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THE AMERICAN NATURALIST [Yol.XLVI 



found in one-half of the straight fertilized Hipponoe 

 eggs ; it is found in half of the Toxopneustes eggs which 

 have been fertilized by Hipponoe speim, and it never 

 occurs in Hipponoe eggs fertilized by Toxopneustes 

 sperm. 



There is then absolutely no appeal from the fact that 

 in Hipponoe the spermatozoa are of two classes, one 

 with, the other without, this idiochromosome. 



The second case that I shall submit is that of Lytechi- 

 nus, better known to us as Toxopneustes. Here I am 

 able to give evidence gained by a study of straight fertil- 

 ized eggs, artificially parthenogenetic eggs, fertilized 

 enucleated egg fragments, and four crosses, the recip- 

 rocal Hipponoe X Toxopneustes and Arbacia X Toxop- 

 neustes crosses. 



The study of straight fertilized Toxopneustes eggs 

 showed two classes of zygotes, one with two V-shaped 

 chromosomes, the other with three. This seemed in 

 agreement with conditions in Echinus, although obviously 

 there was no chance of tracing the source of the hetero- 

 chromosome in such material. 



Those who have devoted any attention to the chromo- 

 somes in Echinoids know the difficulties involved in such 

 a study. The chromosomes are all small; most of them 

 have the form of short, straight or slightly bent rods, 

 while but three or four, at most, may be distinguished by 

 peculiarity of form. Difficulties attend the study of even 

 these chromosomes. The V's in Toxopneustes, for ex- 

 ample, best seen in the anaphases of division, may have 

 both arms brought into contact and so resemble a single, 

 somewhat thickened rod. 



Since this is true, it should be easier to determine size 

 differences when only the haploid number is present than 

 when the full number of chromosomes is in the division 

 figure. 



So far as I am aware, none of the cytological work on 

 artificial parthenogenesis in Echinoids has been done 

 with the idea of an individuality of form of chromosomes 



