i96 



GENERATION. 



able that during the unusual sexual excitement which the female generally 

 experiences after a monthly period, the action of the internal organs, attend- 

 ing and following coitus, presents the most favorable conditions for the 

 penetration of the fecundating elements, and this may explain the more fre- 

 quent occurrence of conception as a consequence of intercourse at this time. 

 Union of the Male with the Female Element of Generation. The first 

 important step toward a positive knowledge of the mechanism of fecunda- 

 tion was the discovery of the spermatozoids, in 1677 ; the second was the 

 demonstration, by Spallanzani, in his experiments upon artificial fecunda- 

 tion, that when the seminal fluid is carefully filtered, the liquid which passes 

 through has no fecundating properties, the male element remaining on the 

 filter ; and the third was the demonstration of the presence of spermatozoids 

 within the vitelline membrane. 



In the ova of certain animals, an opening, called a micropyle, has been 

 demonstrated in the vitelline membrane (Barry, Keber). This has been seen 

 in the ova of rabbits, although its existence is to be inferred, only, in the 

 human ovum. In the ova of the nephelis vulgaris, or common leech, Eobin 

 has seen spermatozoids, to the number of several hundreds, penetrate the 

 vitelline membrane, always at one point, continuing their movements upon 

 the surface of the vitellus. " Almost always, when the penetration has 

 ceased, a bundle of spermatozoids is arrested in the micropyle." The pene- 

 tration of spermatozoids has been observed in the ova of other animals, in- 

 cluding the rabbit (Newport, Coste, Bischoff, Weil and others). Weil has seen 

 spermatozoids wedged in the substance of the zona pellucida, has added 

 blood to a specimen under observation, and has restored the movements of 



the spermatozoids while in this position. 

 He has also seen, in some instances, per- 

 fectly formed spermatozoids in the very 

 substance of the vitellus. As the sperma- 

 tozoids pass to the vitellus, there is a re- 

 traction of its substance, leaving a space 

 between it and the vitelline membrane, 

 which soon becomes filled with a clear 

 liquid. 



All direct observations upon the lower 

 forms of animals have shown that several 



FIG. 291. Ovum of the nephelis vulgaris, criprmH+nrrvirlsj arp nppp^arv fnr flip fppnn- 

 showing penetration of spermatozoids S P 61 



SF^SSfef**^ ofMeyite' 1 ; m 9 ni - dation of a single ovum; but physiologists 



fied 800 diameters (Robin). * J 



have no definite idea of the number required 



in mammals, much less in the human subject. It is not known what be- 

 comes of the spermatozoids after they have come in contact with the vitellus. 

 All that can be said upon this point is that there probably is a direct union 

 between the two generative elements, soon to be followed by the series of 

 changes involved in the first processes of development. 



There are many questions connected with hereditary transmission, which 

 if they were susceptible of any thing approaching a positive, scientific explana- 



