50 THE EARLY DEVELOPMENT OF MAMMALS. 



entrance of the spermatozoon into the ovum and the formation of the male pro- 

 nucleus; (3) fusion of the pro-nuclei to form the segmentation nucleus. 



Meeting of the Sexual Elements. In all amniota the seminal fluid is trans- 

 ferred from the male to the female passages during coitus, and spermatozoa are 

 thereafter, in mammals, found in the uterus. In default of actual knowledge it 

 is commonly believed that the spermatozoa make their way by their own motions 

 into the Fallopian tubes. The ovum, meanwhile (probably, in mammals, while 

 completing its maturation), travels down the tube. The meeting-point, or site 

 of impregnation, in placental mammals is about one-third way down from the 

 fimbria to the uterus. The exact spot is remarkably constant for each species. 

 Nothing is known by direct observation as to the site of impregnation in man, 

 but there is no reason to suppose, as has unfortunately been often done, that the 

 site is either variable or essentially different from that in other mammals. 



The Entrance of the Spermatozoon into the Ovum. It is probable, in mam- 

 mals at least, that only one spermatozoon enters the yolk of the ovum and ac- 

 complishes its fertilization. It has been observed in those animals in which, as in 

 the rabbit, there is formed a more or less considerable space between the yolk 

 and the zona radiata, that a number of spermatozoa appear in this space, but 

 apparently only one actually fuses with the substance of the ovum. The manner 

 in which additional spermatozoa are excluded, after the first has entered, is still 

 under discussion. The hypothesis has been suggested that the attractive power 

 of the ovum is annulled or weakened by the formation of the male pro-nucleus 

 from the spermatozoon which first enters. With our present knowledge the 

 assumption appears unavoidable that the ovum exerts a specific attraction upon 

 spermatozoa of the same animal species. Recent authorities incline to the view 

 that this attraction is of a chemical nature, for it has been observed that certain 

 chemical substances may attract very strongly unicellular organisms capable of 

 free locomotion. The phenomenon is called chemotropism. According to this 

 interpretation, the attraction of the ovum for the spermatozoon would be termed 

 chemotropic. 



At the time of fertilization the ovum in the Fallopian tube is surrounded 

 by a number of spermatozoa. In the case of the rabbit, perhaps by a hundred, 

 more or less. They are all, or nearly all, in active motion, for the most part 

 pressing their heads against the zona radiata. Several of them may make their 

 way through the zona into the interior. According to Hensen, only those sper- 

 matozoa which enter the zona along radial lines can make their way through. 

 Those which take oblique courses remain caught in the zona. The female pro- 

 nucleus is already present, either formed or at least forming as a membranate 

 nucleus. A single spermatozoa makes its way into the yolk proper, passing a 



