Forms Having an Extra Chromosome. 31 



small chromosome fragment, others may have lost a fragment ; and 

 this renders probable another type of germinal change, in which the 

 redistribution of fragments of chromatin may take place. It was 

 formerly pointed out (Gates and Thomas 1914) that all these 

 "irregularities" will become the basis of germinal changes if they 

 are perpetuated by mitosis. Some of the differences between 15- 

 chromosome types may yet be explained on this basis. 



This brings us to another subject closely connected with the 

 extra chromosome, but which concerns also the question of 

 chromosome constancy in (Enothera. Hance (1918), in the paper 

 already referred to, has studied variations in the number of 

 somatic chromosomes in (E. scintillans. The writer 1 first pointed 

 out such variations in the somatic cells of CE. lata, the number 

 ranging from 12 to 20 or 21. Hance finds that the number of 

 separate bodies in metaphase ranges from 15 to 21, the higher 

 numbers being produced by transverse segmentation of certain of 

 the chromosomes. This matter is of minor importance and will 

 be referred to later. A fact of more interest is that more or less 

 constant differences are found between the lengths of the different 

 chromosome pairs in the group of 15. The pairs of chromosomes 

 form a graded series, each somatic pair differing in length from 

 the next longer by about 9%. This conclusion is based on careful 

 measurements of enlarged drawings of the metaphase chromo- 

 some group in 114 somatic cells from different individuals, selected 

 on account of their special clearness and flatness. Of these cells 

 51 or 45% contained 15 chromosomes, and in each case the sum of 

 the lengths of all the chromosomes was approximately the same, 

 showing that increase in number was due to transverse fragment- 

 ation. This demonstration of constant differences in the length 

 of the somatic chromosomes is an important advance in our 

 knowledge of (Enothera chromosomes. While differences in size 

 have been previously observed, they have been assumed to be due 

 merely to fluctuation, not being great enough to demonstrate 

 except by comparison of measurements from many cells. 



Hance assumes that the odd or extra chromosome in 

 CE. scintillans is the shortest of the 15. It might equally well be 

 the longest so far as his data show. His only direct evidence is 

 the difference in total length of the chromosomes in the two 

 telophase groups of 7 and 8 chromosomes respectively in the 

 heterotypic mitosis of the pollen mother cells, This difference is 



1 Gates 1912b, 



