462 ARTHUE ROBINSON AND RICHARD ASSHETON. 



grammatically ; and a careful examination of thin sections of 

 anural ova, in the stages preceding the completion of the 

 circular blastopore, shows clearly that the descriptions given of 

 the gradual extension of the epiblast over the yolk represent 

 theory rather than fact. 



The changes observable are, firstly, that the cells of the 

 superficial layer of the yolk gradually become more distinctly 

 pigmented ; and, secondly, that they thereafter divide into 

 smaller and more distinct segments, which arrange themselves 

 into two layers, a superficial layer of somewhat cubical deeply 

 pigmented cells, and a deeper layer of less pigmented and more 

 rounded cells, irregularly arranged into two or more rows : the 

 latter are separated by a distinct space from the subjacent 

 remainder of the yolk-cells. This transformation and re- 

 arrangement proceeds backwards towards the blastopore, stop- 

 ping about "352 mm. from that aperture, first on its dorsal 

 and then on its lateral and ventral borders, where an area of 

 fusion remains, which is for convenience divided into a 

 dorsal, a ventral, and two lateral lips or borders. 



We conclude, therefore — (1) That the segmentation of the 

 anural ovum does not result in the formation of a vesicle, the 

 roof of which is epiblast and the floor modified hypoblast, but 

 that at the end of the segmentation the primary layers of the 

 ovum are only partially formed. The roof of the segmenta- 

 tion cavity is epiblast, but the floor or yolk is not modified 

 hypoblast. It consists of indifferentiated germ-cells, the true 

 characters of which are not at first recognisable. 



(2) That the yolk is not enclosed by the gradual extension 

 over it of a previously differentiated epiblast, but that the 

 superficial layer of yolk-cells becomes gradually difierentiated 

 into the two layers of epiblast, leaving a remainder of the 

 yolk. This consists of hypoblast and mesoblast, which are not 

 at first separated from each other. 



The formation of epiblast from yolk-cells does not take 

 place over the whole surface of the yolk, for at the posterior 

 pole of the ovum there is a circular patch of yolk-cells from 

 which no epiblast is formed. The cells in question form the 



