POLYEMBRYONIC DEVELOPMENT IN TATUSIA 647 



have arisen through the shifting of cells from the thick pole of the 

 vesicle (figs. 1, 21, 22, 59-61; pp. 598-603). 



16. Soon after its origin, each primary diverticulum gives rise 

 to two secondary diverticula or buds. One of these buds appar- 

 ently is but an extension from the tip of the primary diverticu- 

 lum, while the other takes its origin from the left-lateral portion 

 of the tip of the diverticulum. The embryonic buds extend 

 toward the Trager down along the inner side of the entodermal 

 portion of the blastocyst wall as tube-like processes, which in- 

 volve not only the thickened lateral plates of ectoderm, but also 

 portions of the thin endothelial-like wall of the vesicle (figs. 23- 

 29, 62-74; pp. 603-610). 



17. The part of the ectodermal vesicle which remains after the 

 embryonic tubes are given off becomes the common amniotic 

 vesicle. It retains for some time connections with the embryonic 

 tubes by means of the amniotic canals, which are differentiated 

 from the proximal parts of the original diverticula. The charac- 

 teristic paired condition of litters of T. novemcincta is the result 

 of the method by which two secondary buds arise from each of 

 the primary diverticula. The common amniotic vesicle eventu- 

 ally degenerates and disapppears (figs. 5, 80-84; pp. 611-613). 



18. Each embryo differentiates within the secondary diverti- 

 culum, deriving its ectoderm from a portion of the lateral plate 

 which was carried down into the diverticulum, and its entoderm in 

 loco from the primitive entodermal sac or yolk-sac. The embry- 

 onic mesoderm arises from a typical primitive streak region in 

 each embryonic primordium (figs. 75-84; pp. 614-617). 



19. The region of the so-called Rauber's layer forms the. seat 

 of attachment of the blastocyst. This region is soon trans- 

 formed into a syncytium, from which the embryonic nuclei pass 

 over into the mucosa, destroying it by their phagocytic or his- 

 tolytic action (figs. 31-33; pp. 617-619). 



20. The more superficially situated nuclei of the embryonic 

 syncytium organize the Trager epithelium, which, together with 

 the overlying mesothelial layer, forms the lower portion of the 

 chorionic wall. The Trager proper, which forms a thickened 

 annular zone about the base of the attached vesicle, gives rise 



