434 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 /is." [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- 



