134 bulletin: museum of comparative zoology. 



who noted that one-half the spermatids received one more chromatic 

 element than the others. They have since been described for a large 

 number of forms but so far have been found only in the tracheates and 

 arachnoids, with the possible exception of Sagitta. In insects they 

 have been described by Montgomery ('98, Pentatoma, :01, :01% :06, 

 Hemiptera, :05, Syrbula), Paulmier ('99, Anasa), McClung ('99, 

 Xiphidium, :00, Hippiscus, :02", Locustidae), de Sinety (:01, Or- 

 thoptera), Voinov (:03, Cybister) Baumgartner (:04, Gryllus), Gross 

 (:04, Syromastes), Stevens (.05'^, Stenopelamtus, Blatella, Tenebrio), 

 Montgomery (:05, Syrbula), Wilson (:05 to :06, Hemiptera), Nowlin 

 (:06, Coptocycla), Stevens (:06'\ Coleoptera, Aphrophora, Cacoecia, 

 Euvanessa), Zweiger (:06, Forficula), Otte (:06, Locusta), and 

 Gutherz (:07, Gryllus, Pyrrhocoris). But Morgan (:06) and Stevens 

 (:05, :06) have failed to find any allosomes in the aphids. The 

 allosomes in myriapods have been described by Blackman (:05, 

 Scolopendra) and Medes (:05, Scutigera); in arachnids by Wallace 

 (:05, Agalena), Montgomery (:05, Lycosa) and Berry (:06, Epeira). 



Until we learn more about the allosomes, they may for convenience 

 be separated into two classes with subdivisions in each class (cf. 

 Gutherz :07), although it is an open question whether in some cases 

 the different tj'j^es have any direct relation with each other. The 

 different types of allosomes may be distinguished as follows: 



A. Monosovies. These are allosomes which are unpaired in the 

 spermatogonia and are usually more or less compact in the sperma- 

 tocytes. They divide in only one of the two maturation divisions. 

 These have been variously called accessory chromosomes, heterotropic 

 chromosomes, odd chromosomes, etc. 



The monosomes may be di\'ided into two groups as follows : — 



1. Monosomes which do not di^•ide in the first maturation di\'ision, 

 but do divide, probably equationally, in the second division. This 

 type, with the possible exception of Syrbula (Montgomery :05), 

 apparently occurs universally throughout the Orthoptera, and, with 

 the exceptions noted, is the only ij])e of allosome occurring in this 

 group. Similar elements have been described in the Hemiptera by 

 Wilson (:05^', :06, x\rchimerus, Banasa), Montgomery (:06, Calocoris), 



.and Stevens (:06''^, Aphrophora); in several Coleoptera by Stevens 

 (:06''); in myriapods by Blackman (:05, Scolopendra) and Medes 

 (:05, Scutigera); and in arachnids by Berry (:06, Epeira). 



2. Monosomes which divide equationally during the first division, 

 but fail to divide during the second. Such elements have so far been 

 found only in the Hemiptera, where they have been described by 



