254 Edmund B. Wilson: 
the actual origin of the first type; for I was able to show 
(Studies I—IlI) that in different species of Hemiptera a series of 
gradations exists between forms in which Y is nearly as large as 
X (Mineus, Nezara hilaris) and those in which it is very small 
(Lygaeus, Nezara viridula, ete.). Its final disappearance would leave 
X without a synaptic mate as an odd or unpaired chromosome. 
These relations will be made clear from Figs. 1—3. 
er Eher. Re 
13 17") 
/roleror Svromasles Ascartis 
Anasa Homo lumbricoides 
Sol or 0 0 
8 8 % ” 
MNezara Fuschistus  Nezara Thyanla 
vircdula Coenus Ahularıs calceala 
Y N u u ) 
x ” 6 
Roreonofa Prionidis Gelastocoris  Acholla 
Fulchra Senea m ullispin 054 
Fig. 3. 
Essentially similar relations between the gametes and sex- 
production have since been determined in many other species 
of Hemiptera (Wilson, Stevens, Boring, Montgomery, 
Payne, Morrill), in Coleoptera and Diptera (Stevens), Odonata 
(Lefevre and Me Gill) and Orthoptera (Wassilieff, Gutherz, 
Jordan, Davis, Morse), more recently in the nematodes 
Heterakis (Boveriand Gulick) and Ascaris (Boring, Boveri, 
Edwards). Conditions have now been made known that are 
more complicated, but in principle identical with the foregoing, 
in which the X-chromosome is double or multiple. Thus arises 
a compound ”X-element‘‘ consisting of several components (Fig. 3); 
but in respect to sex-production this group acts as a unit, and 
in all cases that have been fully worked out shows the same 
relations to sex as when it is a single chromosome. The compound 
