DIFFERENTIAL CONDENSATION 311 



of the affected chromosomes to enter into a complete resting stage 

 between divisions. In the male of the coccid Gossyparia spuria 

 half the autosomes already show differential condensation in the 

 late blastula stage (Schrader, 1929). In some groups the unpaired 

 X chromosome shows progressively increased condensation during 

 the resting stages in the spermatogonia as meiosis approaches, and 

 consequently shows very clearly at prophase the spiral form of the 

 chromosome thread from the preceding division (Tryxalis, Brunelli, 

 1910, 1911 ; Rhahditis, Boveri, 1911 ; Perla, Junker, 1923 ; 

 Mecostethus, Janssens, 1924 ; Tettigonia, Winiwarter, 1931). 

 Frequently at the premeiotic metaphases the X chromosome is 

 then understained (D., 1936), and in Rattus and Mecostethus it 

 is understained at meiotic metaphase (Roller and D., 1934 ; White, 

 1936). We have the reversal that we noticed in the behaviour of 

 heterochromatin. Generally, however, the differential condensation 

 does not show itself before the prophase of meiosis, and is seen only 

 in the more rapid condensation of the affected chromosomes after 

 zygotene. From this is derived the name " chromatin -nucleoli " 

 by which they were early described. The precocious chromosomes 

 do not contract further than the rest at metaphase, but during the 

 ensuing interphase and at second metaphase, even in Chorthippus 

 they continue to show exceptional condensation (D., 1936 d). 

 Correlated with the over-staining of the precocious chromosomes are 

 two other anomalies of behaviour at pachytene. The X chromo- 

 some is bent back on itself and only gradually straightens out as it 

 loses its over-condensation at diplotene. It seems to shed a coat 

 of nucleolar substance and at the same time another change occurs. 

 During pachytene the X chromosome seems frequently to attract 

 to itself the other condensed chromosomes which stick to it by their 

 ends. They separate from it at the same time that it straightens 

 out (Janssens, 1924; D., 1936). In parenthesis we may notice 

 that the temporary end to end contacts of chromosomes in the 

 prophase of meiosis in female Icerya (Ch. XI) and haploid Triticum 

 monococcum (Katayama, 1935) are perhaps analogous to this. 

 The ordinary repulsions between chromosomes therefore fall into 

 abeyance during the persistence of the differential staining pro- 

 perties. This is perhaps also true of the sex chromosomes of the 



