192 Chromosomes in Pieris and Abraxas 



now more obviously composed of an achromatic sheath enclosing a 

 double mass of chromatin. There are thus at this stage fourteen 

 chromatin threads and a double nucleolus. The last stage represented 

 in my series is the .contraction of the threads into short thick chro- 

 mosomes, which at the close of the process are conspicuously split 

 (Fig. 5), so that the ordinary chromosomes are no longer clearly dis- 

 tinguishable from the " chromatin nucleolus " which must be regarded 

 as a double, equallj' paired, heterochromosome such as is described by 

 Miss Cook {loc. cit.) in the spermatogenesis of several species. I have 

 not been able to determine with certainty how the doubleness of the 

 thick chromosomes arises ; in Abraxas, and also in Pygaera bucephala, 

 the ovary of which I have examined for comparison, and in which the 

 double chromosomes are most beautifully shown, the appearance strongly 

 suggests that the doubleness is produced by the contraction of looped 

 threads, which break apart at the bend of the loop and give rise to a 

 double rod. 



Certain exceptional conditions should be mentioned. At various 

 stages from the oogonia onwards degenerating cells, singly or in groups, 

 are not infrequent, in which the nucleus develops into a more or less 

 compact staining mass. The same thing is not infrequent in spermato- 

 genesis of various insects. 



Among cells shortly after the synizesis stage in one ovary a single 

 nucleus is present in which, in addition to chromatin-nucleolus and 

 plasmosome, there are about 25 short thick chromatic threads like those 

 of the last stage in the production of the 14 double chromosomes de- 

 scribed above. 



As will be seen below in the description of Abraxas, such cells with 

 the diploid number of shortened chromosomes are abundant in the latter 

 insect, and appear to arise by the more or less complete separation of 

 the chromosomes which have paired in synapsis. In Abraxas these 

 cells probably do not give rise to eggs, but become nutritive or follicle- 

 cells. In Pieris the nuclei of all cells at this stage, whether they will 

 ultimately become eggs or follicle-cells, normally show the reduced 

 chromosome-number; the single cell found with the double number 

 is quite exceptional. 



Finally it should be mentioned that in cells of the ovarian epi- 

 thelium enclosing the developing oocytes, mitotic figures occur with 

 many more than thirty chromosomes ; a similar reduplication of the 

 chromosomes in the ovarian sheath appears also in Abraxas, and has 

 been seen in other orders of insects. 



