A STUDY OF SOMATIC CHROMOSOMES. 



II. THE CHROMOSOMES IN EMBRYOS OF Epilachna borealis AND 



Diabrotica vittata. 



W. E. HOY, JR., 

 PRINCETON UNIVERSITY AND THE UNIVERSITY OF ROCHESTER. 



Epilachna borealis. 



The oogenesis of Epilachna has not been described. Miss 

 Stevens ('06) briefly described the spermatogenesis of this beetle. 

 There are 18 chromosomes in the spermatogonia. One of these 

 chromosomes, the y, is extremely small, its mate, the x, is larger. 

 In each plate there are 2 chromosomes noticeably larger than the 

 others. In the first maturation division the ^-chromosome passes 

 undivided to one pole of the spindle, while the y passes to the 

 other pole. The spermatozoa, then, differ in that one half 

 contain a small y, and the other half a larger x. Each contains 

 one large macrochromosome (autosome). 



Although the oogonial divisions and the maturation of the 

 egg have not been worked out, the anticipated result of fertiliza- 

 tion would be two sorts of embryos with respect to their chromo- 

 some content, one with two x's, and one with an xy combination. 

 This expectation is fulfilled in the divisions of the embryonic 

 cells, as I shall show. The stages of development studied were: 

 (i) Late cleavage, and (2) shortening of the elongate embryo. 

 In all about 60 plates have been recorded. 



Figs. 1-4 are from embryos having the xy combination in their 

 chromosome complexes, these being the smallest chromosomes 

 in the plate. Fig. I is a late cleavage division figure and shows 

 the same characteristic elongation of the chromosomes as was 

 seen in the case of Anasa at a similar stage of development 

 (Hoy, '16). Figs. 2 and 3 are from cerebral neuroblasts, and 

 are both from the same individual. Fig. 4 is from an ectoderm 

 cell in the thoracic region. It will be seen that in all these 

 figures there are two macrochromosomes comparable to those 



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