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 Sinéty (: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 (ef. 
Gutherz :07), although it is an open question whether in some cases 
the different types have any direct relation with each other. The 
different types of allosomes may be distinguished as follows: 
A. Monosomes. '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, ete. 
The monosomes may be divided into two groups as follows: — 
1. Monosomes which do not divide in the first maturation division, 
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 type of allosome occurring in this 
group. Similar elements have been described in the Hemiptera by 
Wilson (:05”, :06, Archimerus, 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 
