A STUDY OF SOMATIC CHROMOSOMES. 333 



maturation division, and immediately separating without fusion. 

 The .T-chromosome divides equationally in the first maturation, 

 but in the second division passes undivided to one of the poles 

 of the spindle. In this way a dimorphism of the spermatids is 

 established, and later, a dimorphism of the spermatozoa, with 

 respect to their chromatin content, one half containing 10, and 

 the other half 1 1 chromosomes. 



The oogonia on the other hand contain 22 chromosomes, four 

 larger than the others, and two very small w-chromosomes. In 

 the metaphase of the first oocyte division (Morrill, '10) there 

 are eleven chromosomes, many of them appearing as tetrads. 

 Two of the chromosomes are larger than the others, the w-chro- 

 mosomes are fused and appear as a single tetrad, and all of the 

 chromosomes divide equally. In the metaphase of the second 

 maturation division there are n dyads, two larger and one (the 

 w-chromosome) smaller than the others. 



To express, then, the possible combinations of the chromo- 

 somes at fertilization, we have the following formulae, letting x 

 stand for the .r-chromosome, M for the macrochromosomes (the 

 large chromosomes), meso for the mesochromosomes 1 (those 

 intermediate in size), and m for the w-chromosome : 



1 . cf gamete (x -f- M + 8 meso + m) + 9 gamete (x + M 

 + 8 meso -f- m) = 9 embryo (2x + 2M + 16 meso + 2m}. 



2. cf gamete (M + 8 meso + w) + 9 gamete (x + M + 8 

 meso + m) = cf embryo (x -f 2M -f 16 meso + 2m). 



The results summarized in these two equations were deter- 

 mined by Wilson and Morrill, and are confirmed by my investi- 

 gations. 



Wilson ('06) figures a group of chromosomes from a dividing 

 follicle cell of the ovary. There are 22 chromosomes, corre- 

 sponding in their size relations to the anticipated result mentioned 

 above. Although he gives no figures, he states that the cells 

 in the ectoderm of the larvae contain approximately the same 

 number of chromosomes as do the oogonia. He also figures a 

 plate from a cell toward the periphery of a larval ovary in which 



1 It has become necessary to find some word by which chromosomes not char- 

 acterized by their size or behavior can be designated. The term "autosome" is 

 too comprehensive. Accordingly I have used the term mesochromosome, the prefix 

 being derived from the Greek adjective juecros meaning intermediate in size. 



