SEXUAL CHARACTERISTICS OF THELIA 621 
of the Membracidae, and in more detail the available genera of 
the tribe Telamonini, to which Thelia belongs. The outstand- 
ing result is that, whereas the female genitalia show a remark- 
able conservativeness to a general type, even in the most widely 
separated forms, the male genitalia are not nearly so stable. 
All the genital appendages of the male, the oedagus, the claspers, 
and ventral valves, show a diversity of form not approached by 
the female gonapophyses. Even the abdominal sclerites of the 
male become involved and form accessory apparatus to the 
genital appendages. The greater inconstancy of the male is, I 
believe, due to the fact that new genes may find expression in in- 
dividuals possessing a single x-chromosome in all their cells which 
could not find expression in the presence of two x-chromosomes. 
Factors to be important in heredity must arise in the germ 
cells. They must arise therefore either in the spermatogonia 
and oogonia or in spermatocytes and oocytes. New genes must 
have their origin in cells which are male, or cells which are female. 
It is conceivable that a mutation might arise in a germ cell of a 
male which could not arise in a female germ cell owing to greater 
stability and conservativeness of the latter, due to its chromatic 
constitution. Likewise, the expression of any new gene must 
often be entirely dependent upon those genes already established 
in the heredity of the individual. Those genes already fixed in 
the evolution of the family, genus, or species form, as it were, 
the internal environment for the new gene. The production of 
characteristics in the development of an individual are believed 
to come through a series of changes in the protoplasmic mass 
governed and controlled by the genes located in the chromatin. 
Each step must be dependent upon those steps which preceded, 
and thus the most recent characteristics must be dependent on 
the changes produced in the cells by the genes older phylo- 
genetically. 
The egg of an insect even before fertilization becomes highly 
differentiated. This subject has recently been presented by 
Hegner (’17). The regions of the cytoplasm become special- 
ized, polarity and bilaterality are established. We look upon 
the force which determines and brings about this organization 
