STUDIES OF FERTILIZATION 425 
never absent. The simplest interpretation of the smaller cleavage 
center is that it represents: the smaller sperm aster, although 
there is a brief period when it is not demonstrable, owing (possibly) 
to defect in the cytological technique. It is my opinion, then, 
that the sperm amphiaster becomes the cleavage amphiaster in 
Nereis, as in so many other animals. 
5. Discussion 
If any apology is needed for presenting so strictly a morphologi- 
eal study of such an apparently threadbare subject as the fertili- 
zation of the ovum, I might say that the impulse to make it came 
from an experimental study, and that it is necessary to the experi- 
mental results which follow. It has, moreover, yielded some 
details of observation which deserve to go on record on their 
own account. 
The conclusion that the cleavage centers arise from the sperm 
centers is In agreement with many other studies. But I am 
unable to accept the usual conclusion that the sperm centers 
arise around a centrosome introduced by the spermatozo6n into 
the egg, and that, therefore, the sperm centrosome is the fertiliz- 
ing agent of the spermatozo6n, and the sperm nucleus concerned 
exclusively with amphimixis. The crux of the problem is pre- 
cisely here on the question of the origin of the sperm centers. 
The fact that the middle piece of the spermatozo6n which usually 
includes the spermatid centrosome does not penetrate the egg in 
Nereis is evidence of a certain amount of value only. Defects in 
cytological technique may always be invoked to explain failure 
to observe the introduction of a centrosome by the spermatozoon. 
Little as I may be inclined to admit this, it is necessary to grant 
some force to this objection where such delicate cytological 
details are involved. It would, however, I believe, be recognized 
as crucial evidence that the sperm centrosome is not necessary to 
fertilization, if a distal fraction of the sperm head alone were 
proved to form a sperm aster, a certain portion of the base of the 
sperm head as well as the middle piece being prevented from 
entering. Such results are described in the second part of this 
paper. 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 12, NO. 4 
