198 ORGANISED FLUIDS. 



velopment of the seminal animalcules are best seen side by 

 side. 



The spring is by far the most suitable period for the 

 study of the development of the spermatozoa, and birds, 

 especially those of the order Passeres, present the best ex- 

 amples in which to trace their evolution, because in them the 

 seminal animalcules are large, and the reproductive function 

 is excessively active for a brief and determined period. 



Wagner * has shown that from the commencement of 

 the time of moulting, and through the entire winter, the 

 testes of birds undergo an extraordinary degeneration, the 

 spermatozoa and the spermatophori being entirely oblite- 

 rated, and the volume of the testes reduced to at least the 

 twentieth or thirtieth of the size to which they attain in spring. 

 Thus the testis of the common chaffinch is in winter not 

 larger than a millet seed, while in spring it exceeds a pea in 

 size. 



The same degeneration is doubtless experienced during 

 winter, although to a less extent, by most animals of the class 

 Mammalia. 



THE SPERMATOZOA ESSENTIAL TO FERTILITY. 



The spermatozoa do not exist in the testes of mammalia at 

 all periods of life : thus they do not make their appearance 

 in that organ in man until the period of puberty, and they 

 disappear gradually as old age advances. It is impossible 

 however to determine the time at which they are first de- 

 veloped, or at which they cease to exist in that organ, be- 

 cause the period of puberty differs in different individuals, 

 and some men are aged in constitution when others of the 

 same years are hale and robust. Certain it is, that some men 

 retain the power of engendering until a very advanced age, of 

 which fact the celebrated Parr presents a memorable example, 

 he having become a father at the extraordinary age of 142. 



The number also of the spermatozoa contained in the 

 seminal fluid varies in different individuals, and is usually in 



* Elements, pp. 28. and 29. 



