346 EDMUND B. WILSON 



INTRODUCTION 



In this paper are described observations on certain phases of 

 the maturation-process in Oncopeltus fasciatus (DalL), Lygaeus 

 bicrucis (Say), and some other Hemiptera, together with the 

 results of a comparison of these species with some other insects, 

 with Tomopteris and with Batracoseps. It was my original 

 object to clear up the relations of the sex-chromosomes in Onco- 

 peltus and to trace as completely as possible their history in the 

 maturation-process; but in doing this it was found necessary to 

 take into consideration many other features of the spermatogene- 

 sis, and I will take this opportunity to present some conclusions 

 based on a broader studj^ of these problems on which I have long 

 been engaged. 



In respect to the sex-chromosomes, Oncopeltus is of especial 

 interest because it stands on the border line between species in 

 which the X- and F-chromosomes are visibly unequal in size, 

 and in which a corresponding visible difference appears between 

 the diploid chromosome-groups of the two sexes, and those in 

 which such sexual differences can not be seen.^ In my fourth 

 'Study' ('09 a) Oncopeltus was classed with Nezara hilaris as 

 an example of the latter class of cases; and much theoretic impor- 

 tance has been ascribed to both these forms as indicating the pos- 

 sibility or probability that the spermatozoa are really sexually 

 dimorphic even when no visible evidence of this is shown by the 

 chromosomes. In my seventh 'Study' ('11a) ) I showed, con- 

 trary to my original account, that a dimorphism of the sper- 

 matid-nuclei is in fact visible in Nezara ; but in regard to Oncopel- 

 tus judgment was reserved as I was still baffled by apparently 

 contradictory data. I am now in a position to clear up these 



1 The first account of Oncopeltus was given by Montgomery ('01), who described 

 the sex-chromosomes ('chromatin-nucleoli') as of equal size in the male, and found 

 that the}' remain always separate, without fusing at the time of general synapsis, 

 and divide separately in the first spermatocyte-division. Subsequently ('06) he 

 added that these chromosomes (now called 'diplosomes') conjugate to form a 

 bivalent after the first division, and undergo disjunction in the second division. 

 In my fourth 'study' ('09 a) I briefly confirmed these accounts, and stated that the 

 female diploid chromosome-groups are not to be distinguished by the eye from the 

 male. 



