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J. T. CUNNINGHAM. 
of the vitellus (see fig. 5, PI. X). It is evident from what has 
been said that this plane is one which at an earlier stage would 
have passed through the axis of the spindle formed by the similar 
branches of the T. On this point Van Beneden insists strongly, 
that the plane of division as observed by him passes through the 
axis of the spindle, and not, as described by other observers, 
through the equator. Each of the chromatic discs divides into 
two, one half going into the polar globule ; thus the chromatic 
part of the latter also consists of two discs. In the polar 
globule there is very little, if anything else, besides half the 
prothyalosoma. 
During the formation of the polar globule a thick layer of 
homogeneous substance is thrown off from the vitellus and 
lines the oospermatic membrane ; this is the first perivitelline 
envelope. The polar globule remains attached to the internal 
surface of this envelope. 
The half of the prothyalosoma which remains within the 
vitellus is called the deuthyalosoma (i. e. deuterohyalosomaj ; it 
contains of course two chromatic discs, or two groups of chro- 
matic globules. 
Changes in the Spermatozoon during this Period. 
— The tail gradually becomes smaller, and its refringent body 
diminishes and finally disappears. In one female the refrin- 
gent body was observed first to have separated from the 
spermatozoon, and at last to have escaped from the vitellus 
altogether and to lie between it and the perivitelline envelope. 
This, however, is exceptional ; the refringent body seems 
usually to be absorbed by the vitellus. Ultimately the sper- 
matozoon in the centre of the ovum consists of a chromatic 
globule round which is a clear perinuclear layer, itself sur- 
rounded by a layer of granular or reticulated protoplasm. 
Formation of the Second Polar Globule. — Imme- 
diately after the expulsion of the first polar globule appears in 
the vitellus a second complicated figure, whose developmeut 
culminates in the formation of the second polar globule. The 
elements which enter into the composition of this figure are : 
(1) The deuterohyalosoma and its two chromatic discs. 
