No. 2.] CENTROSOMES IN THE ANNELID EGG. IQI 
and await the entrance of the spermatozoon (Nereis), some 
remain in the metaphase of the first maturation-amphiaster 
(Chzetopterus), while others proceed with the formation of the 
polar globules and the reconstitution of the egg-nucleus before 
fertilization (sea-urchin). If it is the function of the centro- 
somes upon being brought into the egg “to organize the 
machinery of mitotic division,” 1 its task must be very different 
in different eggs; for in one it must first organize the machinery 
for the two maturation-divisions, in another it finds this 
machinery -already organized but in a state of rest, and in a 
third it has to organize only the machinery for the first cleavage- 
mitosis. 
In Cheetopterus the behavior of the sperm-centrosomes is 
strictly orthodox, according to Boveri’s doctrine ; they are the 
centrosomes of the first cleavage-spindle, and give rise by 
division to those of the succeeding spindles, while the egg-cen- 
trosomes totally disappear. Nevertheless, it does not seem 
necessary to conclude that it is by virtue of the presence of the 
sperm-centrosomes, rather than of the sperm-nucleus, that 
the maturation-processes are resumed, even if we grant that 
the spermatozoén brings in the centrosomes. An adequate 
demonstration that the sperm-centrosomes are actually carried 
into the egg by the entering sperm is, moreover, extremely 
difficult to make, and it is perfectly possible, as Miss Foot has 
urged, that in many cases these structures arise de novo out of 
the egg-cytoplasm in the vzczuity of the middle-piece of the 
spermatozoon. 
The foregoing considerations lead to the conclusion that the 
centrosomes in fertilization are neither vehicles of hereditary 
qualities nor the active agents which organize the machinery 
of mitotic division, but that they may be, like other centro- 
somes, ‘the expression rather than the cause of cell-activities.”’ 
1 Wilson, “ The Cell,” p. 171. 
