37 



C. W. METZ. 



flies 1 have shown that the chromosomes regularly lie in clusters 

 during metaphase, and that each cluster consists of the three, four, 

 or more homologous members. 2 In prophase (Holt, '17; Metz, 

 '16, Figs. 109, no) the association of homologues is closer than 

 in metaphase. Here again, however, there is some doubt about 

 the exact nature of the association. Holt's observations on Cnlc.v 

 seem to lead to two different views on this question, without indi- 

 cating which is correct. According to one view sister chromo- 

 somes associate more closely than homologues that are not sisters. 

 Evidence cited on p. 612, for instance, " . . . suggests that a 

 parasynaptic union of sister chromosomes takes place in the telo- 

 phase. . . ." This is supported also by her Figs. 13, 14, and 15, 

 in which each original pair of chromosomes seems to be repre- 

 sented by two associated bodies, each of which is presumably made 

 up of four or more sister chromosomes. If this is the case, the 

 association between sister elements is closer than that between 

 homologous, non-sister elements. But the statement is also made 

 (p. 613) that "it is believed" that in multiple complexes the 

 chromosomes conjugate " in three groups of homologous individ- 

 uals " (three being the haploid number in Culc.r}. The latter 

 statement implies that there is no distinction between homologous 

 and sister chromosomes. 



In the observations of the writer Cl.c.) on FuccJlia no attempt 

 was made to analyze the details of the prophase association or to 

 discriminate between sister and non-sister homologues. 



The present paper is concerned with additional evidence on this 

 point. The data are taken from some exceptionally clear prophase 

 figures obtained in tetraploid ovarian cells of SarcopJiaga. 3 



The normal chromosome group of Sarcof>liaga consists of five 

 pairs of long chromosomes and one pair of short (sex) chromo- 

 somes. These are represented in Fig. i, taken from a spermato- 

 gonium. In the early prophase, when the association of homo- 



i Holt's observations were on Culex ; those of Metz were on Sarcophaga and 

 Fucellia ; those of Bridges were on Drosophila. 



- That the associating chromosomes are homologous cannot be doubted in 

 the light of present cytological and genetical knowledge. 



3 The species has not been identified in this case, but all of my Sarcophaga 

 material, including several identified species, shows essentially the same 

 (normal) chromosome group throughout. 



