No: 2.) CENVTROSOMES IN THE ANNELID EGG. 189 
is composed of both male and female elements; for the original 
sperm-centrosome and egg-centrosome divide into two, and each 
daughter-centrosome derived from the sperm fuses with one of 
the daughter-centrosomes derived from the egg. The obvious 
inference which has been drawn from these phenomena is that 
“fertilization consists, not only in the adding together of the 
two pronuclei derived from individuals of different sexes, but 
also in the fusion of four half-centres derived from the father 
and the mother into two new bodies, the astrocentres ” (centro- 
somes).! Obviously, if the permanent presence and fusion of 
the sperm- and egg-nuclei indicate that these structures are the 
vehicles of hereditary properties, the same function may be 
predicated of the centrosomes. 
The results of Guignard’s researches on the fertilization 
of the lily and Conklin’s work on the gasteropod Crepidula 
agree in all essential respects with those of Fol on the sea- 
urchin. According to almost every other observer, however, 
either the sperm-centrosome or the egg-centrosome, or both, 
disappear before the formation of the cleavage-amphiaster— a 
phenomenon manifestly incompatible with Fol’s interpretation 
of their functions in fertilization. The fusion of the sperm- and 
egg-centrosomes in fertilization is not of universal occurrence, 
and the inference that these structures are vehicles for the con- 
veyance of hereditary qualities is unwarranted. 
But the observations which discredit Fol’s interpretation do 
not confirm, in every case, Boveri’s theory of fertilization. 
Wheeler showed that in Myzostoma there is no indication that 
a centrosome is brought in by the spermatozoon or that a 
centrosome or aster subsequently develops in connection with 
the sperm-nucleus. On the other hand, the egg-centrosomes 
derived from the second maturation-spindle accompany the egg- 
pronucleus as it approaches that of the sperm. During the 
approach of the pronuclei the egg-centrosomes move away 
from each other as though they were to form the poles of the 
cleavage-amphiaster; but during a certain brief period “it is 
extremely difficult or even impossible to make out the egg- 
centrosomes.” It is probable that these bodies — certainly not 
1 Fol (91), p. 274; Conklin ('94), p. 18. 
