MATURATION AND FECUNDATION OF THE OVUM. 127 
the formation of the pronuclei : the later stages were studied 
in eggs which had continued to develop after the females were 
placed in alcohol. 
The female pronucleus at first contains two chromatic 
discs, which are, as has been seen, probably derived directly 
from the two discs of the germinal spot. After each division 
the chromatic discs regain their size. Each disc is really 
made up of four smaller globules united by a substance com- 
posed of moniliform threads. The acromatic membrane of the 
pronucleus is also composed of very delicate moniliform threads, 
and still more minute fibrils unite the membrane to the 
chromatic elements. The changes which the pronucleus under- 
goes are exactly similar to those of a cell nucleus about to 
divide. The chromatin in the discs diffuses along the achro- 
matic fibrils, and ultimately is found in fibrils at the surface of 
the pronucleus, the central part being almost empty except 
for scattered threads. The pronucleus is usually divided by 
a partition into two parts. The male pronucleus is formed 
from the chromatic elements in the centre of the spermato- 
zoon and the perinuclear layer : the chromatic layer outside this 
is thrown off. A membrane appears round the perinuclear 
layer and then the same changes take place as in the female 
pronucleus (see fig. 9, PI. X). 
The female pronucleus approaches the male : the chromatin 
in each comes to form a single thick sinuous fibril which 
breaks up into two V-shaped loops. The four loops arrange 
themselves as a star, on each side of which are achromatic 
fibrils forming a karyokinetic spindle. At each pole of the 
spindle appears a star derived from the vitellus. The loops at 
the equator now divide longitudinally, and the closed ends of 
the two groups so produced diverge from one another. As the 
groups diverge achromatic fibrils appear connecting them : 
these are produced from the loops as they divide, and are not 
parts of the original spindle as has been supposed by other 
authors. Hence Flemming’s notion that the chromatic loops 
travel along the achromatic fibrils to the poles. As was 
described in cells by Pfitzner the chromatic loops have a moni- 
