STUDIES OF FERTILIZATION 423 
physiology, by ascribing certain very general functions and powers 
to this minute cell element. 
The alternative interpretation, which I would present, is that 
the appearance of the sperm aster in a position definitely oriented 
with reference to the sperm nucleus may be a nucleo-plasmic 
reaction localized by polarity of the nucleus. 
In any case, if the centrosome theory is to be retained for Nereis, 
it becomes necessary to assume that, in this form, the centrosome 
is contained, not in the middle piece, but in the nucleus of the 
spermatozoon. In the experimental study, which forms the 
second part of the present paper and study number 4 of the series, 
I shall present crucial evidence against this assumption; discus- 
sion of the matter may therefore be postponed. 
After the appearance of the sperm aster, the nucleus penetrates 
yet more deeply within the egg and leaves the fertilization cone 
behind near the inner margin of the yolk (fig. 11), where it soon 
disappears. So far asI know, it is a perfectly unique cell organ, 
except in its earliest stages, and it is certainly surprising to find 
it in so highly differentiated a condition, and with apparently so 
active a function in the penetration of the spermatozo6n, as it is 
in Nereis. 
d. Division of the sperm aster. The sperm nucleus moves 
towards the center of the egg, and the aster separates from it a 
little and divides, forming an amphiaster. A secondary spindle 
then arises between one or both of the sperm-centrosomes and the 
inner centrosome of the second maturation spindle (figs. 12 and 
13). This has been very fully described by Bonnevie (’10). Shortly 
after the sperm amphiaster is formed, it is noticeable that one of 
the asters.and centrosomes is decidedly larger than the other 
(figs. 13 and 14a). Bonnevie has also called attention to this 
fact, but has not, as I believe, sufficiently emphasized the fact 
that it is a secondary condition. I must dissent a little from her 
view that the sperm aster does not always divide prior to the com- 
pletion of the second maturation division; in my material, at least, 
the amphiaster is invariably formed prior to this time. 
e. The germ nucler and origin of the cleavage centers. After the 
second polar body is formed, the egg aster gradually dwindles and 
