IN MAN AND OTHER MAMMALIA. 375 
STRUCTURE OF THE OvuM. 
In man and in the cat the mature ovum measures about the ;4 oth part of 
an inch in diameter. 
The zona pellucida, so named by Von Bakr, appears when examined under 
low powers as a strong, perfectly transparent, homogeneous lamella sharply 
differentiated from the yelk. The membrane possesses considerable elasticity, 
and when torn to pieces with a needle, the yelk substance with the germinal 
vesicle escapes in a stream. 
Generally, the external surface is exactly parallel to the internal, but in 
certain specimens I have seen fine processes of the membrane projecting out- 
wards in a radiating manner between the epithelial corpuscles which invest the 
ovum. 
In the zona pellucida investing the ovum in the rabbit’s ovary, when ex- 
amined under very high powers, fine radiating lines may be seen running through 
it from its external to its internal surface. I have not seen these lines in the 
zona pellucida in the cat. They were first described by J. MULLER and Remax 
in the vitelline membrane of the ova of fish. In an extremely thin section of 
the zona in the rabbit’s ovum I have not been able to see them, but in thick 
sections they are readily found. I believe these radiated lines do not indicate 
any special structure of the zona, but are produced by the membrana granulosa 
cells which lie in contact with each other, the lines of contact being seen 
through the zona as fine linear marks. 
The zona resists the action of acetic acid, and its chemical characters are 
not accurately known. . 
It is extremely difficult to say how the zona is formed. By most continental 
observers it is considered to be a product of the follicular epithelium, but from 
the circumstance that in many instances in the cat’s ovary I have discovered it 
as a well-marked structure round the ovum when little or no trace of the fol- 
licular epithelium existed, I am inclined to the belief that it is formed by a 
hardening of the external part of the yelk of the ovum. I regard the ovum as 
a large cell, of which the zona is the cell wall and the germinal vesicle is the 
nucleus. 
The parts of the ovum within the zona pellucida consist of the yelk substance 
| and the germinal vesicle. The yelk substance forms by far the greater part of 
| the mature ovum, and constitutes a large spherical mass in which a number 
| of minute granules are suspended. In the eggs of young cats these granules 
| and small bright bodies are found collected together at the external part of 
| the yelk near the zona pellucida. As already mentioned, the primordial eggs 
do not possess a cell wall at first, but lie in the young Graafian follicles with 
| their yelk substance in close contact with the fibrous wall of the stroma which 
