DEVELOPMENT OF THE ALBINO RAT 59 



forming the roof of the vesicle, not shown in the figure, and 

 known as the parietal or transitory ectoderm. In the floor or 

 germ disc, there is evident a single layer of cells bordering the 

 segmentation cavity or blastocele and possessing a more gran- 

 ular protoplasm, which stains a little more intensely in Congo 

 red. Their differentiation and characteristic reaction to stain- 

 ing agents is at this stage of development not quite so distinct 

 as in slightly older stages. This layer of cells, similar to that 

 described by Sobotta for the blastodermic vesicle of the mouse 

 in essentially the same stage of development, he has termed the 

 yolk entoderm, 'Dotter entoderm,' a designation which is here 

 followed. In the more superficial layer or layers of cells no 

 characteristic differentiation is observed. In no portion of the 

 floor of this vesicle was a distinct covering or trophoblast layer 

 recognized. 



In the vesicle, a section of which is reproduced in B of this 

 figure (rat No. 100, 6 days), the floor or germ disc presents 

 essentially the same structure as that shown in A. The vesicle 

 shown under B, was also folded, especially its roof, which was 

 drawn to one side and was thus not cut through its entire length 

 in the section figured. Furthermore, the section chosen for 

 drawing does not pass quite through the center of the germ disc, 

 but a little nearer to one of its edges, which probably accounts 

 for the fact that there is recognized for the greater part only a 

 single layer of cells, superimposed over the yolk endoderm, 

 which layer is continuous with the parietal or transitory ecto- 

 derm forming the roof of the vesicle. The cells forming the 

 yolk entoderm constitute a single layer and are quite distinctly 

 differentiated; one of the cells shows a mitotic phase. The roof 

 of the vesicle formed by the parietal or transitory ectoderm, is 

 composed of a single layer of flattened cells with flattened nuclei, 

 the form and structure of which is more correctly shown in the 

 right half of the roof wall, which in the section is cut transversely, 

 while the left half, owing to the folding, is shown as cut obliquely. 



In C of figure 23 (rat No. 99, 6 days), there is shown a greatly 

 compressed blastodermic vesicle, taken from a series* of cross 

 sections of the uterine horn. In this figure there is reproduced 

 the fifth of a series of 10 sections of 10 fj. thickness; therefore, 



