30 WALTEE HE APE. 



embryonic life into periods which I propose shall be regu- 

 lated by the following conditions : 



Stage A. The period of the development of an ovarian into 

 a fully-segmented ovum. 



Stage B. The further development of the fully-segmented 

 ovum prior to the formation of mesoblast. That is to say, the 

 stage in which the hypoblast and epiblast are definitely formed. 



Stage c. The formation of the mesoblast; and, 



Stage D. The formation of the medullary groove and noto- 

 chord, and the structure of the neurenteric canal. 



Stage A. 



I have been hitherto unable to satisfy myself as to the details 

 of the process of segmentation in the ovum of the mole, but 

 have been fortunate enough to obtain a fully-segmented ovum. 

 It was found at the upper end of the uterus, and its structure 

 is as follows ; 



It is formed of a number of distinct cells, each surrounded 

 by a cell-wall, and containing a nucleus. The cells are 

 arranged in two layers (vide fig. I), which 1 propose to name 

 (1) the outer layer (o.l.), and (2) the inner mass [t. m.). The 

 cells of the outer layer are more or less cubical in form, and 

 are placed side by side in a single row. The main portion of 

 each cell is composed of hyaline protoplasm, but along its inner 

 border the protoplasm is finely granulated. 



The cells of the inner mass are slightly smaller than those 

 of the outer layer. They are irregularly pergonal in shape, 

 and the protoplasm of which they consist is filled with large 

 and small granules, rendering the cells opaque and dense. 



The single row of outer layer cells closely invests and com- 

 pletely surrounds — except at one point — the inner mass of 

 segments. At this point (5/. of B.), however, there is a break 

 in the continuity of the outer layer, and here one of the 

 granular cells of the inner mass projects on to the surface. 



The ovum is surrounded by a thick membrane, the zona 

 radiata {z.), which is in its turn enclosed in an irregular layer 

 of hyaline gelatinous material (m. c.) derived from the uterus. 



