STUDIES ON CHROMOSOMES 385 



and the two univalent sex-chromosomes. The later may at once 

 be recognized by (1) their smaller size, (2) more compact texture, 

 and (3) simple, rod-like form and longitudinal split (figs. 108 to 

 110, photos. 20 to 23). In both sections and smears all grada- 

 tions are seen between these nuclei and those of the growth-period 

 which remove all doubt as to the identity of these chromosomes 

 with the chromatic 'nucleoli' of the latter. On the other hand, 

 it is easy to trace these chromosomes step by step through the 

 later prophases into the two small chromosomes of the first divi- 

 sion. In these stages the double rods are seen progressively 

 shortening until they assume the dumb-bell shape in which they 

 enter the spindle (figs. 112-114). It is clear from the transi- 

 tional stages that the transverse constriction of the dumb-bell 

 corresponds to the original longitudinal split of the rod before its 

 shortening, while the long axis of the dumb-bell represents the 

 original transverse axis of the rod. The apparent 'transverse' 

 division of the dumb-bell is therefore in reality a longitudinal 

 division. 



We may with advantage consider at this point some very inter- 

 esting features presented by the X-chromosome in Lygaeus bicru- 

 cis^i especially during the stages preceding the prophases. In the 

 earliest stage (a) this chromosome is an elongate, almost vermi- 

 form body, which appears homogeneous in structure (figs. 69, 

 70). In Stages b to d (figs. 71 to 73) it is shorter and thicker, 

 and still usually appears homogeneous, though in much extracted 

 preparations of Stage c it may appear longitudinally divided. In 

 Stage e (synizesis) it is considerably shorter and shows no sign of 

 division (photo. 12). In Stage/ it is again more elongated and 

 unmistakably split lengthwise (photos. 13 to 15), and in this con- 

 dition persists throughout the whole growth-period, gradually 

 shortening in the prophases until it assumes a dumb-bell shape, 

 quite as in Oncopeltus (photo. 25). 



At every period from the post-synaptic spireme onwards many 

 cases may be found in which the double rod appears nearly or 

 quite homogeneous (figs. 84, 98, 99, 100 a, b: photos. 14, 15); but 



" This account applies only to this species. The facts in L. turcicus are very 

 different, as already mentioned (see Wilson, '05 b, '06). 



