THE BIOLOGY OF THE CELL SURFACE 



animals breed. Like many other forms, Nereis exhibits a 

 lunar periodicity in its breedins; behavior and is sexually 

 mature only during the period from full to new moon of 



each lunar cycle from June to 

 September (at Woods Hole, 



Mass.).^ 



Although "ripe" eggs of 

 Nereis limbata are available 

 only during this particular 

 moon-phase, their abundance 

 and the clock-like precision of 

 their development make them 

 ideal objects for observation 

 and experiments on fertiliza- 

 tion. The fertilization-pro- 

 cess as seen in the living egg 

 is as follows: 



When discharged or re- 

 moved from the female the 

 egg measures about lOO by 80 

 microns. It reveals in optical 

 section at the centre a large 

 formation, the germinal 

 Fig. 22.-Drawings from photo- vesicle. Around this are 



graphs of Nereis eggs in a suspension of 



Chinese ink in sea-water (after Lillie). greenish spheres, the yolk, 

 a, before insemination; Z), three minutes among which are larger re- 

 after insemination, c- . IT 1 •! 1 



irmgent bodies, the oil drops. 

 Beyond the area of yolk and oil is a rim, the ectoplasm, 

 made up of coarse strands disposed in a somewhat radial 

 fashion which extend to the clearly discerned vitelline mem- 

 brane. The eggs die in this stage with germinal vesicle and 

 ectoplasm intact, unless fertilized or experimentally treated 

 by means of inducing parthenogenesis. 



^Lillie and Just, igi^; Just, /p/^, igzga. 



