GENERATION AND DEVELOPMENT. 



249 



nation, and their actual contact with the ovum is necessary for its devel- 

 opment; but concerning the manner of their action nothing is known. 



The seminal fluid is, probahly, after the period of puberty, secreted 

 constantly, though, except under excitement, very slowly, in the tubules 

 of the testicles. From these, it passes along the vasa deferentia into the 

 vesiculae seminales, whence, if not expelled in emission, it may be dis- 



FlG. 404. 



FIG. 405. 



FIG. 404. Spermatozoon of Salamandra Maculata. Fresh mounted in glycerin, x 950, reduced 

 one half. 



FIG. 405. Human Spermatozoa. X 2500. (H. Gibbes.) 



charged, as slowly as it enters them, either with the urine, which may 

 remove minute quantities, mingled with the mucus of the bladder and 

 the secretion of the prostate, or from the urethra in the act of defaecation. 

 Vesiculae Seminales. The vesiculm seminales (Fig. 406) have the 

 appearance of outgrowths from the vasa deferentia. Each vas deferens, 



