STUDIES OF FERTILIZATION 447 
internal phase of fertilization must proceed from the funda- 
mental fact of the difference in behavior of the sperm nucleus 
and the egg nucleus in the cytoplasm of the egg. The former 
induces aster formation and karyokinesis; the latter does not. 
Let us recall the facts in the case of Nereis briefly again: (1) 
Even minute fragments of the sperm nucleus cause the formation 
of an aster withacentrosome capable of division. (2) If the sperm 
nucleus be prevented from entering, the egg nucleus may indeed 
form chromosomes but no aster arises, provided that both polar 
bodies are formed.2 (3) We may add from experiments on other 
forms that in the absence of the egg nucleus the sperm nucleus 
behaves the same as in its presence. 
Clearly, then, there is some difference, associated with their 
sex-origin, between these two nuclei; and the most direct form 
of interpretation of this difference is that which identifies it with 
the fundamental sex characters which inhere in every cell. In 
other words, the sperm nucleus has the character maleness, what- 
ever that may be, and neither the egg nucleus nor cytoplasm pos- 
sesses this character. It makes no difference that half the sper- 
matozoa may carry the factor for femaleness and half for maleness. 
The distinction between character and factor is clear. It may be, 
on the other hand, that ova and spermatozoa acquire In the course 
of gametogenesis special differentiating properties that are the 
cause of the fertilizing power of the spermatozoon. 
However we may conceive the demonstrated difference between 
the sperm nucleus and the egg nucleus, it is obvious that there is a 
lack of interchange between the egg nucleus and the egg cytoplasm 
that conditions the inhibition of the unfertilized egg. In some 
way, then, the maturation divisions of the egg must have removed 
certain reacting constituents of the germinal vesicle, or have 
brought about certain cytoplasmic changes in the egg, because 
we have perfect karyokinetic phenomena in the maturation divi- 
sions and a sudden cessation thereafter. 
2 The monaster that arises after suppression of the first polar body with preven- 
tion of entrance of the sperm nucleus forms a special problem which we need not 
consider here. 
