EEPRODUCTION AND HEREDITY 303 



lamellae arranged successively one behind the other. According 

 to Weismann, this structure is the rule. He classes each of these 

 tiny plates as an id, and each id is, according to him, not only the 

 carrier of certain heritable characteristics, but contains already 

 the entire hereditary substance, the ' rudiments ' of a complete 

 organism. Most chromosomes or idants contain therefore the 

 entire hereditary substance several times because they are con- 

 structed of several ids. Only in animals with minute globular 

 * nuclear-loops ' can they become identical with an id. 



Each id is said to possess a very complex structure and to 

 be composed of smaller units, the determinants. These deter- 

 minants are ' those parts of the germ-substance which determine 

 a * heritable part ' of the body, i.e., it depends upon their presence 

 in the germ whether a certain part of the body, be it cell-group, 

 single cell, or cell-part, is formed specifically, and the variations 

 of which induce only these definite parts to vary.' 



The regions which are singly determinable by the germ are 

 apparently differing in extent, ' according to whether we have 

 to do with large or small, simple or complex organisms.' The 

 unicellular Infusorians probably possess special determinants 

 for a number of their cell-organella ; in the lower uniformly con- 

 structed multicellular animals the determinants govern probably 

 larger cell-groups and are therefore present in comparatively 

 small numbers, whilst in the higher animals, in particular, insects 

 and vertebrates, the number of determinants must be enormous,, 

 and certainly exceed thousands or even hundreds of thousands, 

 because each independent heritable variation of the organism, 

 no matter how minute, is controlled by one of these determinants. 



We see, for instance, in many families that there occurs in 

 the skin in front of the ear a tiny dimple, barely the size of the 

 head of a pin. Weismann was able to observe the transmission of 

 such dimple from grandmother to son and several grandchildren. 

 In his opinion there must, therefore, be present in the germ-plasm 

 of the relevant persons a corresponding minute heritable part, 

 a determinant which effects the unusual development of this 

 little spot. Similarly, a white tuft in a dark head of hair, which 

 is transmissible by heredity, is said to possess a corresponding 

 determinant in the germ-plasm. 



These determinants, however, are not by any means the 



