42 



EMBRYOLOGY 





are produced, viz., the mature ovum and three polar bodies, each of which con- 

 tains two chromosomes, i. e., one-half the number present in the nuclei of the 

 somatic cells of members of the same species. The nucleus of the mature ovum 

 is termed the female pronucleus. 



THE SPERMATOZOON. 





The spermatozoa or male germ cells are developed in the testes and are present 

 in enormous numbers in the seminal fluid. Each consists of a small but greatly 

 modified cell. The human spermatozoon possesses a head, a neck, a connecting 

 piece or body, and a tail (Fig. 6). 



Head 



Connecting piece 



[-"Perforator 



Taill 



End-piece < 



5 Neck 



Head-cap 



-Anterior centriole 

 ""Posterior centriole 



Spiral thread 

 Mitochondria sheath 



Terminal disc 



'Axial filament 



FIG. 6. Human spermatozoon. Diagrammatic. A. Surface view. B. Profile view. 

 and connecting piece are more highly magnified. 



In C the head, neck, 



The head is oval or elliptical, but flattened, so that when viewed in profile 

 it is pear-shaped. Its anterior two-thirds are covered by a layer of modified proto- 

 plasm, which is named the head-cap. This, in some animals, e. g,, the salamander, 

 is prolonged into a barbed spear-like process or perforator, which probably facilitates 

 the entrance of the spermatozoon into the ovum. The posterior part of the head 

 exhibits an affinity for certain reagents, and presents a transversely striated appear- 

 ance, being crossed by three or four dark bands. In some animals a central rod- 

 like filament extends forward for about two-thirds of the length of the head, while 

 in others a rounded body is seen near its center. The head contains a mass of 



