A STUDY OF SOMATIC CHROMOSOMES. 34! 



CONCLUSIONS. 



1. The somatic number of chromosomes in Anasa tristis is 

 constant, with the one exception, namely, that double chromo- 

 some groups occur in certain investing cells of the ovary. 



2. There are two classes of embryos, one with a complex of 

 21 chromosomes, the other with a complex of 22 chromosomes in 

 all the cells of the various tissues of the body which have been 

 examined, with the exception noted. 



3. The chromosomes in the somatic cells are of the same 

 number and type as those in the germ cells. The 21 -chromosome 

 type corresponds to the spermatogonial complex, and the 22- 

 chromosome type to that of the oogonia. Accordingly, embryos 

 having 21 chromosomes in the somatic cells are males, those 

 with 22 chromosomes, females. 



4. The male somatic chromosome complex consists of 3 macro-, 

 16 meso-, and 2 w-chromosomes, which is the same as is found 

 in the spermatogonia. 



5. The female somatic chromosome complex consists of 4 

 macro-, 16 meso-, and 2 w-chromosomes. This corresponds to 

 the oogonial complex. 



6. These facts show that there is both an identity of number 

 and an identity of type in the chromosome complexes. The 

 identity of type is further emphasized by the fact that definite 

 pairs of chromosomes can be demonstrated in these groups. 



7. The double chromosome groups do not represent fragmenta- 

 tion or transverse splitting of the chromosomes, for the female 

 formula is doubled and this evidently has been brought about 

 by the failure of the cells in question to divide after a division of 

 the chromosomes had taken place. 



REVIEW OF LITERATURE. 



The following table is composed of the counts which have 

 been made of the chromosomes in somatic cells. In a great 

 many cases these counts have been incidental to a study of the 

 sex cells, and have served, in the main, merely to substantiate 

 the counts of the diploid number of chromosomes in the sex cells. 

 It is an unfortunate fact that the term "somatic number" is 

 frequently used for that of "diploid number" of the germ cells. 

 In a few cases it has been impossible to determine whether the 



