420 OUTLINES OF CHORDATE DEVELOPMENT 



of placentation, which frequently do not parallel the usual 

 ordinal classification, so that even within a single order (e.g., 

 Ungulata, Primates) there may be divergences which con- 

 siderably exceed the range of the morphological traits upon 

 which the orders are based. 



Before attempting to describe any of the actual details of the 

 structure and development of the placenta, we must give a 



amn. 



vase, ompft. 



. s.t 



bU. ompfv. y.spl 



FIG. 174. Diagram of the arrangement of the fcetal membranes in the 

 Marsupial, Perameles. From Hill. The ectoderm is represented by a light con- 

 tinuous line, the endoderm by a dotted line, and the mesoderm by a heavy line. 

 amn, Amnion; all. c., allantoic cavity; all. mes., allanto-chorionic mesenchyme; 

 all. s., allantoic stalk; bil, omph.. ectodermal and endodermal wall of yolk-sac; 

 ch., margin of true chorion; cos., exoccelom; coe. IL\, inner wall of allantois; proa, r., 

 persisting remnant of proamnion; s. t., sinus terminalis; vase, omph., three-layered 

 portion of yolk-sac wall; y. c., cavity of yolk-sac; y. spl., invaginated splanchno- 

 pleural wall of yolk-sac. 



general outline of the ontogenetic history of the embryonic 

 membranes and appendages. We may consider first the 

 method by which the young embryo or "ovum" (blasto- 

 dermic vesicle) effects its primary relation with the uterine 

 wall. In the remaining pages it is understood that what is 

 said is limited to the Eutheria. 



