692 REPRODUCTION AND GROWTH 



secretions to be mixed with the spermatozoa in the seminal fluid; and the 

 male organ, the penis, by which the seminal fluid is introduced into the female 

 organs. 



A. THE TESTES 



Sexual maturity, or puberty, appears in the man at about the age of fifteen. 

 The testes begin to increase in volume and to secrete seminal fluid. At the 

 same time the foreskin becomes loosened from the glans penis, and the rest 

 of the body exhibits many changes : the bones and the muscles become stronger 

 (cf. below, page 709) ; the larynx increases in size, in consequence of which 

 the voice becomes fuller and deeper, etc. 



When the testes are removed by castration before sexual maturity, these 

 changes do not take place which proves clearly that they are occasioned by 

 the testes. 



The general character of the individual also is changed by castration, as 

 may be seen best perhaps by comparison of the disposition of an ox with that 

 of a bull. It follows that all the characteristics by which a man is distin- 

 guished from a woman depend essentially upon the testes and their activity 

 (cf. also page 357). 



The spermatozoa are minute bodies consisting of a thick head and a slender 

 tail, which, in virtue of the whiplike movements of the tail, are capable of 

 independent motion. The speed of their locomotion, considered with refer- 

 ence to their size, is rather high, namely, 0.05 to 0.15 mm. per second. 



The spermatozoa are formed by a peculiar transformation of certain cells 

 in the testes, known as spermatids. According to Lode, in 1 cu. mm. of 

 human seminal fluid there are about 60,000 spermatozoa. The quantity of 

 seminal fluid discharged at a single ejaculation may be estimated at about 

 3 cc. ; whence the total number of spermatozoa in a single ejaculation would 

 be in the neighborhood of 180,000,000 a perfectly enormous number in view 

 of the fact that but a single spermatozoon is necessary for fertilization of 

 the ovum. 



Foges reports the interesting observation that the testis of a cock trans- 

 planted into the abdominal cavity will continue to produce spermatozoa, from 

 which we may conclude that spermatogenesis is in part at least independent of 

 the nervous system. 



B. THE ACCESSORY SEXUAL GLANDS 



In a castrated animal the accessory glands atrophy, showing that they 

 must play some essential part in the sexual functions. If castration be 

 performed before sexual maturity, they do not develop at all. 



Nothing is known at present as to the special functions of the separate 

 glands ; but from the fact, established by comparative anatomical studies, that 

 there is considerable variation in their relative sizes in different species, we 

 may surmise that they all have an essentially common purpose. 



The so-called seminal vesicles are not properly a receptacle for the seminal 

 fluid, although they do always contain a greater or less number of spermato- 



