1958 ] Woodland — Thermobia domestica (Packard) 19 
cytoplasm accumulates around from two to six sperms, their 
distribution is such that they are about equidistant from 
each other and from the surface of the egg. Just as much 
cytoplasm accumulates around a sperm whether it is the 
only one accumulating* cytoplasm or whether five others are 
doing likewise. Accumulation of the sperm plasm ceases at 
about the time the oocyte nucleus has completed its first 
maturation division. By that time the sphere of sperm 
plasm is about 45 microns in diameter. 
As cytoplasm accumulates around a sperm head, the latter 
undergoes a gradual decrease in length, with accompanying 
uncoiling and moderate increase in breadth. The Feulgen 
reaction indicates a simultaneous concentration of desoxyri- 
bonucleic acid toward the anterior end of the shortening 
sperm head (Fig. 6). Shortening continues until all that is 
visible within the sperm plasm is a minute nucleus about 
11/2 microns long. This male pronucleus is formed at about 
the time that accumulation of sperm plasm ceases. While 
the egg nucleus is undergoing the second maturation divi- 
sion, the male pronucleus gradually enlarges (Fig. 7) until 
it is about 8 or 10 microns in diameter (Fig. 9). 
The female pronucleus then migrates from its peripheral 
position to the sperm plasm (Fig. 9). When fixed before it 
reaches the sperm plasm, it is found to be rather irregularly 
elongated in the direction in which it was presumably 
travelling. Cytoplasm has never been identified around it. 
Both male and female pronuclei are approximately the same 
size and their finely divided chromatin appears very similar. 
Fortunately they can easily be distinguished when the 
Feulgen reaction is followed by light green counterstain. 
The nuclear sap of the male pronucleus shows great affinity 
for light green, while that of the female pronucleus shows 
practically none at all. It appears further that the move- 
ment of each nucleus within the sperm plasm just prior 
to fixation can be detected. For adjoining each nucleus fixed 
at this stage is a palely-staining streak which is interpreted 
to indicate the path over which the nucleus has just passed 
(Fig. 10). 
As the female pronucleus enters the sperm plasm the 
sperm nucleus starts to move toward it. The two pronuclei 
