DIFFERENTIATION, HEREDITY, SEX 



291 



ters that are family possessions and may or may not be inherited 

 by individuals. These individual characteristics are, in many 

 cases, comparatively late developments. The early characters 

 are those of the larger group; those of the species appear later, 

 and finally the family and individual traits. The whole sub- 

 ject of Mendelism has developed into an extremely complicated 

 system, in directions largely unforeseen. And yet it is hardly 

 too much to say that the cytology of the germ cells and their 

 nuclei has on the whole fairly kept pace, and it is in most in- 

 stances quite possible to parallel the facts of Mendelism with the 

 facts of chromosome behavior. We shall return briefly to this 

 subject in a more appropriate connection. 



FIG. 137. The structure of chromosomes. A, after K. C. Schneider, others 

 after Bonne vie. A. Nucleus from epidermis of Salamander larva, in telophase. 

 B. Prophase of first cleavage of Ascaris megalocephala bivalens. C. Nucleus 

 from cleavage stage of same. D. Interkinesis in Amphiuma. 



Let us now repeat, from this particular point of view the 

 general ideas regarding the chromosomes mentioned in Chapter 

 II, emphasizing certain topics and adding a few details which 

 bear directly upon the relation of the chromosomes to the proc- 

 esses of development and heredity. 



The chromosomes are not structurally homogeneous masses, 

 but are built up of certain granules which often have a definite 

 arrangement giving the chromosome as a whole a general 

 structure. This structure has been variously described (Fig. 

 137); in some cases it seems to be a cylinder of chromatin 



