124 THE AMERICAN NATURALIST [Vol. LVI 



Most of the constitutional changes which go on in the 

 living organism seem to center in the proteins of the 

 protoplasm. Metabolism is largely a question of the dis- 

 ruption and reconstruction of the various cell-proteins. 

 In the cell, moreover, the characteristic protein-complexes 

 themselves determine the nature of the synthesis that shall 

 go on. When synthetic activity is more than sufficient to 

 make good metabolic waste, growth results, and when such 

 increase becomes overgrowth and takes the form of a de- 

 tached individual, we pronounce it reproduction. We are 

 then in position to talk about inheritance — the fact that a 

 new individual possesses the properties and, under similar 

 conditions, therefore, will express the activities and take 

 on the appearances of the earlier or parent form. Thus 

 the germ-cell is a reduplication of the zygote from which 

 it sprang, a detached bit of living matter made up largely 

 of certain characteristic protein-complexes. Even the 

 simplest protein is a huge molecule built up of a series 

 of different kinds of amino-acid " nuclei " which in differ- 

 ent proteins differ in numbers, kinds and arrangements. 

 Certain of them seem necessary to all proteins, others 

 are present in only some proteins. Each amino-acid, be- 

 sides being linked to its fellow, has a replaceable hydrogen 

 atom which may be exchanged for any one of several 

 radicals or side-chains." Furthermore, the ordinary 

 native proteins are secondarily compounded of a number 

 of the simpler blocks formed of linked amino-acids. The 

 unitary amino-acids which enter the blood as protein 

 digestion-products are used as building units again, each 

 cell selecting the kind of units it requires for replace- 

 ments in its own proteins. 



We have no reason to believe that the proteins of the 

 germ-cells have any mysterious powers associated with 

 them that are not shared by any or all of the somatic 

 cells. The modem view of embryogenesis and histo- 

 genesis no longer finds it necessary to picture troops of 

 pangenes departing from their home in the nucleus at 

 just the proper time to take possession of the cytoplasm 



