood-vessels. The endothelia of the 



* Amer. Journ. Anat., xis. (1916) pp. 277-303 (2 pis.). 

 Oct. 18th, 1916 2 



