ey 
No. 2.] CENTROSOMES IN THE ANNELID EGG. 209 
the fusion of the pronuclei. During the metaphase of the 
cleavage-amphiaster the sperm-centrosomes divide, and the 
daughter-centrosomes at either pole move apart (an incipient 
central spindle developing between them) and form the poles of 
the cleavage-spindles of the two blastomeres. This process is 
repeated in each subsequent mitosis, and the centrosome can 
be demonstrated, lying in the midst of an aster, at every phase 
of mitosis, even including the so-called “resting stage.” It 
follows, therefore, that the centrosomes of the cleavage-cells are 
derived directly from the sperm-centrosomes, — a fact irrecon- 
cilable with Fol’s theory of the “ quadrille.” 
The behavior of the sperm-centrosomes is in harmony with 
Boveri's theory of fertilization, but is not necessarily a confir- 
mation of it; for the karyokinetic activities which are revived 
upon the entrance of the sperm are those leading to the forma- 
tion of the polar globules. The machinery for these mitotic 
divisions is already organized, and it is quite as likely that the 
stimulus which starts it going emanates from the sperm-nucleus 
as that it emanates from the sperm-centrosomes. 
There are nine chromosomes in each maturation-amphiaster 
and eighteen in the cleavage-spindle. In the metaphase of each 
cleavage-amphiaster the numerous nucleoli lie scattered among 
the chromosomes and remain at the equator of the spindle until 
they completely degenerate. As mitosis progresses, the chro- 
mosomes split longitudinally and the halves move toward 
opposite poles of the spindle, where they form new nuclei. 
During the telophase of each mitosis a distinct Zzwzschen- 
korper is present, which gradually fades away as the reconstituted 
nuclei approach the “ resting stage.”’ 
It is a pleasure to acknowledge the many courtesies extended 
to me by Dr. Whitman and other officers of the Marine 
Biological Laboratory, and to express my appreciation of the 
kind assistance of my friend Dr. W. M. Wheeler. 
BROWN UNIVERSITY, 
September 26, 1897. 
