1881.] 



and Early Development of the Mole. 



19a 



slightly flattened cells is separated off from and underlies the main 

 portion of the inner mass of cells ; this layer is the hypoblast. The 

 main portion of the inner mass of cells is undergoing at the same time 

 a change in structure, inasmuch as some of the polygonal 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 millim. 

 by "81 millim., 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 hematoxylin. 



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 millim. by "81 millim.), 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 processes ; 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, preserved 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 different interpreta- 

 tion, the layer of cells above the cavity being formed of flat outer 

 layer cells with a few more or less isolated cells of the inner mass. 



In a vesicle of about '97 millim. 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 trans- 

 formed 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 described specimens lay above the- 



