POLYEMBRYONIC DEVELOPMENT IN TATUSIA 



655 



Tro 



Tr Ep 



19 



Fig. 19 Median section in a right-left plane of specimen No. 234. The devel- 

 opment of the mesoderm has made considerable progress, a^wfc^JMMBtattezsspfr^be 

 MM* V ? the point where the entoderm joins the chorionic ecto- 



derm (trophoblast). X 142. 



Fig. 20 A. section similar to the preceding, from specimen No. 256. The vesicle 

 has collapsed and thus appears very much flattened in the figure. The chorionic 

 ectoderm has sloughed off and disappeared. The mesodermal vesicles have ex- 

 panded until they now fill up the entire extraembryonic cavity. X 113. 



