THE 
AMERICAN NATURALIST 
SEPTEMBER, 1889. 
SEGMENTATION OF THE OVUM, WITH ESPECIAL 
REFERENCE TO THE MAMMALIA. 
BY CHARLES SEDGWICK MINOT. 
(Concluded from page 481.) 
"PjURING all these early stages the cells (segmentation 
spheres) are naked, i.e., without any membrane ; the nuclei, 
when not in karyokinetic stages, are large, clear, and vesicular ; 
the yolk granules are small, highly refractile, and more or less 
nearly spherical ; they show a marked tendency to lie in the cell 
half-way between the nuclear and the edge of the cell, or when 
the cells are large, around the nucleus and at a little distance 
It is at about this stage that the ovum passes from the Fallo- 
pian tube into the uterus, where it dilates into what is known as 
the blastoderviic vesicle. This dilatation is due principally to the 
multiplication and flattening out of the cells of the outer layer, 
and of course involves the expansion and consequent thinning of 
the zona pellucida, (compare Figs. 8 and 13.) The inner mass 
meanwhile remains passively attached to one point on the cir- 
cumference of the vesicle, (Fig. \\,i.ni) By this process the 
thin fissure between the inner mass and the outer layer becomes 
a considerable space, (Fig. 1 3). 
The blastodermic vesicle continues to expand, and in the 
rabbit and mole there is a corresponding enlargement of the 
tubular uterus at the point where the vesicle is lodged. " It is 
