126 



CYTOLOGY 



CHAP. 



The paired arrangement of homologous chromosomes is most strik- 

 ingly shown in the Diptera (Metz, 1916), in some species of which homo- 

 logous chromosomes are very closely approximated. This culminates 

 in Culex, where they are often so closely applied to one another as to 

 be distinguished only with difficulty. Fig. 56 shows the chromosome 



FIG. 56. 



The chromosomes of Culex as figured by different workers, showing the close approximation of homologous 

 chromosomes. (A, B, from Stevens, J.E.Z., 1910 ; C, from Taylor, Q.J.M.S., 1915 ; D, E, from Whiting, 

 J.M., 1917 ; F, G., from Hance, J.M., 1917 ; H, I, J, from Metz, J.E.Z., 1916.) A, spermatogonial pro- 

 phase ; only one of the three chromatin threads is visibly double ; B, spermatogonial metaphase, three pairs of 

 separate chromosomes ; C, somatic prophase ; D, spermatogonial prophase ; E, spermatogonial anaphase ; 

 F, ovarian (diploid) prophase ; G, spermatogonial metaphase ; H, I, J, prophase, metaphase, and anaphase 

 of somatic mitoses. 



complex of this mosquito (where 2n = 6) as depicted by various workers. 

 As will be seen, the homologous chromosomes in prophase (Fig. 56, 

 A, C, D, F, H) are generally intimately applied to or twisted round one 

 another, quite as closely as are the daughter halves of split prophase 

 chromosomes in the somatic mitoses of many other forms (Figs. 3, 8). 



