THE MALE ORGANS OF GENERATION. 597 



become distended with their contents, they project into the peritoneal 

 cavity. Each of the sexes is then under the influence of a correspond- 

 ing excitement. The unusual development of the reproductive organs 

 reacts upon the general system, producing a peculiar condition, known 

 as "erethism." The female, distended with eggs, feels the stimulus 

 which leads to their expulsion ; while the male, bearing the weight of 

 the enlarged testicles and the accumulation of newly-developed sper- 

 matozoa, is impelled by a similar sensation to the discharge of the 

 spermatic fluid. The two sexes are led by instinct at this season to 

 frequent the same situations. The female deposits her eggs in some 

 spot favorable to their protection and development; after which the 

 male, apparently attracted and stimulated by the sight of the new-laid 

 eggs, discharges upon them the spermatic fluid, and thus effects their 

 impregnation. It is in this way that fecundation takes place in nearly 

 all the osseous fishes. 



In instances like the above, where the male and female generative 

 products are discharged separately, their subsequent contact would 

 seem to be dependent on fortuitous circumstances, and impregnation, 

 therefore, liable to fail. But, in fact, the simultaneous excitement 

 of the sexes, leading them to ascend the same rivers and to frequent 

 the same localities, provides with sufficient certainty for impregnation. 

 The number of eggs produced by the female is also very large, the 

 ovaries being often so distended as nearly to fill the abdominal cav- 

 ity ; so that, although many eggs may be accidentally lost, a sufficient 

 number are still impregnated to provide for the continuation of the 

 specie-. 



In cartilaginous fishes, as in sharks, rays, and skates, an actual con- 

 tact takes place between the sexes, and the spermatic fluid of the male 

 is introduced into the female generative passages. Thus the eggs are 

 fecundated within the body of the female, and in many species go 

 through with a nearly complete development in this situation and the 

 young are born alive. 



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

 means of the anterior limbs, which retain their hold by spasmodic con- 

 traction. This continues for one or more days, during which time the 

 mature eggs, after being discharged from the ovary, are passing through 

 the oviducts. As they are expelled from the anus, the spermatic fluid 

 is discharged upon them, and impregnation takes place. 



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

 female generative passage at the time of copulation, by means of an 

 erectile male organ. Of these animals, some lay their eggs immedi- 

 ately after fecundation, others retain them until the embryo is partly 

 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 secreted fluids 



