410 THE EVOLUTION THEORY 



on to show what the theory can accomplish in clarifying our under- 

 standing of the phenomena of reproduction and heredity. I shall at 

 the sam« time give a brief exposition of some of the most important 

 of these phenomena. 



First, a few words in regard to the development of the repro- 

 ductive cells. We may leave aside in the meantime the question 

 whether they are sexually differentiated or not ; we are only concerned 

 just now with the main problem : How is it possible for the organism 

 to produce germ -cells, that is, cells which contain the complete germ- 

 plasm with all its determinants, when the building up of the body 

 in ontogeny, according to our theory, involves a disintegration or 

 segregation of the determinant-architecture into smaller and smaller 

 groups'? It is impossible that specific determinants^ should arise de 

 novo, just as an animal cannot arise otherwise than from its germ, nor 

 a cell otherwise than from a cell, nor a nucleus otherwise than from 

 an already existing nucleus. If vital units ever originate de novo at 

 all, it is only conceivable in the case of the very simplest biophors, as 

 we shall see later when we come to speak of ' Spontaneous Generation.' 

 Specific biophors and the determinants composed of them have behind 

 them a phylogeny, a history, which conditions that they shall arise only 

 from their like. 



Thus we see that germ-cells can only arise where all the deter- 

 minants of the relevant species arranged as ids are already present. 

 If we could assume that the ovum, just beginning to develop, divides 

 at its first cleavage into two cells, one of which gives rise to the 

 whole body (soma) and the other only to the germ-cells lying in this 

 body, the matter would be theoretically simple. We should say, the 

 germ-plasm of the ovum first doubles itself by growth, as the nuclear 

 substance does at every nuclear division, and then divides into two 

 similar halves, one of which, lying in the primordial somatic cell, 

 becomes at once active and breaks up into smaller and smaller groups 

 of determinants corresponding to the building up of the body, while 

 the germ-plasm in the other remains in a more or less ' bound ' or ' set ' 

 condition, and is only active to the extent of gradually stamping as 

 germ-cells the cells which arise from the primordial germ-cell. 



As yet, however, only one group of animals is known to behave 

 demonstrably in this manner, the Diptera among insects ; in all others 

 the cell from which the germ-cells exclusively arise, the ' primordial 

 germ-cell,' makes its appearance later in development, usually during 

 embryogenesis and often very early in it, after the first few divisions 

 of the ovum, but sometimes not till long after the end of embryo- 

 genesis, and not even in the individual which arises from the ovum. 



