34 FORMATION OF THE LAYERS. 



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

 floor underofo 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 

 ■gi^ 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. The}^ seem to 

 have been mistaken by Dr Schultz^ 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 com- 

 pletely 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 blasto- 

 derm 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. VI. fig. A) the blastoderm 

 at this stage appears slightly oval, but the departure from 

 the circular form is not very considerable. The long axis of 

 the oval corresponds with what eventually becomes the 

 long axis of the embryo. From the yolk the blastoderm is 



1 Luc. cit. 



