596 REPRODUCTION. 



mixture for only one second proved to be impregnated, and were after- 

 ward hatched at the usual period." 



Thirdly, the spermatozoa must come in direct contact with the egg 

 or* its envelopes. Spallanzani first demonstrated this by attaching ma- 

 ture eggs to the concave surface of a watch-glass, which he placed, in- 

 verted, over a second watch-glass containing spermatic fluid. The eggs, 

 exposed in this way for several hours to the vapor of the fluid without 

 touching its surface, were afterward found to have failed of impregna- 

 tion ; while others, which had been moistened with the same spermatic 

 fluid, became developed into tadpoles. 



Finally, in the act of fecundation the spermatozoa penetrate, through 

 the vitelline membrane, to the vitellus. This fact, first observed by 

 Barry * in the rabbit, has, subsequently been seen by Newport f in the 

 frog, by Bischoff, by Coste, by Robin J in a species of leech, by Flint 

 in the pond snail, and by Weil,|| in repeated instances, in the rabbit. 

 According to seme of these observations, the penetration of the sper- 

 matozoon takes place by a small orifice or "micropyle" in the vitelline 

 membrane, as first indicated by Barry. In others no such orifice has 

 been visible ; the spermatozoa appearing to perforate the vitelline mem- 

 brane by the impulsive movement of their filamentous extremity (New- 

 port). Such a mode of penetration is not inadmissible, since it is known 

 that the much larger embryos of tasnia and trichina make their way 

 without difficulty through the substance of the intestinal mucous mem- 

 brane. 



After their arrival in the vitelline cavity, the spermatozoa disappear 

 as distinct organic elements. Their substance unites with that of the 

 vitellus ; and thenceforward the fecundated egg is derived from both 

 male and female organisms. The greater portion of its material is pro- 

 duced by the female ; but that which is supplied from the seminal fila- 

 ments of the male is equally essential for the production of an embryo. 

 The offspring, accordingly, may exhibit resemblances to either or both 

 of the parents, since it originates from both the generative products. 



Union of the Sexes. In most animals there is a periodical develop- 

 ment 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 

 one sex, the testicles of the other increase in size, and become turgid 

 with spermatozoa. The accessory organs of generation at the same 

 time exhibit an unusual activity of nutrition, increasing in vascularity 

 and preparing to perform their part in reproduction. 



In fishes, as a rule, the testicles occupy, in the abdomen of the male, 

 the same relative position as the ovaries in the female ; and, as they 



* Philosophical Transactions. London, 1840, p. 533, and 1843, p. 33. 

 f Ibid., 1853, p. 271. 



J Journal de la Physiologic de 1'Homme et des Animaux. Paris, 1862, tome v., 

 p. 80. 



Physiology of Man, New York, 1874, vol. v., p. 352. 

 II Strieker's Medicinischer Jahrbiicher. Wien, 1873, p. 18. 



