NEW TYPES OF CHROMOSOME DISTRIBUTION. 157 



and derived from univalent spermatogonial chromosomes. The 

 case of the large nucleolus of Gelastocoris is not so clear, but as 

 previously stated, there is a possibility that the differential chro- 

 mosomes are embedded in the nucleolus, although not as distinct 

 and separate individuals as in the Reduviidae. The differential 

 chromosomes in the Reduviidse, are without exception (as far 

 as examined) embedded in a plasmosome. This plasmosome 

 may be pale (Priouidus, Sinca] in which case the chromosomes 

 stand out clearly, or it may stain rather densely (Diplocodus, 

 Conorhiniis} so that the chromosomes can be seen only faintly 

 or not at all. In this respect they differ from the chromosome 

 nucleoli formed by the accessory, m- and idiochromosomes. The 

 accessory chromosome is, in the early part of the growth period, 

 in contact with a pale plasmosome, but later separates from it. 

 The idiochromosomes also may be associated during a part of 

 the growth period with a plasmosome. The m-chromosomes 

 may lie in any position and have no relation to the plasmosome. 



Origin of the Differential Chromosomes. 



In a study of the differential chromosomes, one of the most 

 interesting questions which arises, is the origin of the asym- 

 metrical chromosome distribution. It seems almost a certainty 

 that these peculiar forms of distribution have not been present 

 since the origin of the species, but have arisen sometime within 

 its history. 



In a discussion of this question, Montgomery ('06) is inclined 

 to the belief that the m-chromosomes and idiochromosomes are 

 not radically different structures, but are rather extremes of a 

 series of modifications. Of their origin he says, " First a pair 

 of autosomes (chromosomes) became modified so as to retain 

 their compact nature during the growth period, still maintaining 

 their approximate equivalence in volume. Because such allo- 

 somes are usually very small, we might conclude also, that they 

 arose from the smallest pair of autosomes. In the next change 

 would appear a growing disparity in size, which, if our last 

 assumption be correct, would be due not to one becoming smaller 

 and the other becoming larger, but rather to one retaining its 

 original volume and to the other becoming much larger. This 



