160 WALTER HEAPE. 
membrane, the vitelline membrane (vide Reichert No. 18, 
Meyer No. 17, and van Beneden No. 4). In the ovum 
drawn in fig. 1, this membrane may be seen where a space 
exists here and there between the zona and the ovum. 
In fig. 2 no space was to be distinguished with the magnify- 
ing power used (Zeiss p) for the drawing, but in fig. 7, whic 
is a drawing of a portion of the circumference of the same 
ovum with a higher magnifying power (Zeiss, imm. 3), a 
narrow space is clearly shown between the ovum and the zona, 
and a very fine membrane is there discernible closely covering 
the ovum. This membrane is, however, most clearly visible 
in fig. 8, which is the drawing of an ovum in which maturation 
has taken place; in this specimen there is a considerable space 
between the vitelline membrane and the zona, the former 
being rendered still more evident on account of the contrac- 
tion of the material of the ovum itself within the vitelline 
membrane. The space between the vitelline membrane and 
the zona radiata I propose to call the circum-vitelline 
space. 
The development of the membranes, about which there has 
been considerable discussion, I propose to consider in a future 
paper. . 
The Yolk: 
The ripe ovarian ovum itself is composed of food-yolk of two 
kinds—(1) homogeneous, partially transparent, vesicular bodies, 
(2) minute highly refractive granules of various sizes,—and of 
a network of protoplasm which divides the yolk into rounded 
or cubical masses such as I have endeavoured to represent in 
figs. 2 and 7. The two kinds of yolk are similar to those 
described by most of the observers of Mammalian ovarian ova. 
It is worthy of remark, however, that I found no globules in 
the Mole’s ovum similar to those described by Beneden and 
Julin (No. 6), and figured by those authors in their paper 
(No. 7) on the ova of Cheiroptera. 
The difference in the density of the yolk in various Mam- 
malian ova is very remarkable and would, I suspect, if examined 
