126 
J. T. CUNNINGHAM. 
As before ; each half of the deuterohyalosoma contains two 
plates of chromatin, each composed of four globules. 
Even before the second polar globule is expelled, the female 
pronucleus has, in place of a distinct membrane, a periphery 
of punctations. At the time of the expulsion the achromatic 
fibrils outside the divided deuterohyalosoma disappear. The 
author believes that the second polar globule is the exact 
equivalent of the female pronucleus, and that the membrane 
round it is an achromatic nuclear membrane. 
Changes in the Vitellus during the Formation of 
the Second Polar Globule. — The external layer of the 
vitellus becomes first clear and reticulated in structure, and 
then becomes separated from the vitellus to form the second 
perivitelline envelope. After its separation the vitellus con- 
tracts, and a perivitelline space is produced. In ova prepared 
with alcohol the second envelope becomes fluid and cannot be 
distinguished from the perivitelline space. The second polar 
globule remains attached to the surface of the vitellus. At 
the period of the expulsion of the second globule, as at that of 
the first, two concentric circles appear on the surface of the 
vitellus round the polar body. The outer one divides the 
vitellus into two parts, one of which contains the female 
pronucleus, the other the spermatozoon. 
No important change occurs in the spermatozoon during 
this period; often a radial striation of the vitellus is visible 
round the male element. 
Formation of the Pronuclei. — The expulsion of polar 
globules is quite independent of the presence of the spermato- 
zoon. In the rabbit the first polar globule is formed while 
the ovum is still in the ovary, and the membrane formed after 
this, known as the vitelline membrane, is to be regarded as 
the homologue of the first perivitelline envelope in Ascaris. 
These processes take place in the rabbit even when spermatozoa 
are prevented from reaching the uterus. The influence of the 
spermatozoon on the ovum begins with the conjugation of the 
pronuclei. 
In the living female the ova do not develop further than 
