Studies on Chromosomes. 395 



the same number of chromosomes, but differ in respect to one of 

 them. In this respect these genera differ from all those that 

 possess an accessory chromosome (Pyrrochoris, Anasa, Alydus, 

 etc.), since in the latter case one-half of the spermatozoa receive 

 one chromosome fewer than the other half. It is remarkable 

 that two types of dimorphism apparently so different should 

 coexist within the limits of a single order of insects. We are thus 

 led to inquire into the relation between the idiochromosomes and 

 the accessory; and this inquiry must also include the "small 

 chromosomes" of Paulmier and the "chromatin-nucleoli" of 

 Montgomery. It will conduce to clearness if the second part of 

 this question be considered first. 



It is evident from the figures and descriptions of Montgomery 

 ('98, '01, I, '01, 2, '04) that the bodies I have called idiochromo- 

 somes are identical wnth some of those that this author has de- 

 scribed under the name of "chromatin-nucleoli" (which are 

 usually also small chromosomes or microchromosomes); but it is 

 now manifest that the bodies described under the latter name are 

 not all of the same nature. It is evident that two types of these 

 bodies may be distinguished that differ markedly in their behavior 

 during the maturation-mitosis. One of these, typically repre- 

 sented in Anasa, Alydus and Protenor appear in the spermat- 

 ogonia in the form of two equal microchromosomes ("chromatin- 

 nucleoli") which like the other chromosomes sooner or later unite 

 in synapsis to form a bivalent body that lies at the center of the 

 equatorial plate of the first mitosis. Both divisions accordingly 

 show exactly one half the spermatogonial number of chromosomes 

 but it is a very noteworthy fact that the final conjugation of the 

 two microchromosomes is long deferred, taking place in the propha- 

 ses of the first division (as was first observed by Montgomery in Pro- 

 tenor and some other forms, more recently by Gross in Syromastes 

 and by myself in Anasa and Alydus). In this case there seems to 

 be no doubt that the first division of the bivalent body thus formed 

 is a reducing division. It appears to be further characteristic of 

 this type — at least in all the forms mentioned — that a true acces- 

 sory chromosome is associated with the microchromosomes, and 

 that only one-half of the spermatozoa receive one-half the somatic 

 number of chromosomes, the other spermatozoa receiving one 

 less than this. The distinction between the accessory and the 

 microchromosomes or "chromatin-nucleoli" first demonstrated 



