CHROMOSOME STUDIES ON THE DIPTERA. 259 



THE MATURATION DIVISIONS. 



No special interest attaches to these and they will be passed over 

 rapidly. The first division is reductional for autosomes and X- 

 chromosome. The former appear as dyads with little evidence of 

 a tetrad structure. Such a structure probably exists, however, and 

 further extraction would perhaps bring it out, for in early anaphase 

 each component of the dyad is itself clearly double. The X- 

 chromosome goes to one pole in the first division and divides in 

 the second. Metaphases of the first division and of the two types 

 of second division are shown in Figs. 14 to 16. 



OOGENESIS. 



Knowing the unusual behavior of the chromosomes during 

 spermatogenesis, it would be of especial interest to determine 

 whether or not similar phenomena occurred during oogenesis, but 

 this has proved to be a very difficult task. Owing to the nature 

 of oogenesis only a few, rather widely separated stages can be 

 found in any one ovary, and the story has to be pieced together 

 from an examination of many specimens. The orientation of 

 stages, especially the early ones, is very difficult. Consequently 

 I am only able to record a few observations at this time. 



The chromosome group of the female, as expected, consists of 

 five pairs differing from that of the male only in the presence of 

 two X-chromosomes instead of one (Fig. 22). 



Near the apex of the ovarian tubules, in the region of transition 

 from oogonia to oocytes and nurse cells, are found nuclei with 

 chromosomes such as those represented in Fig. 23 (probably 

 oogonial) and in Figs. 24 to 28. In the latter very obvious loops 

 are present, bearing a strong resemblance to those in the spermato- 

 cytes. These chromosomes are not pulling apart on the spindle, 

 as their outlines might suggest, but are in resting nuclei, or at 

 least nuclei that are not actively preparing to divide. Whether 

 they represent the final generation of undifferentiated ovarian cells 

 or represent an early growth stage of oocytes and nurse cells, I 

 am unable to determine. If the latter, then they may correspond 

 to spermatocytes in the aggregated stage b. In any case, the con- 

 dition seems to parallel that found in the male, for I have not 



