The Germ Cells of Diptera 361 



(Fig. I, /zi and h.^. Apparently a side-to-side pairing or conju- 

 gation of homologous chromosomes, with the possible exception 

 of the unequal pair, occurs prehminary to each spermatogonial 

 mitosis. The twelve chromosomes separate, and each divides 

 longitudinally in metakinesis. Whether they pair again in the 

 telophase or not until the prophase of another cell-division is not 

 evident. 



The heterochromosomes remain condensed and are found side 

 by side during the whole growth stage, while the other chromo- 

 somes pass into a more or less diffuse condition (Fig. 2). In the 

 prophase of the first spermatocyte mitosis there are five thick 

 V-shaped chromosomes and a pear-shaped mass of chromatin 

 which in metakinesis proves to be the unequal pair of hetero- 

 chromosomes (Fig. 3). The V-shaped chromosomes all divide 

 longitudinally and the larger and smaller heterochromosomes 

 separate as seen in Fig. 4. In the interval between the first and 

 second divisions a nuclear membrane forms, but the chromosomes 

 do not change greatly. Figs. 5 and 6 show the two kinds of 

 daughter nuclei, one containing the larger, the other the smaller 

 heterochromosome. In the second spermatocyte mitosis the V- 

 shaped chromosomes again divide longitudinally and the hetero- 

 chromosomes divide as shown in Figs. 7 and 8, so that in all stages 

 they are clearly distinguishable from the ordinary chromosomes. 

 The resulting spermatozoa fall into two equal classes, dimorphic 

 as to the heterochromosomes, as in similar cases among the Hem- 

 iptera and Coleoptera. In most of the flies studied there was no 

 difficulty in finding oogonia in which the number and relative 

 size of the chromosomes could be determined. Only one such 

 was found in Musca, that shown in Fig. 9. Here a part of the 

 chromosomes are still paired; others have separated, but the 

 members of each pair of ordinary chromosomes are not far apart; 

 while the two equal heterochromosomes are on opposite sides of 

 the group {h). Here again we have what may be regarded as a 

 partial synapsis of homologous chromosomes. The relation of 

 the heterochromosomes in the two sexes is the same as in many of 

 the Coleoptera (Stevens '05 and '06) and the Hemiptera heterop- 

 tera (Wilson '05 and '06), an unequal pair (large and small) in 



