FERTILIZATION 



275 



germinal vesicle: it is very large and contains in addition to its chromatin 



a considerable amount of material which appears to take do part in the 

 formation of the chromosomes when division ensui After the cyto- 

 plasmic differentiation is complete and the oocyte has reached its full 

 size — even after the spermatozoon has entered In many 

 cases — the oocyte nucleus undergoes two divisions in rapid 

 succession at the periphery of the egg, which at this point 

 buds off two small nucleated cells, the polar bodies (Fig. L06 

 The first polar body may or may not divide again. The 

 details of chromosome behavior in these two mitoses have been 

 described in the preceding chapter. The reduced or haploid 

 number of chromosomes left in the egg organize tip 

 nucleus ("female pronucleus"), rendering the egg ready for 

 the sexual fusion. 



Fig. 104. Fig. 105. 



Fig. 104. — Diagram of typical flagellate spermafc 



P, perferatorium; A, acrosomc; A", nucleus; M, middle piece; /. ixial fUamci 



cytoplasmic sheath; E, end piece. (After Wilson.) 



Fig. 105. — The differentiation of tin- oocyte in Hyd 



A, very young oocyte lying betweeri ectodermal cells al right. B .\th 



period, with yolk globules, X 500. {After Downing, 1909. 



The time relation of the mat mat ion of i hi and t he en1 ranee of t he 



spermatozoon varies considerably in differenl animals. In echinoderms 

 and some other forms maturation is complete. 1 before the spermato 

 penetrates. In some other animals it proceeds as far as the metaphase 

 of the heterotypic mitosis (Chcetopterus, Cerebratulw of the propha 

 the homceotypic mitosis (many vertebrates), bul does do! go further 

 unless penetration occurs. In the marine annelid. A i . finally, tl 



