THE MORPHOLOGY OF NORMAL FERTILIZATION 
IN PLATYNEREIS MEGALOPS 
EK. E. JUST 
THREE PLATES (THIRTY FIGURES) 
1. INTRODUCTION 
In a previous paper on Platynereis megalops, which described 
the egg-laying habits, it was stated that insemination takes 
place in the body cavity of the female and, further, that the 
eggs will not fertilize when inseminated in sea water. The 
present paper is a description of the normal fertilization proc- 
ess in Platynereis. An experimental analysis of fertilization 
in Platynereis appears elsewhere (Just, ’15). 
2. NORMAL FERTILIZATION OF PLATYNEREIS 
The living egg. The egg of Platynereis is compressed and 
irregular in shape while in the body cavity. Those eggs which 
happen to be uninseminated when laid gradually round out in 
sea-water as almost perfect spheres equatorially, but with a 
rather shorter polar axis. The large, centrally placed, germinal 
vesicle is slightly elongated -in the polar direction. The largest 
eggs, fully rounded out, measure 180 to 200 uw. They are almost 
perfectly transparent, have an equatorial ring of oil drops, and 
a well marked transparent exoplasm or cortical layer of proto- 
plasm with very faint granules forming a delicate mesh. In 
short, the living egg closely resembles that of Nereis; it is 
larger (but cf. Wilson, ’92) not so deeply pigmented, and lacks 
the characteristic yolk spheres of the Nereis egg. 
A. Fertilization in the living egg 
We may consider the fertilization of the egg under the follow- 
ing heads: (1) insemination, (2) penetration of the sperm, and 
(3) copulation of the germ nuclei. 
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