FORMATION OF THE LAYERS. 247 



noted that at first its floor is formed by the lower layer cells 

 and not by the yolk, and that its roof is constituted by both the 

 lower layer cells and the epiblast cells. The relations of the 

 floor undergo considerable modifications in the course of de- 

 velopment. 



The other features of the blastoderm at this stage are very 

 much those of the previous stage. 



The embryonic swelling is very conspicuous. The cells of 

 the blastoderm are still disposed in two layers : an upper one 

 of slightly columnar cells one deep, which constitutes the epi- 

 blast, and a lower one consisting of the remaining cells of the 

 blastoderm. 



An average cell of the lower layer has a diameter of about 

 tfijj- inch, but the cells at the periphery of the layer are in some 

 cases considerably larger than the more central ones. All the 

 cells of the blastoderm are still completely filled with yolk 

 spherules. In the yolk outside the peculiar nuclei, before spoken 

 of, are present in considerable numbers. They seem to have 

 been mistaken by Dr Schultz 1 for cells: there can however be 

 no question that they are true nuclei. 



In the next stage the relations of the segmentation cavity 

 undergo important modifications. 



The cells which form its floor disappear almost completely 

 from that position, and the floor becomes formed by the yolk. 



The stage, during which the yolk serves as the floor of the 

 segmentation cavity, extends over a considerable period of time, 

 but during it I have been unable to detect any important change 

 in the constitution of the blastoderm. It no doubt gradually 

 extends over the yolk, but even this growth is not nearly so 

 rapid as in the succeeding stage. Although therefore the stage 

 I proceed to describe is of long continuance, a blastoderm at 

 the beginning of it exhibits, both in its external and in its 

 internal features, no important deviations from one at the 

 end of it. 



Viewed from the surface (PI. 8, fig. A) the blastoderm 



usually so called. Its relations are the same as those of my segmentation cavity at 

 this stage. This paper came into my hands at too late a period for me to be able to 

 do more than refer to it in this place. 

 1 Loc. cit. 



