92 ARTHUR ROBINSON, M.D., AND RICHARD ASSHETON, M.A. 



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, stopping 

 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 segmentation 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 differentiated 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 yolk-plug, and when the epiblast 

 formation has arrived from all sides at their margin, the circular 

 blastopore or anus of Rusconi is first definitely established. 



During the period of differentiation of the epiblast the archenteron 

 has also been forming. The first indication of this cavity is the 

 appearance of a curved area of pigmentation (fig. 1 a and fig. 2 A R) 

 of semilunar outline amidst the yolk-cells at the posterior pole of the 

 ovum above the equator. The convexity of the pigmented area is 

 directed forwards amidst the yolk-cells, towards the segmentation 

 cavity. The angles are turned ventro-laterally. The centre of the 

 concavity of the semilunar area corresponds to the dorsal lip of the 

 blastopore. As the convexity of the area extends forwards towards 



