Studies on Chromosomes. 391 



is longitudinally split. All intermediate stages may readily be 

 observed between this stage and the next earlier one (e) in which 

 the large idiochromosome is an elongate split thread that may 

 extend through more than half the diameter of the nucleus (Fig. 

 6b). It is, however, at once distinguishable from the other chro- 

 mosomes by its straighter course, greater thickness and deeper 

 staining capacity,^ which renders it very conspicuous among the 

 thread-like chromosomes. No plasmosome can now^ be seen. 

 The small idiochromosome can still clearly be distinguished in 

 many of the nuclei as a short rod staining like the larger one. 

 Finally, in the contraction-phase (stage b) when all the chromo- 

 somes are massed together, the large idiochromosome still unmis- 

 takably appears as a very distinct deeply staining rod, sometimes 

 nearly straight, more usually curved, and frequently horse-shoe 

 shaped, at one side ot the chromosome-mass (6<^). Neither 

 plasmosome nor small idiochromosome can now be made out. 

 In Coenus at this period (Fig. 5a) both idiochromosomes (or the 

 single chromosome-nucleolus) still appear as compact, deeply 

 staining spheroidal bodies, the larger one typically having a small 

 plasmosome attached to it at one side. 



The early history of the large idiochromosome proves most 

 clearly that the chromosome-nucleolus into which it afterward 

 condenses is a modified chromosome (as Montgomery first showed 

 in Euschistus variolarius) and one that forms a connecting link 

 between the ordinary chromosomes and the more usual forms of 

 "chromatin-nucleoli" in Hemiptera, described by Montgomery, 

 or the accessory chromosome of Orthoptera; for in none of these 

 latter is the condensation to a compact body so long delayed. 

 Paulmier ('99) showed, however, that in Anasa the compact chro- 

 mosome-nucleolus of the synaptic period . afterward elongates 

 considerably and appears as a rather short longitudinally split 

 rod, similar to that of Lygaeus at stage d (Paulmier, Fig. 22) 

 afterward again condensing into a compact tetrad. In Lygaeus 

 there can be little doubt that the central cavity of the spheroidal 

 chromosome-nucleolus, often visible in stages e-g, represents the 

 original longitudinal split. It is therefore hardly open to doubt 

 that the division of the large idiochromosome in the first mitosis is 

 an equation-division, and the same is probably true ot the small 



'This has been somewhat exaggerated in the engraving. 



