196 Chromosomes in Pier is and Abraxas 



After the oogonial stage, the nuclei begin to enlarge and pass 

 through a synizesis stage closely similar to that of Pieris, except 

 that the nucleolus is less conspicuous and is sometimes difficult to 

 find. On emerging from this condition the nuclei, now considerably 

 enlarged, contain interlaced chromatic threads, the number of which 

 cannot be accurately determined. The nucleolus at this stage varies 

 somewhat in appearance ; it usually consists of an achromatic mass 

 enclosing a double mass of chromatin, but may be almost or completely 

 divided into two parts, and each half is sometimes seen to contain a 

 compound mass of chromatin. In the rather later stages this com- 

 pound nucleolus, consisting of a number of globules or irregular lumps 

 of chromatin, and usually clearly divided into two more or less separate 

 parts, is very commonly seen in sections from which the stain is 

 rather thoroughly washed out or which are stained with Breinl's 

 stain (Figs. 13 a, 15«). The individual chromatin globules sometimes 

 appear double, but I doubt whether this is more than accidental. 



The chromatin threads contract and thicken, but do not in general 

 assume the meridional arrangement often seen in Pieris. They appear 

 to become bent into loops (Fig. 12), and these shorten to form horse- 

 shoe or even ring-like figures (Fig. 1.3). The nuclei are now so large 

 as to extend through two sections, but careful counts show that the 

 number of the contracted loops is not far from 27. From this stage 

 onwards, or possibly from one somewhat earlier, a differentiation appears 

 to set in among the cells. In the majority, the chromosomes contract 

 still further and appear to break apart at the bend of the loop, giving 

 rise to a double body, the halves of which are commonly completely 

 separated, so that the nucleus contains about 27 pairs of short, thick 

 chromosomes, and an additional larger pair apparently derived from the 

 chromatin-nucleolus. Each member of a pair may itself show signs of 

 doubleness, possibly owing to the precocious appearance of the longi- 

 tudinal split of the next division, traces of which may sometimes be 

 seen in an earlier stage (Fig. 13). In many nuclei the two members 

 of each pair are completely separated, giving the diploid number of 

 small irregular chromosomes scattered in the nucleus (Fig. 14). I was 

 at first inclined to believe that the.se single chromosomes came together 

 in pairs, giving rise to the stage in which the reduced number of 

 doubles is seen, but a careful study of the various stages has convinced 

 me that in the nuclei here described the paired condition precedes that 

 with the full number of singles. 



The nuclei just described are the most numerous class at the lower 



