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 Traiger 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 Traiger proper, which forms a thickened 
annular zone about the base of the attached vesicle, gives rise 
