60 G. CARL HUBER 



the third dimension of the vesicle is approximately 100 At. It 

 is evident that had this vesicle been cut in a favorable plane at 

 right angles to the present series, or parallel to the mesometrial 

 plane, its form would have approached that of a circle. I have in 

 my possession one vesicle of this stage of devlopment, similarly 

 compressed, cut parallel to the plane of compression, in which 

 almost the entire roof falls within a single section of 10 n thick- 

 ness. The structure of the vesicle shown in C is very similar 

 to that shown in A and B of this figure. The normal form of this 

 vesicle is quite readily reconstructed from a study of the series 

 of sections into which it has been cut. The cells of the yolk 

 entoderm are evident. The parietal or transitory ectoderm 

 constituting the roof consists of a single layer of much flattened 

 cells, with relatively few nuclei, having, as seen in cross section, 

 a long ovoid form, which, when seen in surface view present a 

 regular, nearly circular outline (see lowermost nucleus in the 

 figure). In similarly compressed vesicles cut parallel to the 

 plane of compression, the germ disc may appear as consisting 

 of three to four layers of cells. In an imaginary section passing 

 in a plane at right angles to that figured in C, and having perhaps 

 a slight obliquity, the germ disc would appear as if much thicker 

 than that shown in A and B of this figure. Such sections may 

 readily lead to false conclusions. 



It seems evident from a study of the material at my disposal 

 that during the sixth day after the beginning of insemination 

 in the albino rat, the blastodermic vesicle or blastocyst, which 

 has its anlage in the latter part of the fifth day, enlarges relatively 

 rapidly; this largely owing to a distension of the segmentation 

 cavity or blastocele. This enlargement is accompanied by a 

 flattening and extension of the enclosing roof cells and by a re- 

 arrangement of the cells of the floor, which is reduced in thick- 

 ness to a discoidal area, the germinal disc or germ area, forming 

 about one-fifth to one-sixth of the wall of the vesicle and con- 

 sisting of two or three layers of cells. During the rearrangement 

 of the cells which constitute the floor of the vesicle, those adjacent 

 to the segmentation cavity or blastocele differentiate to form the 

 anlage of the yolk entoderm. The remaining cells of the ger- 



