392 Edmund B. Wilson. 



idiochromosome. It is, on the other hand, quite certain that the 

 division of the idiochromosome-dyad in the second mitosis is a 

 reduction-division. The order of the divisions in case of the 

 idiochromosomes is thus the reverse of that which occurs in the 

 other chromosomes, according to Paulmier's and Montgomery's 

 accounts; and as pointed out beyond, it is also the reverse of that 

 which takes place in the division of the small central chromosome 

 in Anasa and Alydus. 



As already stated, I did not in Lygaeus and Coenus, succeed in 

 rinding any certain explanation of the fact that the nuclei of the 

 growth-period may show either one or two chromosome-nucleoli. 



In Brochymena, however, there is very clear evidence on this 

 point. Here, too, the nuclei of the middle growth period show 

 either one or two spheroidal chromosome-nucleoli, the former con- 

 dition being much the more frequent. When both are present 

 they may be widely separated or close together, and both very 

 clearly show a central cavity, which is rendered very conspicuous 

 by the fact that the chromatin is frequently concentrated in a 

 dark zone immediately around it (Fig. *]c-g). When but one 

 is present it is, as a rule, perfectly spherical, hollow, and shows no 

 evidence of a double nature (Fig. 7/, g]. In the early growth- 

 period, however, the single chromosome-nucleolus almost always 

 appears bipartite, being composed of two unequal halves, forming 

 an asymmetrical dyad (Fig. Ja, &) very similar to that seen in the 

 second maturation-division (Fig. yra). At a later period both 

 of the constituents become hollow (and hence appear somewhat 

 larger) and stain less deeply; and all gradations may be observed 

 in the fusion of the two bodies to form a single hollow' body (Fig. 

 Jc, d, /) which is plainly as large as the two separate chromosome- 

 nucleoli (such as may be seen in cells of the same cyst) taken 

 together. In Brochymena, therefore, there can be no doubt that 

 when only one chromosome-nucleolus is present it is to be considered 

 as a bivalent body arising by the fusion or synapsis of the two 

 idiochromosomes. 



Thus far the facts confirm the interpretation given by Mont- 

 gomery ('01, i) who observed in Coenus, Euschistus tristigmus 

 and some other forms that the cells of the growth-period may show 

 either a single "chromatin-nucleolus" or (in Euschistus "appar- 

 ently more frequently") two such bodies that are unequal in size; 

 and this fact he interpreted to mean that the two corresponding 



