110 WALTER HEAPE. 



onal or rounded cells of which it has hitherto been composed 

 now become elongated and columnar. 



The hypoblast in an oval blastodermic vesicle of about "88 

 mm. by "81 mm.^ is still formed of slightly flattened cells 

 beneath the embryonic area, but it has grown and extended 

 beyond that area, so that its outer part lies beneath and in 

 close contact with the outer layer of the blastodermic vesicle ; 

 the cells of this portion of the hypoblast are wide and much 

 flattened, and their nuclei stain deeply with hsematoxylin. 



A cavity appears about this stage of development in the 

 region of the embryonic area between the flattened outer 

 layer and the inner mass, the cells of the latter having now 

 largely become columnar. In the vesicle last mentioned 

 ('88 mm. by '81 mm.), nearly the whole of the inner mass 

 has become transformed from a rounded mass of polygonal 

 cells into a concave plate of columnar cells, forming the 

 floor of a cavity which is roofed over by the cells of the outer 

 layer of the blastodermic vesicle. In this cavity a few cells 

 are placed, which are connected with the outer layer or inner 

 mass, or with both of these, by means of protoplasmic pro- 

 cesses ; I believe these cells to be cells of the inner mass 

 which have not yet become columnar. 



Lieberkiihn states that some of the cells of the inner mass 

 grow round and above the cavity just described, which thus 

 comes to lie within the inner mass. The specimens from 

 which he derives his opinion, however, were, I believe, pre- 

 served in Miiller's fluid. I have myself seen a similar 

 apparent arrangement in such preparations, which upon 

 comparison with sections of vesicles of similar ages prepared 

 in picric acid appear to me to bear a diff^erent interpretation, 

 the layer of cells above the cavity being formed of the flat 

 outer layer cells with a few more or less isolated cells of 

 the inner mass. 



In a vesicle of about '97 mm. diameter the inner mass of 

 cells has still the form of a concave plate composed of two or 

 three layers of for the most part columnar cells ; the flattened 

 cells of the outer layer remain, as in the previously described 

 specimen, closely attached to the zona, and the cells lying 

 in the cavity are fewer, while some of them appear to have 

 been drawn on to the concave plate and transformed into 

 columnar cells. Cells in a transition stage may be seen on 

 the surface of the plate. 



At a later stage the concave plate extends itself, the 

 curvature becoming less, and eventually approaches to and 

 finally comes into contact with the flat cells forming that 

 portion of the wall of the vesicle which in the previously 



