ROBERTSON: CHROMOSOME STUDIES IV. 279 



son, '16; Raybum '17). Such could only arise if the chromo- 

 somes of the group are considered as genetically continuous and 

 related. The same applies to the X-chromosome. Further evi- 

 dence that sex chromosomes may be similary constituted 

 throughout a series of related species is alforded by the work 

 of Metz ('14, 16a, IQb, 16c, 16d) on the Drosophilas, which has 

 shown that not only one plan of structure, in so far as relative 

 sizes, etc., is concerned, may run through the chromosomes of 

 each species of a genus, but, what is still more striking, that 

 each gene has probably the same locus with reference to other 

 genes along the length of the chromosome throughout the vari- 

 ous species. If such be the case in the Drosophilas it seems a 

 probability that the same might be true of the accessory chro- 

 mosome among the genera and species of the Tettigidse. Then, 

 we are justified in surmising that, since in the genus Acridium 

 the sex-determining chromosome has a value of 8, in the genus 

 Tettigidea, where its length is relatively 19, only a portion of 

 this chromosome, considerably less than half, i, e., about eight 

 nineteenths, could be concerned with sex-determination. 



If a comparison of the sex chromosome in the genera of this 

 family leads us to believe that the sex-determining portion in 

 the genus Tettigidea is at most not greater than eight nine- 

 teenths of the total, in the case of the supernumerary accessory 

 described in this "Study" we have evidence, it seems, which 

 limits the size of this portion to a still greater degree, namely, 

 between one fifth and one fourth the length of the chromosome. 

 The reasons for concluding this are as follows : The presence 

 of the chromosome in this particular animal has not produced 

 a female ; hence it must lack the part concerned with sex deter- 

 mination. Bridges ('16), in his splendid work upon nondis- 

 junction of pairing sex chromosomes in Drosophila amphilo- 

 phila, has shown that sex determination may be excluded from 

 one portion of the X-chromosome at least, and therefore 

 limited to a part of it. He found that a small portion of the 

 X-chromosome in the region which carries the genes for 

 "barring of the eye" and "forked condition of the bristles" 

 became nonexistent genetically (pp. 150, 151). Females could 

 exist with one such deficient X-chromosome. The sons of such 

 heterozygous females were only one-half as numerous as the 

 daughters, and did not possess the deficiency. The evidence 

 warranted the conclusion that males could not exist with such 



