904 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



invariably accomplished by the formation of an egg by the female and 

 of a fecundating liquid by the male, which, coming in contact with the 

 egg developed by the female organism, gives to the latter its power of 

 independent growth and development. Sometimes the fluid formed by 

 the male conies in contact with the egg of the female after it has passed 

 from the body of the female, as is the case in fishes, while at other 

 times the male fecundates the egg before its exit and while it is still 

 within the body of the female, where it undergoes a certain part of its 

 developmental changes, as in the bird. Finally, the egg fecundated by 

 the male may be retained within the cavity of the female until it has 

 undergone the first phases of its development. 



Although numerous differences may be met with in different mem- 

 bers of the animal kingdom, these fundamental facts are always observed : 

 on the one side the production of an egg, and on the other the production 

 of a fecundating fluid. 



In all the vertebrates, whether mammals, birds, reptiles, or fishes, 

 generation is accomplished by the union of the two sexes, the sexual 

 organs being found in different individuals. 



In mammals the process of fecundation takes place within the 

 interior of the sexual organs of the female, and copulation is, therefore, 

 essential. In most reptiles a similar process takes place, although in 

 some fecundation is external " that is to say, the male extrudes the 

 seminal fluid upon the eggs as they leave the body of the female. This 

 latter process is also that which holds in fishes. In the fishes the eggs, 

 covered, as in the case of the batrachians, with a soft membrane, are 

 usually deposited along the banks or at the bottoms of rivers or ponds, 

 while the male deposits at variable intervals the fecundating liquid. As 

 a consequence of the exposure to so many different causes of destruction 

 a large number of eggs escape fecundation, but the immense number de- 

 posited b}' the fishes serves, however, to prevent the ultimate extinction 

 of the species. As many as a million eggs have been said to be extruded 

 at one time from the body of the female of various fishes. 



The ovaries of the female fish are two voluminous glands, which 

 almost fill the abdominal cavit}^ of the fish at the time of spawning. In 

 most of the bony fishes the oviducts are continuous with the ovaries and 

 form the extrusory canal. In many of the cartilaginous fishes the 

 abdominal extremity of the duct is free, as is the case in mammals, 

 reptiles, and birds. The oviducts open into the cloaca. 



The testicles form in the male two voluminous glands, opening by 

 means of the spermatic canals either into the cloaca, or by a special 

 opening in the neighborhood of the anus. In certain of the 'cartilaginous 

 fishes fecundation occurs within the body of the female, and there is, 

 therefore, true copulation analogous to that in birds. In these fishes the 



