132 THE BIOLOGY OF INSECTS 



left of the body a testis with sperms and on the right an 

 ovary with eggs. In such a case it is likely that in the 

 first division of the egg nucleus one of the daughter x- 

 chromosomes failed to pass into the cell whence the left 

 side of the body developed, therefore the left side is *' male " 

 and the right '' female." An alternative explanation is 

 due to L. Doncaster (19 14), who noticed that some eggs 

 of moths are binucleate and that both nuclei may be 

 fertilised ; if one were male and the other female in 

 composition, each might give rise to the half of a gyn- 

 andromorph. 



K. Toyama (1906) described gynandromorph moths of 

 the common silkworm (Bomhyx mori), hybrids from a cross 

 of two races, and the caterpillars whence they developed 

 showed a bilateral distinction in the colours (white and 

 dark- spotted) of the larvae of the parents. Instead of the 

 axial division of an insect into right and left halves showing 

 respectively the characters of either sex, gynandromorphs 

 sometimes occur in which the two sets of characters form 

 an apparently irregular " mosaic " pattern. This con- 

 dition might be explained also by the irregular division, 

 though at a later stage, of the sex-chromosome in those 

 cells where the abnormal tissues and organs arose in 

 development. In the course of the breeding experiments 

 with Drosophila many cases of gynandromorphism were 

 observed, and described by Morgan and Bridges (1919). 

 These abnormal insects were often the offspring of cross- 

 breeding and consequently showed divergent body- 

 characters (length of wing, eye-colour, etc.) as well as some 

 of the external (" secondary ") features characterising the 

 two sexes. Many of these Drosophila gynandromorphs 

 were bilaterally male and female, while others displayed a 

 more or less irregular " mosaic," the eye, for example, on one 

 side being divided between the alternative colours that are 

 sex-linked in inheritance. Analysis of the results together 

 with knowledge of the nuclear constitution of the various 

 forms, enabled the investigators to demonstrate that all the 

 characters displayed in such abnormal insects are sex- 



