48 Chemical Basis of Genus and Species 



really happened was that the shoots had a growing 

 point whose cells on the one side consisted of cells of 

 nightshade, on the other side of tomato. ^ We know of 

 no case in which the cell of a graft has lost its specificity 

 and undergone a transformation into the cell of the host. 

 3. Another manifestation of the incompatibility 

 of distant species is found in the domain of fertiliza- 

 tion. The eggs of the majority of animals cannot 

 develop unless a spermatozoon enters. The entrance 

 of a spermatozoon into an egg seems also to fall under 

 the law of specificity, inasmuch as in general only the 

 sperm of the same or a closely related species is able 

 to enter the egg. The writer^ has found, however, that 

 it is possible to overcome the limitation of specificity 

 in certain cases by physicochemical means, and by the 

 knowledge of these means we may perhaps one day be 

 able to more closely define the mechanism of specificity 

 in this case. He found that the eggs of a certain Cali- 

 fornian sea urchin, which cannot be fertilized by the 

 sperm of starfish in normal sea water, will lose their 

 specificity towards this type of foreign sperm if the 

 sea water is rendered a little more alkaline, or if a little 

 more Ca is added to the sea water, or if both these 

 variations are effected. Godlewski has confirmed the 

 efficiency of this method for the fertilization of sea- 

 urchin eggs with the sperm of crinoids. 



^ Baur, E., Einfiihrung in die experimentelle Vererhungslehre, Berlin, 

 1911, p. 232. 



^ Literature on this subject in Chapter IV. 



