A STUDY OF SOMATIC CHROMOSOMES. 355 



Farmer and Moore do not hold strictly to the number of 32 

 chromosomes in the cells of the cockroach. Cells were found 

 which differed by one or several chromosomes. 



Variation in the number of chromosomes in Metapodius affects 

 only one class of chromosomes, the "supernumeraries." Wilson 

 considers the 22-chromosome type as the typical. This type 

 possesses an unequal pair of idiochromosomes. The 21 -chromo- 

 some type is derived by the disappearance of the small idiochro- 

 mosome. The numbers above 22 are due to the presence of one 

 to six supernumeraries. This variation of the chromosome 

 number is chiefly between different individuals, for owing to the 

 irregular distribution of these supernumeraries in the maturation 

 divisions of the sperm cells, the number each sperm receives is 

 variable. Further variations may occur in individual cells. 



The count of 19 in Peripatus may be due, according to Mont- 

 gomery, to the fact that not all of the chromosomes were con- 

 tained in one section. This may also answer for the count of 26. 

 The count of 34 was made from two sections, and he suggests 

 that it is possible that parts of some chromosomes were in both 

 sections. In this way some of the chromosomes were counted 

 twice. 



Krimmel finds that in the somatic tissues of Diaptomus the 

 number of the chromosomes varies between the reduced and the 

 diploid number. This is brought about by fusion of some or of 

 all of the chromosomes into tetrads, or the chromosomes may 

 appear bi-partite. "Man wird hier wie bei den generativen 

 Zellen daran denken diirfen, dass in diesen Fallen die Einknick- 

 nung oder scheinbare Querkerbe der in der Reifungsperiode 

 auftretenden Querkerbe entspricht, dass also das Zeichen der 

 synthetischen Aneinanderfiigung zweier Chromosomen noch nicht 

 verschwunden ware" (Krimmel, '10, p. 790). 



Gerould says: "In Phascolosoma mdgare I have uniformly 

 found 10 chromosomes in the late cleavage and in the gastrula 

 . . . each of which I regard as bivalent" (footnote, p. 87). 



In addition to the cells containing four chromosomes in 

 Ophryotrocha, Korschelt found eight chromosomes in some of the 

 cells of the blastula. He believes that this number is due to a 

 transverse splitting of the four chromosomes. Gregoire and 



