38 WALTEE HE APE, 



The opaque inner mass is seen attached to the wall of the 

 vesicle, and in the centre of the mass a lighter coloured space 

 indicates the presence of the secondary cavity. 



The relations of these parts are, however, more clearly seen 

 in fig. 24, which represents a section through the centre of the 

 inner mass of the ovum drawn in fig. 5. The single row of 

 long columnar cells (fig. S3) has given place to a double row of 

 more cubical and broader cells which are continuous with the 

 cells of the outer layer at the circumference of the plate. 



The hypoblast lies free below the inner mass and stretches 

 out laterally beyond the area of the latter. 



The cells filling up the secondary cavity are stellate, and are 

 connected with both the outer layer and inner mass by means 

 of protoplasmic processes ; the size and general appearance of 

 the cells and of their nuclei, however, as well as the manner in 

 which they stain with hsematoxylin, leaves little room for doubt 

 in my mind that they are derived from the former (outer) 

 layer. 



2. The plate of cells now changes its form and becomes 

 flattened out and applied closely to the zona above, the stellate 

 cells within the secondary cavity and the outer layer cells 

 above uniting with it, and the secondary cavity is obliterated. 

 The structure resulting from these changes is the epiblast 

 plate of the embryonic area. 



Reference to figs. 7, 25, 26 and 27, will, I think, substan- 

 tiate this view. 



Fig. 7 is a surface view of the inner mass represented in 

 section in fig. 25. The section is cut along the line of the 

 greatest diameter of the mass, and shows the commencement 

 of the process of the flattening out of the plate. 



The flattening occurs in the first place along one side, the 

 secondary cavity being there much shallower, while elsewhere 

 it is as deep as before. This arrangement gives rise to the 

 appearance seen in fig. 7, in which the light, crescent-shaped 

 area at one side of the inner mass is the deeper portion of the 

 cavity (compare figs. 7 and 25). 



In all the sections of this inner mass only a few cells were to 



