STUDIES OF FERTILIZATION 443 
e. g., injury of certain kinds caused by the centrifuging, possibly 
abnormal maturation, or a general systemic disturbance of the 
cytoplasm. But this does not seem very probable, since eggs 
centrifuged at times when injury to the spermatozo6n is not to 
be anticipated do not exhibit the partial cleavage, at least to the 
same extent. JI would therefore regard it as probable, though 
not proved, that partial sperm nuclei tend to produce more or 
less defective cleavage. 
GENERAL DISCUSSION 
1. The centrosome theory of fertilization 
The centrosome theory of fertilization is still accepted by most 
morphologists in spite of the doubts that have been thrown on 
the theory of the permanence and genetic continuity of the cen- 
trosome by the production of asters in unfertilized eggs (Morgan, 
Wilson, ete.), and by the studies in artificial parthenogenesis. 
The theory that the spermatozodn introduces an extra-nuclear 
centrosome destined to become the organ of cell division of the 
odsperm is confronted for the first time, in the results of the fore- 
going study, with a crucial experimental test. And the result 
that even small parts of the sperm head produce a typical centro- 
some and aster in the egg cytoplasm conclusively demonstrates 
the inadequacy of this conception of fertilization. Solongassimilar 
experiments on other forms are lacking, there is no reason to 
believe that the production of the sperm centrosome depends 
upon any different principle in Nereis than in other forms. 
Meves (’1la) demonstrates that the sperm aster does not arise 
in connection with the so-called middle piece of the spermato- 
zoon in the case of the sea-urchin, but at the base of the sperm 
nucleus itself, the middle piece being already separated from the 
nucleus and lying to one side at the time that the sperm as- 
ter arises. These results, while inconclusive in themse.ves, are 
nevertheless distinctly unfavorable to the existence of an extra- 
nuclear centrosome as the cause of formation of a sperm aster in 
this classical case. Further, one can say without fear of success- 
ful contradiction that in no animal has it been shown that the 
