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A TEXTBOOK OF PHYSIOLOGY 



the ovum, the tail, or flagellum, being (generally) left behind. From 

 the head the male pronucleus is formed (Fig. 477) ; this combines 

 with the female pronucleus to form the oosperm, thus completing the 

 process of fertilization. 



A - B. C. 



FIG. 478. SHOWING THE PRODUCTION OF THE MORULA FROM THE OVUM. (Keith.) 

 A, Ovum after first division; B, after second division; C, morula stage. 



The stimulus supplied by the spermatozoon may be imitated by 

 altering the relation of the cell membrane of the ovum to its 

 environment ; thus it has been shown that an unfertilized frog's 

 eggs may be made to develop into tadpoles by the prick of a pin. 



emunjogenic pole 



exbryogenic pole 

 inner cell mass ..enveloping layer 



vegetative pole 



vegetative pole 

 FIG. 479. THE BLASTULA STAGE. 



enveloping layer 



FIG. 480. THE BLASTOCYST STAGE. 

 (Keith, after Van Beneden.) 



The tadpoles, however, do not develop into frogs. It has been 

 ascertained that the chromosomes in the parthenogenetic larva 

 of frogs are of the reduced type. This is a matter of interest, and 

 probably explains the failure of the method to produce complete 



