48 EVOLUTION AND GENETICS 



factor. It has been found both hi plants (tobacco) 

 and in one animal (Ciona) that the failure to self- 

 fertilize is not due to incompatibility of any sort be- 

 tween the egg and the sperm, but to a physiological 

 block to the penetration of the sperm into the egg. It 

 has also been shown in the case of several marine 

 animals (sea urchins and fishes) that the eggs may 

 be entered by spermatozoa of widely separated spe- 

 cies — belonging even to different families — and 

 start development. The failure to produce a normal 

 embryo is due in some cases to the failure of the 

 sperm to develop normally in the foreign egg; in 

 other cases to the failure of the chromosomes derived 

 from the two sources to become normallv distributed 

 in the cleavages of the egg, and in still other cases 

 to the inability of the introduced chromosomes to 

 function in the cytoplasm of the foreign egg. 



All this is satisfactory and carries us a step fur- 

 ther in an understanding of the problem of the infer- 

 tility between species. We may add a further con- 

 sideration in line with what genetics and embryology 

 lead us to exjiect, namely, that the genetic factors 

 present in the chromosomes of the fertilized egg 

 derived in part from the egg, in part from the sperm, 

 are acting on the cytoj)lasm throughout the process 

 of development. So long as these jDairs of factors are 

 alike or identical (one maternally derived, one pater- 

 nally) the course of development runs smoothly, 

 but if one member of the pair acts in a different way 



