434 A. J. NICHOLSON 
nucleus for a certain period is the rule rather than the exception 
in animal eggs, and Doncaster (5) makes the following observa- 
tion on the subject : * Very commonly the chromosomes. . . 
disappear, and the chromatin becomes scattered through the 
nucleus in the form of fine particles, or for a time it may 
vanish altogether, at least in the sense that it ceases to take 
up stain.’ 
The production of the segmentation nucleus at about the 
period when the egg is laid is the normal occurrence in insect 
eggs, and it is quickly followed by the polar divisions. Don- 
caster (5) observes that ‘in some animals the act of laying 
seems to be the stimulus and in others the polar division only 
occurs when a spermatozoon enters the egg’; but as in 
A. maculipennis oviposition and fertilization are simul- 
taneous, it cannot be stated which acts as the stimulus. 
In conclusion, the more important points with regard to the 
oocyte nucleus of A. maculipennis may be summarized 
as follows : 
1. From the earliest stages separate vegetative and germinal 
portions can be distinguished in the oocyte nucleus. 
2. During the second period of growth the nucleus branches 
throughout the entire oocyte. 
3. The branching nucleus, in conjunction with the nurse- 
cells, takes an active part in the nutrition of the oocyte. 
4, The branching nucleus is almost entirely the product of 
the nucleolus. 
5. The branching nucleus is morphologically and physio- 
logically comparable to the Corpuscles of Balbiani of other 
animals. 
6. The germinal portion of the nucleus, the ‘ chromatin 
residue ’, is the product of the condensation of the spireme 
threads. 
7. The ‘chromatin residue’ becomes invisible for a short 
period and reappears after oviposition as the segmentation 
nucleus. 
