582 J. T. PATTERSON 



of about 0.430 mm. At this time the embryonic spot has not 

 reached its maximum expansion, that is, it has not completely 

 flattened out. Consequently in section the entoderm forms a 

 slightly curved line, due to the bulging out of the mass of embry- 

 onic ectoderm (fig. 14). The ectodermal cells are large, relatively 

 clear, and sharply cut off from the overlying trophoblastic cells. 

 On the inner surface of the mass they are less sharply separated 

 from the entodermal cells, which here and there send processes up 

 between some of the bordering ectodermal cells. Such processes 

 are undoubtedly the last remnants of the migrating entodermal 

 cells to be withdrawn from the embryonic mass. 



The condition of the entoderm is of much interest. It does not 

 as yet form a complete sheet of cells underlying and coextensive 

 with the embryonal ectoderm. In places the entoderm may be 

 wanting for more than the width of a cell. Furthermore, on the 

 right side of the sections of specimen No. 249 the entoderm is fre- 

 quently wanting, indicating that it must have taken its origin from 

 the left portion of the embryonal mass. In some of the earlier 

 blastocysts this same fact was observed. The smaller, deeply 

 staining cells, which give rise to the entoderm, were found to ex- 

 tend over not more than two-thirds of the embryonic mass. 

 Finally, in the largest free blastocyst secured (No. 300) and one in 

 which the entoderm is completely segregated, we find this same 

 excentricity of the entoderm. It was so evident in this specimen 

 that I have gone to the trouble of making a special preparation 

 for the purpose of demonstration by a photograph. The vesicle 

 in question was slightly stained in eosin and imbedded in paraffin. 

 Under the high power of the binocular microscope the non-em- 

 bryonic hemisphere was carefully pared away with a sharp razor. 

 The remaining hemisphere was dissolved out of the paraffin, 

 stained in hematoxylin, and mounted in balsam, with the cut sur- 

 face uppermost. Thus it was possible to get an unobstructed 

 view of the entoderm, and since the embryonic mass had become 

 completely flattened, the entodermal layer lies in a single plane. 

 The exposed sheet of entoderm was then studied and pho- 

 tographed (fig. 42). 



