390 Edmund B. Wilson. 



form is often less distinct (6^). Up to this point both idiochro- 

 mosomes are apparently always present. In the stages now to be 

 considered both are often plainly distinguishable, but quite as 

 frequently this is not the case, the nucleus showing but a single 

 chromosome-nucleolus the size of which proves it to be either the 

 large idiochromosome or the large and small one united. Between 

 these two possibilities I have not been able to decide in Lygieus 

 and Coenus; but decisive evidence is given in the case of Brochy- 

 mena, as described beyond. In stage o- (60) the idiochromosomes 

 (or the single chromosome-nucleolus) appear as spheroidal com- 

 pact bodies, usually not showing a bipartite structure, and in 

 addition one or more pale rounded plasmosomes are often present. 

 In 'Stage /, owing to the loss of staining capacity by the larger 

 chromosomes, the idiochromosomes show with brilliant clearness, 

 since they are still stained intensely black, and may very readily 

 be studied with care (6g-n). When both are present they appear 

 slightly larger than in the later stages, and often the larger one 

 is plainly seen to be hollow (which probably accounts for its larger 

 size) though this is shown still more clearly in Brochymena, as 

 described beyond. A small plasmosome is sometimes attached 

 to it at one side, but in addition to this there is always present a 

 very large pale plasmosome quite free from both idiochromosomes, 

 and free also from the single chromosome-nucleolus when but 

 one of these bodies is present. In stage e the idiochromosomes 

 present the same appearance, but the larger one is now ahunvs 

 attached to the large plasmosome, and often more or less flattened 

 against it (as is also the single chromosome-nucleolus when but 

 one appears) while the small one is almost always free from it 

 (Fig. 6d, e, f). The larger body is as before often evidently 

 hollow. 



Up to this point Lygaeus and Coenus agree almost exactly. In 

 the earlier stages they differ in that Coenus still shows the idio- 

 chromosomes in the form of compact chromosome-nucleoli, 

 while in Lygaeus the larger one certainly, and I believe the smaller 

 one also, assumes the form of a longitudinally split elongate chro- 

 mosome. In stage d in Lygaeus (Fig. 6c) the large idiochromo- 

 some is a rather short, deeply staining rod, longitudinally split, 

 and still attached (usually toward one end) to the plasmosome, 

 which is now considerably smaller. The small idiochromosome 

 is now also more or less elongate, but I cannot be sure whether it 



