STUDIES OF FERTILIZATION 377 
Cytological study of the twelve lots of preserved eggs showed 
stages ranging from the metaphase of the second maturation 
spindle to the prophase of the first cleavage, the earlier stages 
of course being found in lot 1 in each ease. 
In the control lots it was easy to demonstrate the sperm 
nucleus at all stages to the formation of the first cleavage spin- 
dle. The sperm nucleus is rendered particularly conspicuous 
during the second maturation division by the large amphiaster 
that accompanies it (fig. 4), both lying in the yolk-free proto- 
plasm. After the formation of the second polar body the sperm 
amphiaster gradually fades, but the sperm nucleus can be 
recognized by its position and by the remnants of radiations 
up to the time of union of the two germ nuclei; and in the later 
stages its presence may be inferred by the degree of development 
of the cleavage amphiaster and the number of chromosomes. 
There is, therefore, no time from the beginning of the second 
maturation division up to the formation of the first cleavage spin- 
dle when the presence of the sperm nucleus cannot be readily 
demonstrated. 
In the study of the serial sections of the control eggs I found 
no egg in which, all sections being present, the sperm nucleus 
could not be demonstrated. In the serial sections of 27A, the 
sperm nucleus could be recognized in only about 37 per cent of 
the eggs; in 27B in only 10 per cent to 20 per cent; in 27C in about 
25 per cent; in 27D in about 53 per cent; in 27E in about 76 per 
cent. The stages of maturation of lots A to E corresponded very 
closely with the stages of maturation of the control eggs killed 
at the same time. 
It is a relatively simple matter to demonstrate the presence 
of the sperm nucleus, for a single positive observation suffices; but, 
to be sure of the absence of a sperm nucleus from any particular 
egg, it is necessary to examine practically every section of the 
egg, and the absence of two consecutive sections is sufficient 
reason for excluding an egg from the count. This may be one 
reason why the number of eggs in the different lots shown to 
contain sperm nuclei tends to be somewhat larger than the esti- 
mate of the number of eggs that segmented. Another reason 
