No. 2.] CENTROSOMES IN THE ANNELID EGG. 207 
One cycle of karyokinesis has been completed, and the next 
cycle which is concluded with the formation of the four blasto- 
meres is essentially similar. Around each centrosome there 
develops a centrosphere (Fig. 53). The nuclear membrane, 
beginning at the portion nearest the centrosome, breaks down 
(Fig. 54), the chromosomes group themselves in the equatorial 
plate (Figs. 55, 56), and the typical anaphase is established. 
During the succeeding stages, the chromosomes in each blasto- 
mere divide longitudinally and migrate toward the poles of the 
spindle, the nucleoli drop out into the cytoplasm, and the cen- 
trosomes divide and move apart within the growing centrosphere 
(Figs. 57, 58). 
I have followed the karyokinetic processes to the formation 
of sixteen cells. All the phenomena are essentially similar to 
those in the preceding cycles of division. The thirty-two cen- 
trosomes of the 16-cell stage arise by the successive divisions 
of the original sperm-centrosome, while the centrospheres, on 
the other hand, appear and vanish in each mitosis. 
III. SUMMARY. 
During the growing-period of the oocytes a deeply-staining 
paranucleus is developed which contains a reticulum continuous 
with that of the surrounding cytoplasm; but, before the oocyte 
has attained to its full size, this structure becomes entirely 
resorbed into the general cytoreticulum. 
Ripe eggs may be carried in the body-cavity of the worm for 
several days before they are laid. During this time neither 
centrosome nor aster can be distinguished, though the reticu- 
lum is unusually distinct. In a few minutes after the eggs have 
been deposited in sea-water, however, a large number of asters 
are developed by rearrangement of the cytoplasmic network. 
Two of these asters (primary asters) continue to develop and 
finally lie at the poles of the first maturation-spindle, while the 
others gradually vanish. Distinct centrosomes are not demon- 
strable except in the primary asters. If they are present in 
the multiple asters, it is extremely difficult to reconcile their 
presence with the theory that the centrosome is a permanent 
