204 WALTEE HEAPB. 



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. 3 no space was to be distinguished with the magnify- 

 ing power used (Zeiss d) for the drawing, but in fig. 7, which 

 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 in Cheiroptera. 



The difference in the density of the yolk in various Mam- 

 malian ova is very remarkable and would, I suspect, if examined 



