28 



HybridizaUun of Echinoids. 



The evidence here confirms my conclusions reached in 1912, namely, 

 that in Lytechinus (Toxopneustes) the fertilized eggs contained either 

 3 V's or 4 V's, and that this variation in number was due to the fact 

 that the sperms were dimorphic with respect to these chromosomes, 

 carrying either 1 or 2 V's into the egg. 



Figs, 12 to 15 are of anaphase plates from Cidaris eggs activated with Lytechinus 

 sperm. They were drawn and enlarged in the manner described for figures 6 to 11. 

 They have been reduced one-half in reproduction, I'he magnification shown is of 

 about 2,400 diameters. 



/ '»i> , I 



) 



ui<V)|»./ ?~HV' 



1. 



a 



12 



m^^ 



• '/ 





\V /(//,! I// N 



/ 



a 



13 



i 



Fig. 12, a and b, Cidaris 9 X Lytechinus c^ . Two heterochromosomes, 33-35. 

 Fig. 13. a and b, Cidaris 9 X Lytechinus (f . Three heterochromosomes. Much lagging; 

 287-26?. 



The fact that two heterotypic chromosomes in figure 156 lie 

 among the lagging chromosomes, and the knowledge that two of 

 these elements were brought into the egg by the Lytechinus sperm, 

 make it seem probable that the lagging chromosomes are those of 

 Lytechinus. The same type of evidence is given by the sections of 

 the egg of which figure 17 is one. In the half of the spindle shown in 

 figure 17 division is very regular and a V-shaped chromosome may be 

 seen clearly in each plate. In the next section of the egg (not shown 

 in the illustrations), two pairs of lagging V's lying among a very 

 irregular and much twisted mass of autosomes may be seen, the 

 evidence again lending support to the conclusion that the lagging 

 chromosomes are those of Lytechinus. 



The number of chromosomes in these plates varies greatly. In 

 figures 12, a and h, the number is 33-35; in figure 13, 28?-26?; in figure 

 14, 27?-35?; in figure 15, 29-28. The expected number is 38-38 or 

 37-37. Figures 16, a, h, and c, are of a tetrapolar spindle in anaphase. 

 While spindles of this type are usually associated with double fertili- 

 zation, it is evident in this example that the chromosomes of but one 



