FERTILIZATION. 



109 



the protoplasm of the ovum, is thrown out of the egg as the so-called 

 directive bodies or polar cells (fig. 101). The part of it, however, 

 which remains in the ovum retains its significance as a nucleus, and 

 is known as the female pronucleus. This fuses with the single 

 spermatozoon (male pronucleus) which has forced its way into the 

 ovum (fig. 102); and the compound structure so formed constitutes 

 the nucleus of the fertilized ovum, or as it is generally called, the 

 first segmentation nucleus. 



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FIG. 103. Development of a Star-fish, Asteracanthion berylinus (after Alex. Agassiz). 1, 

 Commencing segmentation of the flattened egg at one pole are seen the polar bodies ; 2, 

 stage with two segments ; 3, with four ; 4, with eight ; 5, with thirty-two segments ; 6, 

 later stage ; 7, blastosphere with commencing invagination ; 8 and 9, more advanced 

 stages of invagination. The opening of the gastrula cavity becomes the anus. 



This new nucleus, which divides to give rise to the nuclei of the 

 first segmentation spheres, would appear therefore to be the product 

 of the fusion or conjugation of the part of the germinal vesicle, 

 which remains behind in the ovum, with the male pronucleus, which 

 is a derivative of the spermatozoon which has entered the ovum. 

 Fertilization would appear, therefore, to depend upon the addition 



