CHROMOSOME STUDIES ON THE DIPTERA. 255 



haps, the growth period does not involve an increase of more than 

 about 30-50 per cent, in nuclear diameter. 



The description of the growth period may best be taken up at a 

 point shortly after growth has begun (returning later to a con- 

 sideration of the earliest stages). At this time the chromosomes, 

 with the exception of the condensed X-chromosome, are in the 

 form of long, deeply staining threads, closely applied to the nuclear 

 membrane. There are four pairs of threads, in one of which 

 (apparently the smallest) the two members are usually closely 

 associated throughout their length i.e., the synaptic association is 

 complete. In each of the other three the association is evident at 

 each end but toward the middle the two components diverge to 

 form large loops. Each pair of threads is very long, frequently 

 extending more than half way around the circumference of the 

 nucleus, hence it is practically impossible to represent them all in 

 one figure. Individual pairs, however, are shown in Figs. 7 to 10. 

 Most of these are complete, but in some (e.g., Fig. 9) one or both 

 ends of a pair may be cut off. To one of the three looped pairs 

 (apparently the largest) is attached a large dense body (Fig. 7), 

 which serves to identify this pair throughout the entire growth 

 period. This body (see page 258 for description) is attached to 

 both members of the pair in the looped region and normally lies 

 at a point near one end of the loop a position which it occupies 

 with surprising regularity. 



The three large pairs of chromosomes at this stage give the 

 appearance of having undergone synapsis only near their ends 

 the threads having remained well apart medially for about one 

 third to one half their length: In the smallest pair, as noted above, 

 synapsis is usually complete, but in a few cases a small loop is 

 present. The chromosome on the left in Fig. 9 appears to be 

 such a case. Throughout the remainder (80 per cent, or more) 

 of the growth period the condition of the chromosomes is main- 

 tained with relatively little change. The cells and nuclei grow 

 somewhat and the chromosomes become more condensed and hence 

 more easily examined. The X-chromosome remains condensed 

 throughout. Successive stages are represented by thousands of 

 cells and the transformation during growth is so gradual that 

 scarcely any change is observable from one cyst to another down 



