L\YJON OF THE GERM-CELLS 



201 



only after formation of the polar bodies ; for when in sea-urchins the 

 spermatozoa enter immature eggs, as they freely do, they penetrate 

 but a short distance, and no further change occurs. 



(/') The Ovum. The entrance of the spermatozoon produces an 

 extraordinary effect on the egg, which extends to every part of its 

 organization. The rapid formation of the vitelline membrane, already 

 described, proves that the stimulus extends almost instantly through- 

 out the whole ovum. 1 At the same time the physical consistency 

 of the cytoplasm may greatly alter, as for instance in echinoderm 

 eggs, where, as Morgan has observed, the cytoplasm assumes immedi- 

 ately after fertilization a peculiar 

 viscid character which it afterward 

 loses. In many cases the egg con- 

 tracts, performs amoeboid movements, 

 or shows wave-like changes of form. 

 Again, the egg-cytoplasm may show 

 active streaming movements, as in 

 the formation of the entrance-cone in 

 echinoderms, or in the flow of periph- 

 eral protoplasm toward the region 

 of entrance to form the germinal 

 disc, as in many pelagic fish-eggs. 

 An interesting phenomenon is the 

 formation, behind the advancing 

 sperm-nucleus, of a peculiar funnel- 

 shaped mass of deeply staining 



material extending OUtward tO the during fertilization." [WHITMAN.] 



periphery. This has been carefully P- b - P lar bodies = /' P lar rin s : 



j -i_ i i T- A. /> \ ^.1 j.t cleavage-nucleus near the centre. 



described by Foot ( 94) in the earth- 

 worm, where it is very large and conspicuous, and I have since ob- 

 served it also in the sea-urchin (Fig. 94). 



The most profound change in the ovum is, however, the migration 

 of the germinal vesicle to the periphery and the formation of the 

 polar bodies. In many cases either or both these processes may occur 

 before contact with the spermatozoon (echinoderms, some vertebrates). 

 In others, however, the egg awaits the entrance of the spermatozoon 

 (annelids, gasteropods, etc.), which gives it the necessary stimulus. 

 This is well illustrated by the egg of Nereis. In the newly dis- 

 charged egg the germinal vesicle occupies a central position, the 

 yolk, consisting of deutoplasm-spheres and oil-globules, is uniformly 

 distributed, and at the periphery of the egg is a zone of clear peri- 

 vitelline protoplasm (Fig. 60). Soon after entrance of the sperma- 



1 I have often observed that the formation of the membrane, in Toxopneustes, proceeds 

 like a wave from the entrance-point around the periphery, but this is often irregular. 



Fig. 102. Egg of the leech Clepsine 



