No. 542] CHROMOSOMES IN ECHINOID LARVAE 



the normally fertilized egg, but we may study the chromo- 

 somes of the egg itself, i. e., the maternal chromosomes, 

 in artificially parthenogenetic eggs; we may study the 

 chromosomes of the spermatozoan, i. e., the paternal 

 chromosomes, in fertilized enucleated egg fragments; 

 and we may study the chromosomes in cross-fertilized 

 eggs, identifying here those which have come from one 

 parent or the other. 



The first subject of which I shall speak is the correla- 

 tion of certain chromosomes with sex. 



The conclusions reached by Baltzer, from a study of 

 Echinus and Strong yloccnt rot us, are known to most of 

 those present. They find their readiest expression in the 

 statement that "in Echinoids the female is digametic 

 while the male is homogametic." I have expressed else- 

 where the idea that this generalization is too broad and 

 that the statement should be limited to the cases for 

 which the condition was described; but I must do more 

 than this. I can admit that apparent condition for but 

 one of Baltzer 's cases, that of Echinus, for his own illus- 

 trations for Strongylocentrotus indicate that this form 

 is in agreement with another interpretation, and not with 

 the one given. 



In the forms which I have studied, the male is digametic, 

 the female homogametic, as in the insects. 



The first case that I shall submit is that of Tripneustes 

 esculentus (Hipponoe), in which the evidence is espe- 

 cially clear, although not submitted to the ultimate anal- 

 ysis that has been given to the second case. 



In Hipponoe, I have been able to show, by the study of 

 cross-fertilized eggs, made in connection with straight 

 fertilized Toxopneustes and Hipponoe eggs by two of my 

 students, Miss Heffner and Miss Pinney, that a chromo- 

 some of peculiar form, namely hook-shaped, is trans- 

 mitted by half of the Hipponoe spermatozoa, and that the 

 eggs which receive it must become males. 



The evidence here is clear. Such an element never 

 occurs in straight-fertilized Toxopneustes eggs. It is 



