422 FRANK R. LILLIE 
of the sperm head. An assumption that the spermatozo6n intro- 
duces any differentiated structure at the base of the head could, 
in this case, be due only to a preconception in its favor. 
c. Revolution of the sperm-nucleus and origin of the sperm-aster. 
The original orientation of the sperm nucleus within the egg is 
perfectly apparent, owing to the association of its apex with the 
fertilization cone from the very beginning of penetration. The 
whole complex of fertilization cone and sperm nucleus now rotates 
180 degrees, so that the fertilization cone, which was placed cen- 
trally to the sperm nucleus, comes to lie peripheral to it. In fig. 
10 a (fifty-four minutes after insemination) the revolution 1s shown 
nearly completed. Fig. 10 6 shows the cone-nucleus-aster com- 
plex of the same egg more highly magnified; and figs. 10 ¢ and 
10 d show the entrance point of the spermatozo6n involved, with 
tail (10 c) and middle piece (10 d) external to the vitelline mem- 
brane. The sperm asteris beginning to form in this stage opposite 
to the fertilization cone, thus in the position of the original middle 
piece, which, as we have seen, was left on the egg membrane. A 
minute granule, actually in the substance of the nucleus (fig. 10 6) 
is the point of focus of the rays. In fig. 11 (sixty-four minutes 
after insemination) this granule, now more clearly recognizable, 
is still in contact with the sperm nucleus. The rotation of the 
sperm nucleus is invariable, and no exception has been seen in 
the numerous cases observed; the point of origin of the sperm 
aster is just as invariable. 
The fact that the sperm aster arises from the base of the sperm 
nucleus as it is oriented in the spermatozo6n, not only in Nereis 
but in all other forms accurately determined, is unquestionably 
of fundamental significance. I may, however, be permitted to 
point out that the usual interpretation, which connects the sperm 
aster with the centrosome of the spermatid (occupying this posi- 
tion in the spermatozo6n) is not the only possible interpretation. 
It is important to emphasize this point because such an interpre- 
tation ascribes the fertilizing power of the spermatozo6n in the 
last analysis, not to the spermatozo6n as a whole, but to a minute 
part of it, the centrosome; and it involves a whole theory not only 
of fertilization, but of cell division in general, and much of cellular 
