52 Chemical Basis of Genus and Species 



In exceptional cases circulation may be established 

 and in these a normal embryo may result, but such an 

 embryo is chiefly maternal. 



This incompatibility of two gametes from different 

 species does not show itself in the case of heterogeneous 

 hybridization only, but also though less often in the 

 case of crossing between two more closely related 

 forms. The cross between the two related forms S. 

 purpuratus 9 and S. franciscanus cf is very sturdy and 

 shows no abnormal mortality as far as the writer's ob- 

 servations go. If, however, the reciprocal crossing is 

 carried out, namely that of S. franciscanus 9 and S. 

 purpuratus cf, the development is at first normal, but 

 beginning with the time of mesenchyme formation 

 the majority of larvae become sickly and die ; and again 

 the question may be raised whether or not the begin- 

 ning of sickliness coincides with the development of 

 mesenchyme cells. If we assume that the sickliness 

 and death are due to the formation of a poison, we 

 must assume that the poison is formed by the proto- 

 plasm of the egg, since otherwise we could not under- 

 stand why the reciprocal cross should be healthy. 



All of these data agree in this one point, that the 

 fusion by grafting or fertilization of two distant species 

 is impossible, although the mechanism of the incompat- 

 ibility is not yet understood. It is quite possible that 

 this mechanism is not the same in all the cases men- 

 tioned here, and that it may be different when two 



