POLYEMBRYONIC DEVELOPMENT IN TATUSIA 579 



are in the act of migrating to the surface. The cells lying within 

 the mass (b and d) are rounded in outline, while those in the act of 

 coming to the lower surface are invariably elongated in the direc- 

 tion of migration. 



The evidence obtained from a study of these sections indicates 

 that each group of two or more entodermal cells has, in all proba- 

 bility, arisen from a single primary cell, or entodermal mother- 

 cell (using Hill's terminology). Futhermore it suggests that the 

 primary cell may, from the first, be situated on the surface; and 

 consequently will undergo no migration; or it may migrate to the 

 surface without having undergone division; or again, divisions 

 may have come in before the migration occurs. 



Blastocyst No. 296 came from the proximal part of the left fallo- 

 pian tube, and measured 0.376 mm. in diameter. The embryonic 

 spot had a very even outline and measured 0.161 mm. across. 

 It is distinctly thicker than the last specimen, averaging about 4 

 cells, exclusive of the trophoblast (fig. 10). The arc covered by 

 the embryonic spot is 50, 42'. 



In every way the blastocyst is more advanced than any we have 

 so far described, It is not only larger, but also shows a higher 

 state of differentiation. One feature in particular, although not 

 entirely unique since it has been observed in one or two other cases, 

 is nevertheless worthy of mention. This is the presence of faint 

 structures on the outer surface of the trophoblast (fig. 37). These 

 were observed in the preserved egg and were naturally taken to be 

 the follicle cells, which sometimes adhere to the ovulated mamma- 

 lian egg and persist for some time; but in section they are seen to 

 be protrusions or exudations from the trophoblastic cells, and are 

 probably formed at the time the egg is fixed. 



The number of entodermal cells in the median section is thirteen 

 as against sixteen ectodermal cells. This would seem to indicate 

 that there has been a great increase of entodermal cells, but in 

 several of the other sections they are very much less numerous, 

 owing to the fact that the median section passes through the prin- 

 cipal groups of entodermal cells. 



The difference in size between the ectodermal and entodermal 

 cells is very obvious (fig. 10), and, as compared with the preceding 

 figures, stands in sharp contrast. 



