STUDIES ON CHROMOSOMES 377 



an unsymmetrical dumb-bell shaped body (photo. 12) but are 

 never fused to form a single body. In thirty-six of the foregoing 

 fifty-five cases in Lygaeus, X and Y were attached end to end 

 (photos. 12, 13), in seventeen side by side (photo. 13) 



There is a good deal of variation in the degree of contraction, 

 even in cells of the same cyst ; and this may be due in part to differ- 

 ences of response to the fixing agent. That the contraction figure 

 can not be regarded as an artifact, however, is proved by the 

 fact (which I briefly described in my fourth 'Study') that in some 

 Hemiptera it may be readily seen in the living cells, as has also 

 been shown by other observers. Gates ('08) has suggested, in 

 case of certain plants, that the synizesis is not produced by a 

 contraction of the chromatin-mass but by enlargement of the 

 nucleus due to rapid accumulation of liquid about the chromatin. 

 Such a view can hardly apply to these insects, I think, though 

 studies of the living material would give a more trustworthy 

 result than those upon sections. 



The synaptic knot often lies excentrically in the clear space. 

 Just outside it, or embedded in its periphery lie the sex-chromo- 

 somes, still surrounded in many cases by the vacuole, though this 

 is now less evident. They retain the same appearance as in the 

 preceding stage, except that in Lygaeus they are somewhat shorter 

 than before. In none of these Hemiptera does either sex-chromo- 

 some elongate, or show any definite relation to the nuclear pole 

 at this stage. In this respect these animals differ markedly from 

 some of the Orthoptera, where the X-chromosome becomes elon- 

 gated and takes part in the general polarization of the chromatin 

 in the 'bouquet' stage. There is no evidence of a giving off of 

 material from this chromosome or from the nucleus at this tirhe.' 



4. Stages f and g 



Stage f. The postsynaptic spireme. Pachytene and diplotene. 

 In the stage immediately following synizesis the chromatin- 

 threads quickly spread apart through the nuclear cavity, and are 



' Cf. Moore and Robinson ('04) and Morse ('09) on the cockroach, Buchner ('09) 

 on Gryllus. 



THE JOURNAL OF EXPEKIMENTAL ZOOLOGY, VOL. 13, NO. 3 



