FERTILIZATION IN PLATYNEREIS MEGALOPS 219 
a cone. Twenty-five minutes after oviposition a slender strand 
of protoplasm may be discerned across the perivitelline space 
and beneath the point of sperm entry; even this does not seem 
to be constant, but appears to be formed only in the animal 
hemisphere. This protoplasmic strand lies, first, in a radius 
of the egg, but gradually bends so that it now lies almost tan- 
gential to the egg. It is found after sperm entry. 
After thirty minutes the spermatozoon is engulfed. Often 
the formation of the sperm aster is discernible, the middle- 
piece and tail remaining outside. The maturation asters are 
always visible in the living egg. Mathews (’06) has called 
attention to the difference between the structure of the asters 
of living eggs and of fixed material. The difference is certainly 
striking in Platynereis. Instead of the short stiff astral fibres 
of chrom-osmic material or the long slender ones of mercuric 
fixation, one sees in the living egg beautiful broad rays sweep- 
ing through the cytoplasm. 
3. Copulation of the germ nuclei. About fifty minutes after 
egg-laying, the germ nuclei copulate, the cleavage asters form, 
and at sixty minutes the egg divides unequally. The egg at 
this time exhibits a stratification of protoplasmic stuffs. Dur- 
ing maturation the cytoplasmic currents shift the materials. The 
equatorially placed oil drops, about eighteen in number, grad- 
ually become massed at the vegetative pole, the coarser (yolk) 
granules lie above these; at the clearer animal pole are the male 
and female pronuclel. Beneath the polar bodies the cytoplasm 
is most transparent. The asters are very distinct. One can- 
not get an adequate picture of these structures from sections. 
In the living egg they are incomparably clear; large broad rays 
which bear little resemblance to the short stiff fibres seen in the 
sections. 
The penetration path of the spermatozoon may often be 
followed, the copulation path always followed. The sperma- 
tozoon enters at any point of the egg and through this, as in 
Nereis (Just, ’12), the first cleavage plane passes along the copu- 
lation path of the germ nuclei. 
