418 A, J. NICHOLSON 
I. Branching of the Oocyte Nucleus and 
Segregation of Vegetative and Germina] Parts. 
During the second period of growth of the egg-follicle of 
A. maculipennis the oocyte nucleus undergoes a most 
remarkable development. JI have not been able to find 
a detailed description of a similar development in the case of 
any other insect, but, as will be seen later, it is probable that 
this particular form of development of the oocyte nucleus 
is by no means confined to A. maculipennis. 
After the mosquito has fed on blood the first indication of 
alteration in form of the oocyte nucleus is observed in the 
nuclear membrane. Previously this was spherical in form, 
but now it is seen to be somewhat irregular in outline. This 
irregularity becomes more and more marked as development 
proceeds, till the nuclear membrane is seen to send out a few 
blunt processes into the cytoplasm and the cavity enclosed by 
the membrane appears somewhat larger than it was previously. 
While this has been taking place the nucleolus has also been 
altering somewhat in shape. It loses its spherical form, first 
becoming ovoid and later slightly flattened in a plane at right 
angles to the axis of the egg-follicle, at which stage it begins 
to send out blunt processes (fig. 18). The structure, however, 
is still the same as in the resting stage, that is, it is vacuolated 
and contains a large non-staining central mass. 
The chromatin residue commences to separate from the 
nucleolus at this stage. Its subsequent history will be dealt 
with separately. From this point the more obvious nuclear 
changes are confined to the nucleolus and the nuclear mem- 
brane, the nucleolus and its products forming by far the greater 
part of the bulk of the nucleus. 
It has been noticed that both the nucleolus and the nuclear 
membrane have begun to send out blunt processes. These 
processes rapidly elongate and take on the form of branches, 
which in their turn send out secondary branches. The branching 
of the nucleolus and the nuclear membrane is intimately 
connected, as the nuclear membrane surrounds the nucleolar 
