L. DONCASTER 191 



plates are seen in face, 30 chromosomes are easily counted (Fig. 1). 

 They include eight which are smaller than the rest, and 22 larger 

 chromosomes. The larger ones, however, are not all of exactly equal 

 size, so that it is not easy to separate the smallest of them from the 

 larger members of the eight small ones. The eight small ones usually 

 appear to be graded into two smallest, four rather larger, and two 

 somewhat larger still, but these paii"S cannot always be recognised with 

 certainty. Pairs similar in size often but not always appear near to- 

 gether in the equatorial plate ; this is most conspicuous in respect of 

 the two smallest. Some figures give the appearance of an odd number 

 of small chromosomes, but more careful study suggests that this is due 

 to one member of a pair being seen end-wise, and the other more side- 

 ways. I conclude therefore that there is no evidence for the existence 

 of an unequal pair, and still less for an unpaired chromosome in the 

 female of Pieris hrassicae. Outside the circle of chromosomes in 

 the equatorial plate, two or more small chromosome-like bodies are 

 often visible ; their position and size usually distinguish them without 

 difficulty from true chromosomes. Miss Cook^ has described similar 

 " chromatin granules " in the spermatocytes of Lepidoptera. 



After the last oogonial division, the nucleus begins to enlarge, the 

 reticulum becomes more clearly visible, and then, apparently suddenly, 

 the "synizesis" stage supervenes. The nucleus now contains a very 

 fine thread or group of threads, tightly coiled on one side of the cavity ; 

 the chromatin nucleolus at this stage is single, small, with sharp outline 

 (Fig. 2). 



The synizesis condition ceases as suddenly as it began, giving place 

 to nuclei with the chromatin thread in separate segments, much inter- 

 twined, and noticeably thicker than in the preceding stage (Fig. 3)» 

 The transition from this to the following stages is gradual ; the nucleus 

 enlarges, the threads become less interwoven, and finally become ar- 

 ranged not quite regularly but approximately in a meridional manner 

 round the nuclear membrane (Fig. 4). In the earlier stages it is im- 

 possible to count them, but in favourable cells at this stage it is 

 sometimes not difficult to see that there are 14 separate threads, and 

 as the unravelling of the segments is a quite gradual process there are 

 probably 14 at the close of synizesis, although some of the earlier cells 

 give the impression of a larger number (possibly twice as many). The 

 nucleolus during this process has enlarged and finally again become 

 conspicuously double ; occasionally the two parts are separated ; it is 

 1 Proc Acad. Sei. Philadelphia, 1910, p. 294. 



14—2 



