are FRANK R. LILLIE 
aster arises from the pole of the sperm nucleus opposite the en- 
trance cone, thus in the position of the original middle piece. 
Morgan has recently (’10), with entire justice as it appears 
to me, taken a stand against the current view that penetration 
of the sperm is due to mechanical boring into the egg. He believes 
that the presence of the sperm calls forth a reaction on the part 
of the egg that leads to the absorption of the former. There can 
be no question that the latter conception fits the case of Nereis 
much better than the former. In the first place the spermatozoon 
is absolutely motionless after its attachment to the membrane; in 
the next place the formation of the entrance cone shows a very 
decided reaction on the part of the egg to the presence of the 
spermatozoon; in the third place the retraction of the spermato- 
zoon into a depression of the membrane is due to the retraction 
of the entrance cone; and finally, as I shall show in a subsequent 
cytological study, the inclusion of the spermatozoon within the 
egg appears to be due to activity of the substance of the entrance 
cone, and not to active penetration by the spermatozoon. The 
spermatozoon does not penetrate the egg, it is drawn in or en- 
eulfed. 
b. The later history of the sperm nucleus. The sperm amphi- 
aster is visible in the preparations all through the period of the 
second maturation division (fig 4). After the formation of the 
second polar body the sperm-nucleus begins to enlarge and the 
amphiaster gradually wanes, but it may be recognized up to 
the time of contact of the germ nuclei. The centrosomes of the 
first cleavage spindle then begin to appear. Whether or not they 
are continuous with those of the sperm amphiaster is a question 
which I shall take up in the next study of this series. The cleay- 
age asters rapidly become very large and conspicuous (figs. 5 and 
6). During the growth of the germ nuclei a considerable number 
of large granules staining strongly in iron haematoxylin appear in 
their immediate vicinity. 
The main point of these observations on the normal fertili- 
zation, both in the living eggs and also in section, is to demon- 
strate for elucidation of the experiments following: (1) That 
membrane formation precedes penetration of the spermatozoon 
by along time. (2) That the spermatozoon does not penetrate 
