I50 



FERTILIZATION OF THE OVUM 



where it is very large and conspicuous, and I have since observed it 

 also in the sea-urchin (Fig. 69). 



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 Q^g 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 uni- 

 formly distributed, and at the 

 periphery of the egg is a zone of 

 clear perivitelline protoplasm (Fig. 

 43). Soon after entrance of the 

 spermatozoon the germinal vesicle 

 moves towards the periphery, its 

 membrane fades away, and a radi- 

 ally directed mitotic figure appears, 

 by means of which the first polar 

 body is formed (Fig. 71). Mean- 

 while the protoplasm flows towards 

 the upper pole, the perivitelline 

 zone disappears, and the Q,g^ now 

 shows a sharply marked polar 

 differentiation. A remarkable phe- 

 nomenon, described by Whitman 

 in the leech ('78), and later by 

 Foot in the earthworm ('94), is 

 the formation of " polar rings," a process which follows the entrance 

 of the spermatozoon and accompanies the formation of the polar 

 bodies. These are two ring-shaped cytoplasmic masses which form 

 at the periphery of the egg near either pole and advance thence 

 towards the poles, the upper one surrounding the point at which the 

 polar bodies are formed (Fig. 76). Their meaning is unknown, but 

 Foot ('96) has made the interesting discovery that they are probably 

 of the same nature as the yolk-nuclei (p. 121). 



Fig. 76. — Egg of the leech Clepsiiic, dur- 

 ing fertilization. [Whitman.] 



I>.b., polar bodies ; p.r., polar rings ; cleav- 

 age-nucleus near the centre. 



