192 WM. REES B. ROBERTSON 



rod-shaped chromosomes with those of the first spermatocyte, 

 all measurements of the former have been doubled. Most 

 dependence is to be placed upon results from simple rod-shaped 

 chromosomes of spermatogonia and second spermatocytes. The 

 double chromosomes in the first spermatocyte divisions are less 

 reliable (figs. 29, 31, 33, etc.), because the members of some pairs 

 are more completely separated than others, which affects un- 

 equally the apparent lengths of the chromosomes. The diame- 

 ters also vary. For this reason length measurements do not 

 always accurately represent the size of chromosomes. But 

 the differences in size are so evident to the eye that it is not 

 difficult to decide where each chromosome belongs in the series. 

 To find the 'relative lengths' of the chromosomes, the actual 

 length of each of the six pairs of autosomes was found for all 

 available cells. Then the average length of all the chromosomes 

 in each group (no. 1, no. 2, etc.) was found and the sum of these 

 six averages, for the six groups of autochromosomes, was taken 

 as 100 per cent, this being the average total length of the auto- 

 somes in all the cells of a given class studied. The 'relative 

 length' of any given chromosome was found by comparing its 

 length with the average total length of all six autosomes and is 

 expressed in per cent. The sex chromosome was not included 

 in making up the 100 per cent, but the percentage relation that 

 it bore to the average sum of the autosomes was found in each 

 case. It was kept separate from the other chromosomes be- 

 cause of its great variation in size in different genera, which 

 ranged from no. 1 to no. 5. The autosomes were more uniform 

 in size in this respect and afforded, therefore, a more suitable 

 basis for comparison. 



Explanation of Tables I to XX 



The tables show by horizontal bars the average relative lengths of the six 

 autosomes and the relative length thaf the sex chromosome bears to these in 

 germ cells and in somatic cells for each species of Tettigidae studied. Measure- 

 ments were made from camera lucida drawings of cells illustrated in the plates 

 and from additional cells, not drawn. The intended length of each bar is indi- 

 cated in millimeters at the right of the bar. The upper edge of the bar repre- 

 senting chromosome 6 is marked at centimeter intervals in each table. Consult 

 also the text for further explanations. 



