700 MALE ORGANS OF GENERATION. 



the vitelline membrane, as first indicated by Barry. In others no such 

 orifice has been visible; the spermatozoa appearing to perforate the sub- 

 stance of the vitelline membrane by the impulsive movement of their 

 filamentous extremity (Newport). Such a mode of penetration is not 

 inadmissible, since the much larger embryos of the tsenia and trichina 

 (page 675) make their way without difficulty through the substance of 

 the intestinal mucous membrane. 



After their arrival in the interior of the vitelline cavity, the sperma- 

 tozoa disappear as distinct organic elements. Their substance unites 

 with that of the vitellus ; and thenceforward the fecundated egg con- 

 sists of materials derived from both the male and female organisms. 

 The greater portion of this material is that produced by the female ; 

 but that which is supplied from the seminal filaments of the male is 

 equally essential for the production of an embryo. The offspring, 

 accordingly, may exhibit resemblances to either or both of the indi- 

 vidual parents, since it originates from a union of both the generative 

 products. 



Union of the Sexes. In most of the lower animals there is a peri- 

 odical development of the testicles in the male, corresponding in time 

 with that of the ovaries in the female. As the ovaries enlarge and the 

 eggs ripen in the one sex, so in the other the testicles increase in size, as 

 the season of reproduction approaches, and become turgid with sper- 

 matozoa. The accessory organs of generation at the same time share 

 the unusual activity of the testicles, and become increased in vascularity 

 and ready to perform their part in the reproductive function. 



In fishes, as a general rule, where the testicles occupy, in the abdomen 

 of the male, the same relative position as the ovaries in the female, these 

 organs enlarge, become distended with their contents, and project into 

 the peritoneal cavity. Each of the two sexes is then at the same time 

 under the influence of a corresponding excitement. The unusual de- 

 velopment of the reproductive organs reacts upon the general system, 

 and produces a state of peculiar excitability, known as the condition of 

 "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 spermatozoa, 

 is impelled by a similar sensation to the discharge of the seminal 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 the 

 protection and development of the young ; after which the male, appa- 

 rently attracted and stimulated by the sight of the new-laid eggs, dis- 

 charges upon them the seminal fluid, and their impregnation is accom- 

 plished. It is in this way that fecundation takes place in nearly all the 

 osseous fishes, as the trout, the salmon, and the stickleback. 



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

 products are discharged separately, the subsequent contact of the semi- 

 nal fluid with the eggs would seem to be dependent on the occurrence 

 of fortuitous circumstances, and their impregnation, therefore, liable to 



