STUDIES ON CHROMOSOMES 



IV THE "ACCESSORY" CHROMOSOME IN SYROMASTES AND 

 PYRROCHORIS WITH A COMPARATIVE REVIEW OF THE 

 TYPES OF SEXUAL DIFFERENCES OF THE CHROMOSOME 

 GROUPS^ 



BY 



EDMUND B. WILSON 



With Two Plates and Two Figures in the Text 



Since the unpaired idiochromosome ("accessory chromosome") 

 was first discovered by Henking ('91) in Pyrrochoris apterus L. this 

 species has been reexamined by only one observer, Dr. J. Gross 

 ('07), with results that are in substantial agreement with those that 

 pe had reached in an earlier investigation ('04) on the coreid 

 species Syromastes marginatus L. In both cases his conclusions 

 hre in conflict with the view advanced by McClung ('02), and first 



1 Terminology. With the advance of our knowledge of the chromosomes that form the distinctive 

 differential between the chromosome groups of the two sexes, and between the male producing and the 

 female producing spermatozoa, it becomes increasingly difficult to find a common name that will apply 

 equally to their various modifications. Terms such as the "accessory," "odd" or "heterotropic" chro- 

 mosome, or "monosome," that are based on the condition of these chromosomes in the male only, are. 

 misleading or inappropriate; and some of them are in certain cases inapplicable, even in the male — 

 e. g., in Syromastes, where the "accessory" chromosome is not univalent but bivalent. Such terms as 

 "heterochromosome" or "allosome" (Montgomery) seem to me unsatisfactory, since they designate the 

 w-chromosomes as well as the differential chromosomes, though these are obviously of quite different 

 nature. Since it has now become evident that a univalent "accessory" chromosome in the male is 

 exactly equivalent to what I have called the "large idiochromosome" in other forms, I think these 

 chromosomes should be designated by the same name, and one that will apply equally to both sexes. 

 While there are some objections to the word "idiochromosome" as a general term for this purpose I 

 am not able to suggest a better one; and since it has already been thus employed by some writers, I 

 shall use it hereafter in a broader sense than that in which I first proposed it, to designate the differential 

 chromosomes in general, whether they are paired or unpaired in the male, and whether one or more 

 pairs are present. A univalent or odd idiochromosome in the male will be called the unpaired idiochro- 

 mosome (or simply the idiochromosome), while the word "heterotropic" may perhaps conveniently 

 be used as descriptive of its passage without division to one pole in one of the maturation divisions. In 

 Syromastes, as will appear, the "accessory" or heterotropic chromosome represents a pair of idiochro- 

 mosomes; while in Galgulus there are several pairs of these chromosomes. 

 The Journal or Experimental Zoology, vol. vi, no. i. 



