THE BIOLOGY OF THE CELL SURFACE 



enclosed by the nuclear membrane. The cytoplasm Is 

 sharply differentiated Into two regions: the endoplasm, 

 crowded with yolk spheres, oil drops and granules, the 

 smaller of which are mitochondria; and the ectoplasm with 

 Its radially disposed lines which extend to the plasma- 



/ 



Fig. I. — Section of an unfertilized egg of Platynereis megalops. Surrounding 

 the large nucleus, germinal vesicle, is the cytoplasm, which is clearly marked off 

 into two zones, the endoplasm and the ectoplasm. 



membrane beneath the vitelline membrane. The q^^ of 

 Arbacia, a sea urchin, Is similar to that of Platynereis 

 except that the whole egg Including the nucleus and the 

 endoplasmic bodies Is smaller, the ectoplasm a mere line, 

 and the vitelline membrane an extremelv fine film. In 



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