18 THE ORIGIN OF PLANT AND ANIMAL CHILDREN 
Though the spermatozoa leave the testes by the hundreds 
of thousands, they are so tiny that they and the fluid which 
floats them make a very small quantity. To make a mass 
large enough to be forcibly discharged, fluid is added by se¬ 
creting glands along the route, by the seminal vescicles lying 
behind the bladder, by the 'prostate gland around the urethra 
at the outlet of the bladder, and by Cowper’s glands, a small 
pair opening into the urethra a little lower than the prostate. 
These fluids together with the spermatozoa compose the 
semen. The spermatozoa are the vital part of the semen. 
