A STUDY OF SOMATIC CHROMOSOMES. 357 



in the germ cells by compound chromosomes, it is highly probable 

 that the number of the chromosomes will be found to be the same 

 in all of the somatic tissues. In this type of somatic chromosome 

 behavior we find two distinct cases, one where, as in Phragma- 

 tobia, there is only a splitting up of one or at the most a few 

 chromosomes, and the other where all of the chromosomes in the 

 germ cells are apparently compound, and each breaks up into 

 its component parts in the somatic cells (Hymenoptera, Nema- 

 toda, and perhaps some of the vertebrates). 



The synapsis of homologous pairs of chromosomes in somatic 

 cells was described by Miss Stevens ('08, '10) in the case of a 

 number of species of the Diptera. This synapsis apparently 

 took place in the anaphase or telophase and lasted until the 

 metaphase of the next division. Metz ('14, p. 55-56) confirms 

 this: 'The Drosophilas offer some of the most striking evidences 

 of the actual pairing of chromosomes in the diploid groups, thus 

 far observed. The phenomenon is apparently characteristic of 

 all Diptera, but is nowhere so striking as in this genus. . . . But 

 the most remarkable feature of the whole study is the discovery 

 that the chromosomes not only exhibit a close association in 

 pairs at nearly all times, but that before every cell division the 

 members of each pair become so intimately united that they may 

 be said actually to conjugate. Each pair, with the possible 

 exception of the sex-chromosomes, goes through what amounts 

 to a synapsis in every cell generation, so that in many cases the 

 figures closely resemble those of the haploid groups. Apparently 

 this takes place in every prophase, but a second conjugation may 

 occur during metaphase, just a short time before division." 

 This latter statement he qualifies in his later paper saying that 

 it is "of only occasional occurrence and is not a uniform stage in 

 chromosomal activities" ('16, p. 230). In this paper he presents 

 the results of his study of this chromosome pairing in about 80 

 species of Diptera, where he has found it to be of uniform occur- 

 rence throughout the developmental history of the individual. 

 He concludes that this paired arrangement is "selective to the 

 highest degree," that the members of the pair represent, one a 

 maternal and one a paternal chromosome, and that this pairing 

 shows that the chromosomes (forming a pair) are qualitatively, 



