1 88 PROBLEMS OF FERTILIZATION 



a) Echinoderms: We may consider the data in 

 taxonomic order: (i) species crosses, (2) genus, fam- 

 ily, and ordinal crosses, (3) class crosses, (4) phylum 

 crosses. 



I. Shearer, De Morgan, and Fuchs (1913) crossed 

 three species of Echinus: esculentus, acutus, and mili- 

 aris, in all possible combinations. The fertilization suc- 

 ceeded in all six combinations without any artificial aid, 

 by either increasing the usual concentration of the 

 sperm or changing the chemical composition of the 

 sea-water. Larvae were readily raised from all crosses, 

 but only the crosses E. esculentus ? X £. acutus 6 

 and E. miliaris ? X £. acutus male gave normal sea 

 urchins. 



The cytology of these crosses was studied by Don- 

 caster and Gray (1913) and by Gray (1913). The be- 

 havior of the germ nuclei was normal; but some elim- 

 ination of chromosomes from the first cleavage spindle 

 occurred in certain of the crosses. A curious fact was 

 that this might occur in one reciprocal of a cross but 

 not in the other. Thus in the cross esculentus ? X 

 acutus 6 the cytological events are perfectly normal; 

 but in acutus 9 X esculentus 6 there was an invariable 

 elimination of some chromosomes. In this process vesi- 

 cles formed on certain chromosomes, and often sepa- 

 rated from them and came to lie outside of the spindle; 

 other altered chromosomes were often carried entire to 

 one pole without dividing. It was not possible to deter- 

 mine what relation there might be between chromosome 

 elimination and the character of inheritance. 



Such elimination of chromosomes from hybrid zy- 

 gotes has often been referred vaguely to incompatibility; 



