208 MEAD. [Vou. XIV. 
cell-organ; for the theory would require, in this instance, that 
no less than seventy-five centrosomes should arise by the 
division of the two centrosomes of the cell of the preceding 
generation, and that these centrosomes should be distributed 
throughout the larger portion of the egg-cytoplasm. If they 
are not present in these asters, the centrosomes are only an 
occasional and not a constant or an essential feature of the 
aster. 
The centrosomes of the two primary asters evidently arise 
de novo out of the cytoplasm, and are typical in every respect; 
they lie in the midst of the astrospheres, grow, divide, and 
persist in the next cell-generation. 
Normally the maturation of the unfertilized egg proceeds no 
further than the metaphase of the first maturation-spindle, but 
upon the entrance of the spermatozoon the karyokinetic activity 
is immediately resumed and the maturation is completed. I do 
not know whether the spermatozo6n actually brings the sperm- 
centrosomes into the egg or not; at any rate, they are demon- 
strable in the midst of a minute aster which lies close to the 
sperm-head soon after the latter enters. From this time the 
development of the sperm-aster into the cleavage-amphiaster, 
and the mitotic divisions, resulting in the formation of the polar 
globules, proceed simultaneously in different parts of the egg, 
and appear to be independent phenomena. 
The primary centrosomes lie at the poles of the first matu- 
ration-spindle, and the daughter-centrosomes, arising by the 
division of one of these, move apart and form the poles of 
the second maturation-spindle. During the reconstitution of 
the egg-nucleus and its approach to the sperm-nucleus, the egg- 
centrosome remains in the midst of the fusing vesicles, its posi- 
tion being indicated by the point of convergence of the rays of 
its waning aster. Not only the fact of its disappearance, but 
the fact that, when last seen, the centrosome is in the midst of 
the group of vesicles, renders it in the highest degree improb- 
able that the egg-centrosome takes part in the formation of the 
cleavage-amphiaster. Moreover, the sperm-centrosomes may 
always be seen at the poles of the incipient cleavage-amphiaster, 
and they become more and more conspicuous up to the time of 
