THE GERM-PLASM 3 



being of species which are normally cross-fertilized, it is not a 

 necessary antecedent of vigour, and cannot have arisen as a means 

 to that end. At present we are hardly in a position to discuss 

 this subject. Later, when we have collected a large array of facts, 

 we shall be able to deal with it to greater advantage. 



4. The multicellular individual, then, consists of various kinds 

 of cells (each of which corresponds to a unicellular organism) which 

 perform different functions, each kind its own special function. 

 Some of these cells are germ-cells, the rest are body or ' somatic ' 

 cells. The latter, which are usually much the more numerous, 

 provide the former with shelter and nutrition, much as worker 

 bees shelter and nourish queens and drones. They never unite, 

 never ' conjugate,' in the way that sperms and ova do. Only the 

 germs are marriageable ; and, as we have just seen, in the great 

 majority of animals and plants they observe the degrees of con- 

 sanguinity very strictly, and do not unite except with members of 

 another cell-community, and then only to found a new colony of 

 cells, an offspring. 



5. In each sperm and ovum is a dot, the nucleus. The essential 

 feature of the union of sperm and ovum is now believed to be the 

 union, the intimate mixture, of their nuclei, so that the two nuclei 

 become one. Under high powers of the microscope there may be 

 seen within the nucleus specks and threads of a substance known 

 as chromatin. When the fertilized ovum and its descendant cells 

 divide into daughter-cells, the chromatin, which grows with the 

 cells, displays remarkable movements, and is distributed, ap- 

 parently with great quantitative equality, between the daughters, 

 thus forming their nuclei. Now this chromatin, contained in the 

 germ-cells, is believed with some reason to be the germ-plasm, the 

 ' bearer of heredity,' the substance that carries the ' hereditary 

 tendencies,' 1 which direct development, and thus determine the 

 kind of individuals that shall arise from the germ-cells. A man 

 differs greatly from an elephant because the germ-plasms of the 



1 1 fear the expression ' hereditary tendency ' is somewhat clumsy. I use it 

 for lack of a better and imply nothing mystical. I mean no more by it than those 

 potentialities for development which are carried by germ-cells. These poten- 

 tialities differ, of course, with every species, and indeed with every individual. 

 Thus the hereditary tendencies of a man are those which enable him to develop 

 from a human germ into a human being. He does not develop into a rabbit 

 or a tree, because the hereditary tendencies of human germs are such that rabbits 

 and trees cannot arise from them. From a developmental point of view the germ- 

 plasm is compounded of structures carrying hereditary tendencies. By this, 

 however, I do not mean that definite structures in the germ-plasm carry definite 

 tendencies, each structure a tendency. I know nothing about that. 



