Evolution of Animals 491 



Alike in urodeles and marsupials the rectal and urino-genital 

 canals open into a definite cloaca, though in the latter, as in 

 embryonic higher mammals, it may be shallow or reduced 

 to a common circular fold of integument. In the urodeles 

 and marsupials the oviducts and uteri are paired structures, 

 which in the former open into the cloaca. Now in Salamandra 

 maculosa the eggs are fertilized in the upper region of the ovi- 

 ducts, but then descend into the uterus and there undergo 

 development. The resulting embryos feed on the egg-yolk, 

 but later absorb nourishment through the wall of the uterus, 

 and may undergo gestation for about ten months. The egg 

 capsules with enclosed embryos are then ejected, when the 

 perfect viviparous animal in each at once escapes. In S. atra 

 an even more intimate relation exists between egg, embryo, 

 and the parent uterus, for the several eggs passed from the 

 oviducts into the uterus are gradually absorbed by one or 

 at most two that survive. These develop rich red vascular 

 gills in time, that grow closely against the red richly vascular 

 uterus of the parent. Nutritive and respiratory interchange 

 are thus effected between parent and embryo, till the latter 

 has become fully developed. The former species then might be 

 said to have a uterine placenta, the latter a branchial placenta. 



But in marsupials the usually solitary egg passes down 

 from an oviduct into the uterus. Here "the outer covering 

 of the ovum or false chorion is free from persistent villi, and 

 not adherent to the epithelium of the uterine walls ; for, although 

 fitting into the folds of the latter, it is perfectly and readily 

 separable in its entire extent from them. The umbilical vesicle 

 or yolk sac is large, vascular, and adherent to a considerable 

 portion of the false chorion or subzonal membrane, while the 

 allantois is relatively small, and, although the usual blood 

 vessels can be traced into it, it does not appear to contract 

 any connection with the false chorion, and therefore much 

 less with the walls of the uterus, of such a nature as to con- 

 stitute a placenta. In other forms however, such as the opos- 

 sums, the umbilical vesicle or yolk sac develops temporary 

 villi, which unite with the subzonal membrane, or false chor- 

 ion, to form a disk-like area closely attached to the cells cover- 

 ing the utricular glands of the uterine epithelium and thus 

 forming a so-called yolk sac placenta. The function of this 

 organ is considered to be the transmission of the secretions of 

 the utricular glands to the embryo by means of the umbilical 

 vescicle; the function of the allantois being either respiratory 

 or the absorption of the fluid secreted in the uterine cavity by 

 the utricular glands" (164: 78). 



