MALE ORGANS. OF GENERATION. 701 



fail. But, in point of fact, the simultaneous functional excitement of 

 the two sexes, and the operation of corresponding instincts, leading 

 them to ascend the same rivers and to frequent the same spots, provide 

 with sufficient certainty for the impregnation of the eggs. The number 

 of eggs produced by the female is also very large, the ovaries being 

 often so distended as nearly to fill the abdominal cavity; so that, 

 although many of the eggs may be accidentally lost, a sufficient number 

 will still be impregnated to provide for the continuation of the species. 



In many of the cartilaginous fishes, on the other hand, as in sharks, 

 rays, and skates, an actual contact takes place between the two sexes at 

 the time of reproduction, and the seminal fluid of the male is introduced 

 into the generative passages of the female. Thus the eggs are fecunda- 

 ted while still in the body of the female, and in many species go through 

 with a nearly complete development in this situation and are born alive. 



In the frog, the male fastens himself upon the back of the female by 

 means of the anterior limbs, which retain their hold by a kind of spas- 

 modic contraction. This continues for one or more days, during which 

 time the mature eggs, which have been discharged from the ovary, are 

 passing downward through the oviducts. As they are expelled from the 

 anus, the seminal fluid of the male is discharged upon them, and impreg- 

 nation takes place. 



In serpents, lizards, and turtles, the sperm is introduced into the female 

 generative passage at the time of copulation, by means of a single or 

 double erectile male organ. Of these animals, some species lay their 

 eggs immediately after fecundation, others retain them until the embryo 

 is partly or fully developed. 



In birds, the spermatozoa are introduced into the sexual orifice of 

 the female, and make their way into the upper portion of the oviduct, 

 where they may be found in active motion, mingled with the fluids of 

 this canal. 1 The vitellus is thus fecundated immediately upon its- dis- 

 charge from the ovary, and before it has become surrounded with the 

 albuminous and membranous envelopes supplied by the middle and 

 lower portions of the oviduct. 



Lastly, in the human species and in mammalians, where the impreg- 

 nated egg is to be retained in the body of the female parent during the 

 whole period of its development, the seminal fluid is introduced into the 

 vagina and uterus by sexual congress, and meets the egg at or soon 

 after its discharge from the ovary. A close correspondence between the 

 periods of sexual excitement, in the male and the female, is visible in 

 many of these animals, as well as in fish, birds, and reptiles. This is 

 the case in most species which produce young but once a year, as the 

 deer, the wolf, and the fox. In others, such as the dog, the rabbit, and 

 the guinea pig, where several broods of young are produced during the 

 year, or where, as in man, the generative epochs of the female recur at 

 short intervals, the time of impregnation is comparatively indefinite, 



1 Foster and Balfour, Elements of Embryology. London, 1874, p. 21. 



