588 J. T. PATTERSON 



cavity between the Trager and the ectoderm. Material with 

 which to follow the steps through which the blastocyst passes 

 during this is not at hand, for there is here a slight gap in the 

 series. However, I have been fortunate enough to obtain two 

 blastocysts which show the condition immediately following the 

 inclusion of the ectoderm. The two specimens are nearly of the 

 same age, one being slightly more advanced than the other. 



The younger blastocyst (No. 316) was found attached to a 

 small leaf-like outgrowth from one of the folds of the placental 

 mucosa. The outgrowth lay horizontal to the surface of the mu- 

 cosa, and the vesicle had gained attachment to its under side, 

 close to the edge. One side of the trophoblastic wall caved in 

 during fixation, otherwise the vesicle is in an excellent state of 

 preservation (fig. 16). 



That portion of the trophoblastic wall which has caved in pre- 

 sents nothing different from what was observed in earlier stages, 

 except that the cells are more attenuated, but the opposite wall of 

 the vesicle has undergone a marked thickening, in addition to a 

 great increase in its cellular elements. The cells have, therefore, 

 changed in shape from a flattened squamous type to a distinctly 

 columnar epithelium. All the thickened portion of the wall was 

 originally in contact with the mucosa, but during the process of 

 fixation the leaf -like fold of the mucosa has undergone a great deal 

 of contraction and has shrunk away from the trophoblast, as is 

 evident from its folded appearance. Evidently the thickened 

 trophoblast owes its existence to contact with the mucosa, which 

 in some way specifically stimulates the cells to a striking activity, 

 as shown by the rate of division and change in shape. The spe- 

 cific reaction between the trophoblast and the mucosa must be 

 limited to that portion of the uterine epithelium which covers the 

 placental area, otherwise the wall of a free vesicle would give evi- 

 dence of response before the blastocyst had reached this area. 



It is also a point worthy of note that the columnar cells send 

 out pseudopod-like processes, which eat into the mucosa, giving 

 it a serrated appearance in section. That the placental tropho- 

 blast has therefore a specific action on the trophoblastic cells can 



