v CHROMOMERES 137 



constant in their arrangement in a given chromosome. Fig. 66 shows 

 one example of the corresponding chromosome from thirteen different 

 individuals. It is taken from the pachytene stage of spermatogenesis, 

 and is therefore bivalent, as indicated by frequent signs of duplicity. 

 The five principal chromomeres are numbered 1-5 and it will be seen 

 how noticeably constant in arrangement they are. This regularity 

 extends also to the smaller chromomeres. For instance, in the segment 

 between Nos. 3 and 4 there are always two small granules of about the 

 same size, while there are never any prominent ones between Nos. 2 

 and 3. 



Of course, the constancy is not perfect. A certain amount of variation 

 in the relative sizes of the principal chromomeres, and in the lengths of 



5 



FIG. 66. 



Various examples of the same chromosome in Phrynotettix magnus. (After Wenrich, B.M.C.Z.H., 1916.) 

 A, the chromosome (bivalent) from the pachytene stage of thirteen different individuals. The principal 

 chromomeres are numbered 1-5. * B, successive stages in the contraction of the pachytene chromosome to form 

 the definitive chromosome of metaphase I. 



the segments separating them, can be observed, as well as in the number 

 and arrangement of the smaller granules in between. This variation 

 may be due to several causes, partly to errors of technique for instance, 

 distortion by fixing agents, optical effects, etc. partly to difference in 

 the extent to which fusion of smaller granules to form larger ones has 

 proceeded, but partly probably to real biological variation. The thesis 

 outlined at the beginning of this chapter requires that all genetic differ- 

 ences in organisms should be referred to preceding variation in the 

 idioplasmic elements, and hence it is no more surprising to find variation 

 in homologous chromosomes than in the somatic characteristics of 

 organisms. 



Wenrich finds indeed, in the case of the particular chromosome under 

 consideration, that the chromomere numbered 5 is often absent. In 



