ASSOCIATION OF CHROMOSOMES IN DIPTERA. 371 



logues is intimate, the six pairs resemble six single chromosomes, 

 as shown in Fig. 2 (ovarian cell). The conditions found in 

 tetraploid cells are represented in Figs. 3 to 6. These cells were 

 found scattered about in the somatic tissue of the same ovary from 

 which Fig. 2 is taken. The tetraploid cells are so large that the 





EXPLANATION OF FIGURES.* 



" I am indebted to Miss Ruth Lincks for making the drawings for the 

 figures. 



All figures were drawn to the same scale, with the aid of a camera lucida. 

 Kumbers i and 2 are from diploid cells ; numbers 3 to 6 are from tetraploid 

 cells ; numbers 2 to 6 are from the same ovary. 



FIG. i. Typical metaphase (from a spermatogonium) showing the chromo- 

 somes loosely associated in pairs. The smallest pair is the sex chromosome 

 pair'(XY). 



FIG. 2. Typical prophase (ovarian cell) showing the close association of 

 homologous chromosomes. 



FIG. 3. Tetraploid chromosome group in metaphase showing loose associa- 

 tion in fours instead of twos. The four smallest elements are the sex chromo- 

 somes (XXXX). 



FIG. 4. Late prophase of a tetraploid nucleus showing close association in 

 fours. 



FIGS. 5 AND 6. Early prophases of tetraploid cells showing intimate associa- 

 tion of homologous chromosomes. 



nucleus and some of the chromosomes are usually cut in sectioning ; 

 but a few nuclei have been found entire, or nearly so. One of 

 these, a metaphase, is represented in Fig. 3. This figure brings 

 out the loose association of chromosomes, characteristic of the 



