Studies on Chromosomes. 541 



chromosomes thus uniting are the paternal and maternal homo- 

 logues. In the case of the m-chromosomes, no less clearly than 

 in that of the idiochromosomes, the conjugation is not in anyway 

 an inference but an easily observed fact; and in both cases it is 

 equally clear that the subsequent reducing division separates, 

 with their individuality unimpaired, the same chromosomes that 

 have previously united in synapsis. 



I believe that any observer who will take the trouble to study 

 in detail the history of the chromosomes in these insects must sooner 

 or later in his task acquire the firm conviction that he is dealing 

 with definite, well characterized, entities which show the most 

 marked individual characteristics of behavior, which in some 

 manner persist from one cell-generation to another without loss 

 of their specific character, and which unite in synapsis and are 

 distributed in the ensuing maturation-divisions in a perfectly 

 definite manner. All the facts indicate that these phenomena are 

 the visible expression of a preliminary association, and subsequent 

 distribution to the germ-cells, of corresponding hereditary char- 

 acters. It is evident, therefore, that the time has come when 

 cytologists must seriously set themselves to the task of working 

 out a comparative morphology and physiology of the chromosomes, 

 with the ultimate aim of attempting their specific correlation with 

 the phenomena of heredity and development. 



SUMMARY. 



1. The chromosomes that have been called " heterochromo- 

 somes" in Hemiptera (Montgomery) include three distinct forms 

 that may provisionally be called (a) the paired microchromosomes 

 or m-chromosomes; (&) the idiochromosomes; (c) the "accessory" 

 or heterotropic chromosomes. 



2. The m-chromosomes are usually very small, form a sym- 

 metrical pair in the spermatogonia, and do not unite (in the 

 forms I have studied) to form a bivalent chromosome-nucleolus 

 in the growth-period. At an earlier or later period they condense 

 to form two separate chromosomes that finally pair to form the 

 small bivalent central of the first division, but are immediately 

 separated without fusion. Each divides equally in the second 

 division. 



3. The idiochromosomes are typically unequal, and hence 

 do not form a symmetrical pair in the spermatogonia. They may 



