32 THE TESTIS AND ITS RELATION TO REPRODUCTION 



well formed. In the opossum there are three pairs of bulbourethral glands. 

 The mucous contents of these and other small urethral glands are discharged 

 at the beginning of the sexual climax and, as such, become part of the semi- 

 nal fluid. 



7. Functions of Seminal Fluid 

 a. Amount of Seminal Fluid Discharged and Its General Functions 



As stated previously, the semen or seminal fluid is composed of two parts, 

 the sperm cells (spermia; spermatozoa) and the seminal plasma. The presence 

 of the sperm cells represents the most constant feature, although they may 

 vary considerably from species to species in size, shape, structure, and number 

 present. The seminal plasma varies greatly as to composition and amount 

 discharged. 



The quantity of seminal fluid discharged per ejaculate and the relative 

 numbers of sperm present in man and a few other vertebrate species associ- 

 ated with him are as follows:* 



Voliane of Sini>le Ejaculate, Sperni Density in Semen, 



Species Most Common Value, in CC. Average Value, per CC. 



Boar 250 cc. 100,000,000 per cc. 



Bull 4-5 cc. 1,000,000,000 per cc. 



Cock 0.8 cc. 3,500,000,000 per cc. 



Dog 6 cc. 200,000,000 per cc. 



Man 3.5 cc. 100,000,000 per cc. 



Rabbit 1 cc. 700,000,000 per cc. 



Ram 1 cc. 3,000,000.000 per cc. 



Stallion 70 cc. 120,000,000 per cc. 



Turkey 0.3 cc. 7,000,000,000 per cc. 



* Modified from Mann ('50). 



Two important branches of study involving the semen pertain to: 



(1) the chemical and physiological nature and numerical presence of the 

 sperm, and 



(2) the physiology and biochemistry of the seminal plasma. 



(See Mann, '50, for discussion and bibliography.) As a result of the studies 

 thus far, a considerable body of information has been accumulated. 



The main function of the semen, including the plasma and accessory sperm, 

 appears to be to assist the sperm cell whose chance fortune it is to make con- 

 tact with the egg. Once this association is accomplished, the egg seemingly 

 takes over the problem of fertilization. The seminal plasma and the accessory 

 numbers of sperm appear to act as an important protective bodyguard and 



