326 Dr. E. A. Cockayne on 



female, and the valve and harpe in the male. In the case 

 of these it is generally met with, but there is a tendency 

 in the case of paired organs, which lie contiguous to one 

 another, for the half-sized mass of epithelial cells, which 

 theoretically should give rise to one full-sized member of 

 a pair of paired organs, to produce actually an imperfect 

 and undersized pair instead. 



This occurs most often in the case of the valve and 

 harpe in the male. 



It does not occur in the case of the rod of the ninth 

 segment, because the rods develop far apart, not close 

 together, like the valves. 



In the case of unpaired structures, especially those which 

 are tubular or saccular, the half-sized mass of primitive 

 cells gives rise to a complete tube or sac, which is reduced 

 in size, and often imperfect in form. 



Examples of this kind of structure are the penis in the 

 male and the ostium bursae and bursa copulatrix in the 

 female. 



This explains the narrowness of the bursa and slender- 

 ness of the penis usually found in halved gynandromorphs. 



Entire failure of a part to develop occurs less often in 

 the external than in the internal organs, but in a genetic 

 hermaphrodite, which I described and figured, the whole 

 of the external genitalia of the female side failed to develop, 

 whilst those on the male side showed reduplication. 



They were, however, situated laterally on the male side 

 of the insect and not centrally. Mr. Bethune Baker kindly 

 called my attention to the account he published in these 

 Transactions in 1891, of the external genitalia of a halved 

 gynandromorph of Eronia hippia var. gaea. It is mounted 

 laterally, and the point I wish to bring out is not clearly 

 shown in his beautiful and accurate drawings. He has 

 allowed me to examine the specimen, and I agree with all 

 he says, except that what he regarded as combined valve 

 and ovipositor I consider to be a valve lying over a half 

 ovipositor. Examination of it from above and below with 

 careful focussing has convinced me that the specimen agrees 

 with the majority of gynandromorphs of this kind, and that 

 the arrangement of the parts is that shown in my diagrams 

 (Plates CIII and CIV). On the left side of the terminal 

 segment one can see a half ovipositor with its rod, on the 

 right a narrow uncus, from which runs as a half girdle 

 the tegumen and cingula. The saccus, a structure not 



