406 



VERTEBRATE ZOOLOGY 



COfl 



Perameles with its primitive placenta represents a more nearly prim- 

 itive condition than any other living marsupial so far studied. Such 

 a view would involve the corollary that both modern marsupials 

 and modern placental mammals have been derived from a primitive 

 placental ancestry, possibly akin to the insectivores. 



Conditions in Placental Mammals. Some of the simpler types, 

 such as that of the pig and the horse, are not unlike those seen in 

 the marsupial, Perameles, but in others, as for example the primates, 



the armadillos, etc., the con- 

 ditions are very much modi- 

 fied. In earlier stages, how- 

 ever, the differences are 

 slight. 



The egg of the placental 

 mammal is extremely small 

 and essentially yolkless, yet 

 many changes take place 

 that seem to occur with refer- 

 ence to a large yolk supply. 

 The embryo is developed 

 from a small region of the 

 blastula, and is cut off from 

 the extra-embryonic area, 

 with which it remains con- 

 nected by a slender yolk- 

 stalk. There is a fairly large 

 yolk-sac, without any yolk 

 content, upon which a vitel- 

 line circulation develops up -to the point of blood formation and 

 then goes no further. Amnion, chorion, and allantois form much as 

 in birds, though secondary modifications of all of these membranes 

 are found in various groups. All of these conditions seem to admit 

 of but one interpretation: that the small yolkless mammalian ovum 

 is the lineal descendant of a large-yolked egg similar to that of the 

 monotremes or the reptiles, and that the yolk has been lost in con- 

 nection with the habit of uterine gestation. With all the conserva- 

 tiveness of the typical germ cell, the mammal egg persists in behav- 

 ing much as though it had a large supply of yolk upon which it had 

 to depend for nourishing the embryo. 



FIG. 211. Diagram of the embryo and 

 placenta of the marsupial Perameles obe- 

 sula. Letters as in fig. 209. In addition 

 all. s, allantoic stock; mes, mesenchyme 

 of outer surface of allantois fused with 

 mesenchyme of serous or chorionic mem- 

 brane; st, sinus terminalis; ut, uterine wall. 

 (From Parker and Has well, after Hill.) 



