CHROMOSOMES IN ECHINOID EGGS 21 



We may get at the matter before us most quickly by seeking 

 to ascertain whether in either or both of the Toxopneustes X 

 Hipponoe crosses there is an ehmination of chromosomes. 



I am not able at this time to make a final statement as to the 

 exact number of chromosomes in either Toxopneustes or Hipponoe. 

 In Toxopneustes it is either 36, 37 or 38. In Hipponoe 32, 33 or 

 34. This is sufficiently near to enable us to determine a noticeable 

 elimination. In my illustrations the numbers found (and prepara- 

 tions were selected in which I believe that I was able to count all 

 of the chromosomes that were present) were fig. 1, upper 34; 

 lower 34; fig. 2, 30 and 31 ; fig. 3, 34 and 32; fig. 4, 30 and 31 ; fig. 5, 

 33 and 33; fig. 7, 33 and 34; fig. 8, 38 (?) and 35 (?). 



Similar counts hold for older segmentation stages up to the 32 

 to 64 cell stages, so that we have in this cross no chromosome 

 elimination. The Hipponoe chromosomes are retained and we 

 have in correlation Hipponoe dominance. 



With the reciprocal cross, Hipponoe 9 X Toxopneustes cf the 

 counts range; fig. 12, 22 and 29; fig. 13, 30 and 36; fig. 14, 28 and 

 24; fig. 15, 27 and 24; fig. 18, 31 and 32 (those at center not 

 counted); fig. 17, 30 and 25 (anaphase four cell stage, one at 

 center not counted); fig. 20, sixteen cell stage, 16 and 16 (two 

 at center not counted). 



It will be seen from these examples that in this cross the 

 behavior of the chromosomes is irregular from the beginning and 

 that in some instances there is an elimination of fully half of the 

 chromosomes, presumably Toxopneustes chromosomes, by the 

 time the sixteen cell stage has been reached. 



The facts regarding the chromosomes and the correlation with 

 dominance in these crosses may be stated in general terms, 

 Toxopneustes 9 X Hipponoe d" 



Without chromosome elimination, Hipponoe dominance. 

 Hipponoe 9 X Toxopneustes cf 



With chromosome elimination, Hipponoe dominance. 



In the Hipponoe 9 X Toxopneustes d" cross there is a con- 

 stant elimination of chromosomes up to the sixteen and immedi- 

 ately succeeding cell stages until only (?) Hipponoe chromosomes 

 remain. This elimination is brought about by lagging and failure 



