62 G. CARL HUBER 



section figured, onto the layer of parietal ectoderm. Selenka 

 and Duval, who regard the cells of the primary entoderm as 

 having ameboid properties, are disposed to regard the entodermal 

 cells found lining the parietal ectoderm as having wandered 

 from their seat of origin to the side wall of the vesicle. Sobotta 

 sees no evidence of such wandering of the primary or yolk ento- 

 dermal cells, but suggests that they are drawn to their position 

 on the wall of the vesicle during its increase in size; their wander- 

 ing, therefore, is more relative than absolute. Certain cells 

 nearer the edge of the yolk entoderm, having attachment to the 

 parietal ectoderm, which attachment they retain as the vesicle 

 enlarges, are thought to be drawn from their close relation to 

 the yolk entoderm and to appear as scattered cells lining the 

 parietal ectoderm. Now and then, such cells may divide, re- 

 sulting in further distribution. Sobotta's suggestion seems to 

 me to be more in accord with the observed facts. In vesicle 

 D, the roof, consisting of a single layer of flattened, parietal 

 ectodermal cells, presents several major folds as well as minor 

 folds. The latter particularly account for the variation in 

 thickness of the wall of the vesicle as seen in sections. At the 

 lower left of the figure is seen a portion of the wall as seen cut 

 on the flat, the shape of the two nuclei here shown as seen in 

 surface view may be compared witli the long ovoid form of 

 similar nuclei when seen in cross section. 



Vesicle E of figure 23 (ral Xo. 100, 6 days) presents a stage 

 that is slightly older than the other four vesicles shown in this 

 figure. The floor of this vesicle, the germinal disc, as seen in 

 cross section, present- the form of a triangle with its base rest- 

 ing on the cavity, the blastocele. AYhen compared with the 

 slightly younger stages this portion of the vesicle presents an 

 increase in the number of constituent cells, arranged in irregular 

 layers to the number of five in its thickest portion. The thicken- 

 ing is no doubt in part due to the slight lateral compression of the 

 vesicle, but this does not wholty account for it. The cells con- 

 stituting this thickened germinal disc are for the main of irregular 

 polyhedral form with relatively large nuclei rich in chromatin. 

 A distinct covering layer is not evident. On its under sur- 

 face there is found a single layer of cells of yolk entoderm. The 



