A STUDY OF SOMATIC CHROMOSOMES. 169 



as a tetrad. They are clearly of different sizes, though the same 

 size differences, such as appear in the spermatogonia, can not 

 be observed. 



Accordingly, two classes of embryos would be expected as the 

 result of the fertilization, one containing 21, and the other 22 

 chromosomes in the embryonic cells. These differences have 

 been observed and are shown in Figs. 9-11. 



Figs. 9 and 10 are of the 21 chromosome type and are from the 

 same embryo, corresponding in development to the stage of 3 

 of Epilachna. Fig. 1 1 is from an ectoderm cell of a late blasto- 

 derm stage of the 22 chromosome type. In all these figures one 

 pair of chromosomes is much larger than any of the others. With 

 the exception of the ninth chromosome in Figs. 9 and 10, all the 

 other chromosomes form a paired series graded in size. In Fig. 

 ii there is no unpaired chromosome. 



CONCLUSIONS. 



1. The developing eggs and embryos of Epilachna borealis 

 fall into two classes in respect to their chromatin content, one 

 containing an xy combination, and the other an xx combination 

 of sex chromosomes in all the cells of the body which have been 

 examined, the former corresponding to the chromosomes of the 

 spermatogonia, the latter probably to those of the oogonia. 

 The same size relations are maintained in all the chromosome 

 groups of one type. 



2. Two types of chromosome groups are found in embryos of 

 Diabrotica vittata, one with 21 chromosomes, corresponding to 

 the spermatogonial chromosomes and one with 22, presumably 

 homologous with those of the oogonia. In each of these types 

 the chromosomes do not vary in number and form in different 

 tissue cells which have been studied. 



BIBLIOGRAPHY. 

 Hoy, W. E., Jr. 



'14 A Preliminary Account of the Chromosomes in the Embryos of Anasa 



tristis and Diabrotica vittata. BIOL. BULL., Vol. 27, No. i. 

 '16 A Study of Somatic Chromosomes, I. BIOL. BULL., Vol. 31, No. 5. 

 Stevens, N. M. 



'06 Studies in Spermatogenesis. Pub. Carn. Inst. Wash., 36. 

 '08 The Chromosomes in Diabrotica vittata, Diabrotica soror and Diabrotica 

 i2-punctata. Journ. Exp. Zool., Vol. 5, No. 4. 



