THE MALE GENITAL ORGANS. 361 



D. THE MALE GENITAL ORGANS. 



J. THE SPERMATOZOON. 



The semen, or sperma, is a fluid that, as a whole, consists of 

 the secretion of several sets of glands in which the sexual cells, the 

 spermatosomes, or spermatozoa, which are formed in the testes, are 

 suspended. 



We shall first consider the structure of the typical adult sperma- 

 tosome, taking up consecutively its component parts. Three prin- 

 cipal parts may be distinguished the head, the middle piece, and 

 the tail or flagellnm. The round or oval body of the head termi- 

 nates in a lanceolate extremity. The former consists of chromatin, 

 and is most intimately associated with the phenomenon of fertiliza- 

 tion. The middle piece, which is attached to the posterior end of 

 the head, is composed of a protoplasmic envelop which surrounds a 

 portion of the so-called axial thread. The latter is enlarged ante- 

 riorly just behind the head to form the terminal nodule, which fits into 

 a depression in the head. From the middle piece on, the axial thread 



Fig. 288. Diagram showing the general characteristics of the spermatozoa of 

 various vertebrates : a, Lance ; b, segments of the accessory thread ; c, accessory 

 thread ; d, body of the head ; e, terminal nodule ; f, middle piece ; g, marginal thread ; 

 h, axial thread ; i, undulating membrane ; k, fibrils of the axial thread ; /, fibrils of the 

 marginal thread ; m, end piece of Retzius ; ;/, rudder-membrane. 



is continued into the tail of the spermatozoon, and is here sur- 

 rounded by a transparent substance the sheatJi of the axial thread. 

 The envelop is lacking at the posterior extremity of the tail, where 

 the axial thread extends for a short distance as a naked filament 

 called the end-piece of Retzius. From the middle piece a still finer 

 thread is given off", the marginal thread, which extends at a certain 

 distance from the axial thread as far as the end-piece of Retzius. 

 In its course it crosses and recrosses the axial thread at various 

 points, and may even wind around it in a spiral manner. In all in- 

 stances it is connected with the sheath of the axial thread by a 

 delicate membrane the undulating membrane. Another and still 

 more delicate filament the accessory thread runs parallel with the 

 axial thread along the surface of its sheath and terminates at a cer- 

 tain distance from the end-piece of Retzius. Near the extremity of 

 the flagellum and immediately in front of the end-piece is another 

 and shorter membrane, the rudder membrane, which is continu- 

 ous with the undulating membrane. Maceration reveals a fibrillar 



