FERTILIZATION 



275 



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

 a considerable amount of material which appears to take no part in the 

 formation of the chromosomes when division ensues. 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. 106). 

 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 the egg 

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

 the sexual fusion. 



FIG. 104. 



FIG. 105. 

 FIG. 104. Diagram of typical flagellate spermatozoon. 



P, perferatorium ; A, acrosome; N, nucleus; M, middle piece; F, axial filament; S, 

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



FIG. 105. The differentiation of the oocyte in Hydra. 



A, very young oocyte lying between ectodermal cells at right. B, oocyte after growth 

 period, with yolk globules, X 500. (After Downing, 1909.) 



The time relation of the maturation of the egg and the entrance of the 

 spermatozoon varies considerably in different animals. In echinoderms 

 and some other forms maturation is completed before the spermatozoon 

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

 of the heterotypic mitosis (Chcetopterus, Cerebratulus) or the prophase of 

 the homceotypic mitosis (many vertebrates), but does not go further 

 unless penetration occurs. In the marine annelid, Nereis, finally, the 



