374 ARTHUR ROBINSON. 
General Description of the Various Stages, and Com- 
parison with the Results obtained by Previous 
Observers. 
After fecundation the ovum of the rat, whilst it still retains 
its spherical form, becomes, according to Tafani’s observa- 
tions (51), divided into a number of blastomeres of equal size 
and texture, and then, as the segmentation continues, the 
spherical form is lost and an oval form is assumed. My 
youngest specimen is an ovum in the oval stage. The uterus 
in which it was found was cut into vertical-transverse sections, 
and the ovum, which lies free in the antimesometrial end of 
one of the radii of a triradiate cavity, is divided into eight 
sections, the fourth of which is represented in fig. 1. The 
longest diameter of the ovum measures 64°6 w, and the shortest 
418 u. The long axis of the ovum is at right angles to the 
long axis of the uterine cavity. 
The ovum consists of a number of large cells. There is no 
trace of the polar bodies, which are said by Tafani to be pre- 
sent up to this period. There is no vitelline membrane, but 
the cells of the ovum are closely packed together, and their 
forms are irregular from mutual compression. The protoplasm 
of the cells is very slightly granular, and it stains but faintly 
with carmine. 
Each cell contains a large more or less spherical nucleus, 
which is bounded by a distinct nuclear membrane, and which 
contains a reticulate and nodulated chromoplasm, but no 
nucleoli distinct from the nodes of the reticulum. The ovum 
at this period, about the fourth day, is a solid morula, and its 
cells present no appearance of arrangement into the two 
groups mentioned by Tafani. 
The next stage in my possession is that represented in 
fig. 3. It is a vesicle with an excentric cavity: one wall of 
the cavity, which for convenience may be termed upper, is 
formed by a single layer of flattened cells (ED), whose nuclei 
are comparatively large in proportion to the cell body. The 
