ROBERTSON: CHROMOSOME STUDIES IV. 279 



son, '16; Rayburn '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 afforded by the work 

 of Metz ('14, 16a, IQb, 16c, 16c^) 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 aynphilo- 

 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 w^hich 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 



