FERTILIZATION IN PLATYNEREIS MEGALOPS 225 
aster begins to wane soon after the formation of the homody- 
namic amphiaster and finally disappears. One aster can always 
be found (fig. 29). The opposing nuclear membranes break 
down and one nucleus forms with the single sperm aster. Soon 
a small aster appears on the nuclear membrane (fig. 30), the 
nucleus breaks down, and the heterodynamic first cleavage 
spindle forms. 
3. DISCUSSION 
The case of Nereis and. Platynereis, with respect to the en- 
trance cone offers an interesting parallel with that of Toxop- 
neustes and Arbacia (Wilson and Mathews). In both Nereis 
and Platynereis, however, the middle-piece is left outside the 
egg. The absence of cone-organ in Platynereis makes the ques- 
tion of rotation obscure, whereas in Nereis the evidence is indis- 
putable. 
Bonnevie (’08), in her paper on Nereis, has mentioned cer- 
tain cytological differences between the ‘large and small varie- 
ties’ of Nereis eggs. As indicated above, the time of sperm 
entry is earlier in Platynereis. It is also true that the polar 
bodies are formed earlier, the first cleavage is earlier, and the 
subsequent rhythms are faster, so that the larval stage is reached 
earlier. 
So far as both Nereis and Platynereis are concerned, the rdéle 
of the middle-piece or its contained centrosome, as the chief 
actor in fertilization, is wanting. There are spermatocytes with 
intra-nuclear centrosomes? (see Julin on Styleopsis). But if 
this hypothesis be postulated (cf. Packard in 1914) for Nereis 
sperm, this next step, as was pointed out by Lillie in 1912, should 
also be taken: the centrosome zradient must be quantitatively 
different from its base at the middle-piece to the tip of the 
sperm head: “If intra-nuclear centrosomes are the causes of the 
formation of the sperm aster, not only must they exist at every 
level, but also (that) they must decrease in size from the base 
to the apex of the sperm nucleus!”’ 
2See also Hegner and Newman for intra-nuclear centrosomes in oocytes. 
