Hyhridizalion of Echinoids. 29 



sperm nucleus are involved in division, and that of a sperm carrying 

 but one heterochromosome. The expected number of chromosomes 

 in this case would thus be 19 + 18 = 37; these have all split in division, 

 which would give a total of 74. The number shown in the figure is 80, 

 of which some are evidently fragments. One of the V's has been 

 cut in sectioning. I am unable to designate its parts. 



Chromosomes in Cidaris Eggs Cross-Activated by Tripneustes Sperm. 



Cidaris 9 X Tripneustes cf . 

 Figures 18 to 25. 



In general, the division figures of this cross are more regular than 

 those of the Cidaris-Lytechinus cross. There are some lagging 

 chromosomes, but these do not form so striking a feature as in the 

 preceding cross. Some of the sections show the heterochromosomes 

 especially well. In figure 18, for example, 3 V-shaped and 1 hook- 

 like chromosome are present; in figure 19, 3 V-shaped chromosomes 



tii%\\\ il\ M'^h . i\ 



I 



I 



\ 



ct b 





/ ' ^- • 15 



5/ 



Fig. 14, a and b, Cidaris 9 X Lylcch- 



imi-sd^. Three heterochromo- Fig. 15, a and b, Cidaris 9 X Lytechinusd'. The V- 

 somes. Lagging chromo- shaped form of the heterotypic chromo- 

 somes at center, 27?-35? somes shows clearly 29-28. 



were present in the egg, but no hook-shaped chromosome was present. 

 These two illustrations represent the two classes of eggs that were 

 present in these cultures. ^ In figures 20, b and c, 3 V's; in figures 

 21, a and 6, 3 V's and a hook; in figures 22, a and b, 3 V's; in figures 



23, a and 6, 3 V's and a hook. The sections from which figures 



24, a and b, were drawn are of more than usual interest. Most of the 

 chromosomes appearing in b are longitudinal sections of the chro- 

 mosomes shown in a, as indicated both by position and width of the 

 chromosomes. The third section of this spindle (not shown in the 

 illustrations) contains a much involved mass of chromosomes, v/hich 



