256 



FERTILIZATION 



ooplasmic substances of the germinal disc occur. As a result, the blastodisc, 

 circular during the maturation period, becomes transformed into an oval- 

 shaped affair with the polar bodies situated on one end (fig. 136). The first 

 plane of cleavage is indicated by numerals I-I, and the second plane of cleavage 

 by numerals II-II. A distinct bilateral symmetry thus is established by the 

 rearrangement of ooplasmic materials during the fertilization process, (Com- 

 pare with Styela, Amphioxus, and frog.) 



E. Significance of the Maturation Divisions of the Oocyte in Relation 

 to Sperm Entrance and Egg Activation 



As indicated in the foregoing pages, the maturation divisions of the oocyte 

 vary greatly in different animal species. Figure 137 shows that the time of 

 sperm entrance in the majority of eggs occurs either before or during the 

 maturation divisions, that is, when the female gamete is in the primary or 

 secondary oocyte condition. In some animals, however, the sperm enters 

 normally after the two maturation divisions are completed. 



The correlation between the maturation period of the egg and sperm entrance 

 indicates that the breakdown of the germinal vesicle and the accompanying 

 maturation divisions has a profound effect upon the egg. This conclusion is 

 substantiated by experimental data. For example, A. Brachet ('22) and 

 Runnstrom and Monne ('45, a and b), working on the sea-urchin egg, found 



PERIVITELLINE 

 MEMBRANE 



SPACE 



Fig. 128. Formation of the vitelline membrane in the egg of Ascaris after fertilization. 

 (After Collier, '36.) (A) Heavy cell wall (vitelline membrane) is beginning to thicken. 

 (B) Cell wall is reaching condition of maximum thickness. (C) Egg contracts away 

 from vitelline membrane, leaving perivitelline space filled with fluid-like substance, form- 

 ing the typical fertilized egg of Ascaris as ordinarily observed. 



