102 



DE VEL OPMENT. 



take place in the peritoneal cavity. The spermatozoon becomes buried in the 

 yolk, the tail disappears, and the head, which is really the nucleus of the sperma- 

 tozoon, constitutes the." male pronucleus." This gradually approaches the female 

 pronucleus, which by this time is situated in the centre of the ovum. As soon as 

 they come into contact they fuse into one, and thus fecundation is effected 

 (Fig. 74).' 



/. pr. f. pr. 



f.pr. 



FIG. 74. Fertilization of the ovum of an echinoderm. s. Spermatozoon, m. pr. Male pronucleus. /. pr. Fe- 

 male pronucleus. 1. Accession of a spermatozoon to the periphery of the vitellus. 2. Its penetration. 3. Trans- 

 formation of the head of the spermatozoon into the male pronucleus. 4, 5. Blending of the male and female 

 pronuclei. (From Quain's Anatomy, Selenka.) 



The first result of the fertilization of the ovum is its cleavage or multiplica- 

 tion, it being first cleft into two masses, the germinal vesicle having previously 



FIG. 75. First stages of segmentation of a mammalian ovum ; semi-diagrammatic. (From a drawing by 

 Allen Thomson.) z. p. Zona pellucida, p. gl. Polar globules, u. L'pper cell. I. Lower cell, a. Division into 

 two spheres. 6. Stage of four spheres, c. Eight spheres, the upper cells partially enclosing the lower cells. 

 d. e. Succeeding stages of segmentation, showing the more rapid division of the upper cells and the enclosure 

 of the lower cells by them. 



split up into two nuclei ; so that it now consists of two separate masses of proto- 

 plasm, each containing a nucleus, situated within the original vitelline membrane, 



1 If the student refers to the development of the generative organs, he will find that the 

 ovum of the female and the spermatozoon of the male are derived from fundamentally the same 

 structures, and therefore their fusion is the union of two elements of very similar morphological 

 value. 



