430 



HEREDITY AND DEVELOPMENT 



FIG. 43. The relation between 

 reproductive cells and the 

 " body." The broken 

 vertical line to the left 

 represents a succession of 

 ova from which " bodies " 

 are produced. The other 

 part of the figure indicates 

 a chain of " bodies," suc- 

 cessive generations. For 

 convenience of the diagram, 

 the " bodies " are repre- 

 sented as if larger at each 

 generation. A sperm fer- 

 tilising an ovum at the 

 beginning of each generation 

 is indicated. 



primary-constituent - substance 

 of the whole organism; and 

 the complexes of primary- 

 constituents necessary to the 

 production of a complete indi- 

 vidual I call Ids." [In some 

 cases these " ids " are probably 

 the chromosomes, but many 

 band-like chromosomes (or 

 " idants ") are visibly com- 

 pound, consisting of several 

 ids.] It is through the co- 

 operation of these ids that the 

 precise constitution of the indi- 

 vidual which develops from the 

 fertilised ovum is determined. 



Every one admits that the 

 germ-cell has a complex or- 

 ganisation, with the details of 

 which every year makes us 

 better acquainted. Every one 

 admits that the whole sub- 

 stance of the fertilised ovum 

 cannot be equally important 

 as regards inheritance. Every 

 one admits that small but still 

 visible units the ids or the 

 chromosomes behave as if 

 they were of fundamental im- 

 portance. If we admit that 

 there is a hereditary substance 

 at all, the theoretical interpre- 

 tation begins when we regard 

 these ids as containing a com- 



