IMPREGNATION OF THE OVUM. 39 



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 appar- 

 ently 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 attrac- 

 tion is of a chemical nature, for it has been observed that certain chemical sub- 

 stances 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 spermatozoa 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, and may be either with or without a membrane, according to the species. 

 A single spermatozoon makes its way into the yolk proper, passing a short distance 

 into the interior. It is uncertain whether the whole tail of the spermatozoon enters 

 the ovum or not. In some of the lower vertebrates and in other animals it ap- 

 pears to do so. It is probably always the case that at least the main piece of the 

 tail enters the yolk. The tail, as such, very soon disappears, while the head of 

 the spermatozoon enlarges, probably by the imbibition of fluid from the surrounding 

 yolk. The head of the spermatozoon is rich in chromatin, which forms a series 

 of irregular masses as the head enlarges, producing a network appearance, and is 

 thus converted into a nucleus-like body, the male pro-nucleus. At the same time 

 in some animals the growing head surrounds itself by a membrane. 



We now have a cell which contains two nucleus-like bodies, one derived from 

 the head of the spermatozoon and the other from the nucleus of the egg-cell. They 

 are termed respectively the male and female pro-nucleus. Each pro-nucleus, when 

 it first appears, is small and gradually enlarges, probably in both cases by the im- 

 bibition of fluid. The relative size of the two pro-nuclei varies considerably in 

 different species, and is probably a secondary and relatively unimportant relation. 

 The proportion between the two probably depends upon the time when the male 

 pro-nucleus is formed. If the spermatozoon enters early, while the female pro- 

 nucleus is forming, it may make a pro-nucleus as large as that from the egg- 

 cell. If, on the other hand, the spermatozoon enters late, the female pro-nucleus 

 enlarges, acquires a start, and the growing male pro-nucleus is, therefore, smaller. 



