24 Hyhridizulion of Echinolds. 



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. 



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6 



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c 



Fig. 6, a, b, 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. Tlie figures 

 were drawn 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 

 \Aith 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. 



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a 



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

 spindle. Two sections have been combined in 6. On the slide, 

 the upper part of h appears in one section, then a 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 movement 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. 



