24 Hybridizalion of EcJiinoids. 



From all the evidence obtainable I have concluded that the number 

 of chromosomes present in these eggs is 37 and 38. This difference is 

 due to the fact that half of the eggs contain a single V-shaped chro- 

 mosome, this shape being due to the atelomitic attachment of spindle 

 fibers, and the remaining half a pair of these V-shaped elements. 



'^,//'0 ^''^^"IWS^^ 



I..) 



(I M 



M'/J/' i^Uiinl |V^' 



h 

 a. 



h 



K't'f// 



c 



Fig. G, a, h, and c. Three successive sections of a first-cleavage am- 

 phiaster, 35-41. Two heterochromosomes. 



Figs. 6 to 11 are of anaphase plates from Cidaris X Cidaris eggs. The figures 

 were drawai with the aid of a Zeiss compensating ocular 12 and 2 mm. oil-immer- 

 sion objective. The camera sketches were enlarged 2 diameters and then compared 

 with the sections and finished. These enlarged drawings have been reduced 

 one-half in reproduction, so that the magnification of the chromosomes in the 

 figures described is about 2,400 diameters. 



mv%\ n 



7 



Fig. 7, a and h. Plane of section was slightly oblique to long axis of 

 spindle. Two sections have l)een combined in 6. On the shde, 

 the upper part of h appears in one section, then o as drawn, 

 then the lower part of h. Two heterochromosomes, 33-34. 



In most cases the arms of the V are brought so closely together, during 

 their mo"\'ement toward the poles of the spindles, that the chromo- 

 some has the appearance of a rod of twice the thickness of the remain- 

 ing chromosomes. In one anaphase (fig. 11a) there is a single long 

 chromosome; no V is present. 



