CHROMOSOMES OF MOSQUITO 527 



Upon the disappearance of the nucleolus the separate chromo- 

 somes become apparent as three pairs of bent threads. The 

 nuclear membrane now dissolves and the chromosomes appear 

 as in figures 7, 8, and 9. Two of the pairs are very much longer 

 than the third, but this difference is not always well seen in the 

 figures on account of foreshortening due to twisting up or down. 

 Figure 9 is a slightly later stage than the others and the chromo- 

 somes have shortened and thickened. Figure 10 is a very late 

 prophase. 



2. Metaphase, stages a and b. Figures 11, 12, 13, and 14, 

 represent a very characteristic arrangement of the chromosomes 

 in a flat plate with a somewhat radially symmetrical appearance. 

 This stage may be called stage A of the metaphase. The chromo- 

 somes do not begin their division at the end of this stage but 

 first assume a parallel arrangement, which may be called stage 

 B. In stage A the chromosomes tend to be constricted in the 

 middle and swollen at the ends. The homologues are usually 

 approximated and their middles are nearer to each other than 

 their ends. Since the chromosomes are in an approximately 

 flat plate we may speak of a polar or an equatorial view of the 

 same by analogy with the metaphase. Equatorial views are 

 not favorable for drawing, but the chromosomes may be studied 

 and counted by focusing. 



In the figures of stage A a short line is drawn to represent the 

 polar axis. The number at one end of the line denotes in degrees 

 of the circumference of a circle the slope of the polar axis to the 

 plane of the drawing. The direction of the polar axis has been 

 estimated by focusing upon the metaphase plate. 



The smaller pair of chromosomes is readily distinguishable 

 from the larger pairs in metaphase and anaphase, as it is also 

 in late prophase. There is considerable variation in the size of 

 the chromosomes from different cells, probably depending upon 

 nutritive conditions or previous rapidity of cell divisions. 



There are certain points of interest to be noted here in connec- 

 tion with diffuse bodies and extra elements. In figure 9 there 

 is represented an extra darkly stained element alongside one of 



