THE VITAL PROPERTIES OF THE CELL 301 



sexual cells are able to move, they precipitate themselves upon 

 each other ; if however one cell, as ovum, has become fixed, the 

 reciprocal attraction is evinced by the movements of the sperma- 

 tozoon. But sexual affinity continues to operate even after the 

 two cells have fused, being seen in the attraction which the eers* 

 and sperm-nuclei, with their centrosomes, exercise upon each other, 

 the result of which is, that they come into contact and coalesce as 

 described above. 



Thus two points remain to be proved in this section : firstly, 

 that reciprocal influences between cells requiring fertilisation 

 really do exist ; these we will designate by the name of sexual 

 affinity ; and secondly, that this affinity is only evinced between 

 cells of a definite kind ; and this suggests the question as to what 

 are the special attributes which these cells requiring fertilisation 

 must possess. 



a. Sexual Affinity in General. That sexual cells at a 

 certain distance from one another exert upon one another a 

 definite influence may be concluded from numerous observations, 

 made by reliable observers. I will confine myself to a few especi- 

 ally instructive examples, which have been described by Falken- 

 berg, de Bary, Engelmann, Juranyi and Fol. 



Falkenberg (VII. 10) investigated the process of fertilisation in 

 a low species of Alga, Cutleria* To the receptive ova of Cutleria 

 adspersa which have come to rest, he added actively motile 

 spermatozoids of the nearly allied species Cutleria multifield; these 

 two species can only be distinguished from one another by small 

 external differences. " In this case the spermatozoids, as seen 

 under the microscope, wandered aimlessly about, and finally died, 

 without having fertilised the ova of the allied species of Alga. 

 It is true, that individual spermatozoids, which by chance came 

 into contact with the quiescent ova, remained attached to them 

 for a few moments, but they soon detached themselves again. 

 However, a very different result was obtained as soon as a single 

 fertilisable ovum of the same species was introduced into the 

 vessel containing the spermatozoids. After a few moments, all 

 the spermatozoids from all sides had gathered around this ovum, 

 even when the latter was several centimetres distant from the 

 place at which they were chiefly collected." In doing this they 

 even overcame the attractive force exerted by the rays of light 

 falling upon them, and moved in a direction opposed to the one 

 which they would otherwise have chosen. 



