INDIVIDUALITY OF CHROMOSOMES. 9 



To test this I sketched twenty-one groups of chromosomes 

 that showed a variety of shapes and classified them into rings, 

 crosses, rods, brackets, parenthesis, f's, S's and e's. I found 

 that but two groups showed no rings and eleven groups showed 

 two rings, but in no case were there more than two. Five cases 

 showed no crosses and nine cases showed two. 



It is evident that any one of the shapes represented may be 

 turned in such a way in the cell that it would appear as a straight 

 rod. This might account for the non-appearance of the rings, or 

 any of the other forms in my sketches. I then tried the hypoth- 

 esis that there were two rings in each cell and tested it as follows : 



To find out what per cent, of rings should appear I proceeded 

 thus. The ring is formed of a cylindrical rod. Measuring a 

 number of rings with the eye-piece micronometer I found that 

 the opening was about one third of the diameter. I then 

 moulded a ring of modeling clay making the rim of a rod whose 

 diameter was equal to that of the central opening. By turning 

 this doughnut-like ring on a sheet of paper it was found that the 

 opening was not visible for about 67 of the semicircle. Sixty- 

 seven and one half degrees is three-eighths of a semicircle, hence 

 three rings out of every eight should not show any opening. 

 But as the ring becomes wide enough to be distinguished as a 

 ring before the opening can be seen, it seemed possible to esti- 

 mate the percentage of rings that should be recognizable. By 

 using the above ring of clay and testing several of the co-workers 

 in the laboratory as to the angle at which they could be reason- 

 ably sure that it was a ring and not a rod they were seeing it 

 was found that on an average the ring could not be recognized 

 for fourty-five degrees of the semicircle. Fourty-five degrees is 

 one-fourth of one hundred and eighty degrees, or one ring in 

 four should not be recognizable. 



Of the twenty-one sketched groups ten showed all the ordi- 

 nary chromosomes. If there are two rings in each cell, accord- 

 ing to the hypothesis there should be 20 present, of which i 5 

 should be recognizable. My table shows 13. 



I then sketched 25 cells, all showing the full number of 

 chromosomes, 10. These should have 50 rings, three-fourths 

 of which, or 37^, should appear. After completing my table I 



