STUDIES ON CHROMOSOMES 375 



individually for a considerable distance without branching; and it 

 it my belief, after prolonged study, that the threads do not 

 really branch or form a network, though such an appearance 

 is often given by fine strands of 'linin' (perhaps coagulated nuclear 

 sap) connected with the threads. 



A point to- be emphasized is that these threads do not show the 

 least sign of longitudinal division, and in this respect offer a 

 marked contrast to the longitudinally double threads seen in the 

 post-synaptic stages. 



As Stage c passes into Stage d, the contrast between the sex- 

 chromosomes and the others becomes still more pronounced. In 

 Oncopeltus the former often become nearly spheroidal in shape, 

 and stain so intensely as to appear exactly like chromatic nucleoli. 

 In Lygaeus they stain with equal intensity, but still retain more 

 or less of a rod-like form, particularly in case of the Jf-chromo- 

 some. In Largus (as in Pyrrhocoris) the unpaired X-chromosome 

 becomes as a rule spheroidal. In all these forms the sex-chromo- 

 somes always occupy a peripheral position with respect to the mass 

 of chromatin-threads, sometimes in the clear space outside the 

 latter, more often embedded in its peripheral zone. A very 

 striking fact (to which I formerly called attention in case of Pyr- 

 rhocoris) is that the sex-chromosomes in these stages are always 

 separated from the threads by a vacuole-like space. This is most 

 conspicuous in Largus (figs. 78 to 80) where the vacuole is unusu- 

 ally large and clearly defined; but it also appears in the other 

 forms when seen in the right position. No definite wall to the 

 vacuole can be seen, but the chromatin-threads are often seen 

 encircling its outer limit, as if lying upon a definite substratum. 

 This fact is interesting as indicating that the sex-chromosomes 

 really lie in separate compartments or chambers of the nucleus, 

 even though their walls can not be seen. Is this, conceivably, 

 true of other chromosomes, and may this possibly be the basis of 

 the genetic continuity of chromosomes in general? 



