3U 



Hijbridizalion of Ecliinoids. 



obviously could not have divided normally. The edge of this mass 

 of chromosomes lies between the anaphase plates in figure 246, 



Here again the morphology of the groups of chromosomes confirms 

 the expectation based on our knowledge of the chromosomes in 

 Tri-pneustes (Hipponoe). From the studies of Tennent (1911a) 

 and Pinney (1911) we know that the dimorphism in straight-fertilized 

 eggs of Tripneustes is due to dimorphism of the groups of chromosomes 

 in Tripneustes sperms, the sperms of this species introducing either 

 2 V's or 2 V's and a hook into the egg. 



Fig. 16, a, h, and c, Cidaris 9 X Lytechinus cf . Three successive sections of a tetrapolar 

 spindle 80 chromosomes, inclusive of fragments. X 2,400. 



The expectation as to number of chromosomes in the cross-activated 

 eggs is 19-fl6 = 35, or 19 -|- 17 = 36. The realization as to the number 

 of chromosomes in the sections drawn is: Figures 20, a, b, and c, 

 38-35; 21, a and b, 25-34; 22, a and b, 29-28; 23, a and b, 22-26. 

 It is evident that success in division of the chromosomes in the 

 cross-activated egg is relative. Figures 20, a, b, and c, show a normal 

 division. It is evident that at least two of the rods in the upper 

 anaphase plate are fragments of chromosomes; probably a third is 

 also a fragment. With the rejection of three, the count would be 

 35-35, the expected number following fertilization by the sperm 

 carrying 2 V's only. In figure 21, the plane of section is slightly 

 oblique to the long axis of the spindle. I do not feel confident that 

 I have been able to see all of the chromosomes which are massed 



