590 J. T. PATTERSON 



subsequently become lined with a layer of mesoderm. The con- 

 dition here presented reminds one somewhat of that found in 

 a corresponding stage of the blastocyst of Pteropus edulis 

 (Selenka and Gohre '92), except that in the case of the latter the 

 entoderm continues around the inside of the trophoblast, forming 

 a complete, inner layer to the blastocyst. 



In the older of the two blastocysts (No. 332) the sections pass 

 exactly parallel to the median axis of the vesicle, and consequently 

 the relation of the different parts of the embryo is clearer than 

 in No. 316. The general conditions of the trophoblast are much 

 the same in the two specimens, except that at one point on No. 

 332 there is a knot of cells (fig. 17, k) which, in the living condi- 

 tion, fitted into a corresponding crypt in the mucosa. In later 

 stages we shall see further evidence of similar knots, which rep- 

 resent points on the trophoblast that have been specifically 

 stimulated to cell proliferation. 



The relation of the entoderm to the embryonic ectoderm is re- 

 markably clear in this preparation (fig. 17). In contrast with 

 the preceding blastocyst, the entoderm of this specimen has under- 

 gone one important change, in that it has folded in beneath the 

 ectoderm, forming all but a closed entodermal sac. Only a small 

 pore-like opening (fig. 17) remains to place the extraembryonic 

 cavity in communication with the cavity of the entodermal sac. 

 On the right-hand side (or lower side, owing to the inclination to 

 the right of the blastocyst) a fusion has taken place between the two 

 layers of the entoderm and a portion of the Trager; but this fu- 

 sion, especially with the Trager, covers a very small area, as it no 

 longer exists in the sections a short distance to either side of this 

 one. The loop of entoderm which lies between the fused area and 

 the pore (fig. 49) is probably comparable to the group of cells sit- 

 uated in a similar position in the other specimen. The loop of 

 cells is especially clear in figure 17. 



Returning to a fuller consideration of the ectodermal sphere, 

 we see that even in so young a stage as that of No. 316 it is no 

 longer a solid mass of cells, as must have been the case at first, but 

 in the central portion there are three relatively large and distinct 

 besides several smaller, less distinct vacuoles. In fact, the 



