70 THE GERM-PLASM 



stages of development, exert no influence on the cell. As each 

 determinant consists of many biophors. it must be considerably 

 larger than a biophor, and is probably therefore unable to pass 

 out through the pores of the nuclear membrane, which we must 

 suppose to be very small and only adapted for the passage oL 

 the biophors. Although it is impossible to make any definite 

 statement with regard to the internal structure of the determi- 

 nants, it must be owing to this structure that each determinant 

 only breaks up into biophors when it reaches the cell to be 

 determined b\' it. We may suppose that, just as one fruit on a 

 tree ripens more quickly than another, even when the same 

 external influences act on both, so also one sort of determinant 

 may mature sooner than another, although similar nourishment 

 is supplied to both. 



It must not, however, be overlooked, that a difference in the 

 time of maturation of the determinants in the embryogeny of 

 animals is chiefly to be assumed only in the case of the actual 

 embryonic cells : for the histological differentiation of the cells 

 of the body, and the differentiation of the parts of the latter, 

 occur at about the same time ; that is, not until the organs 

 already exist as definite groups of cells. This is equivalent to 

 saying that the disintegration into biophors occurs when the 

 id only contains the single determinant which controls that par- 

 ticular kind of cell. It is well known how suddenly the histolog- 

 ical differentiation of the cells occurs in the embryogeny of an 

 animal. For a long time the various parts and tissues are very 

 similar to one another, though not perfectly so, and then histo- 

 logical differentiation suddeulv sets in. This is verv markedlv 

 the case as regards the transversely striped muscles of Arthropods 

 and Vertebrates, in which the contractile substance is first seen 

 as a mere narrow ring around the cell, and then gradually 

 becomes thicker, so as to replace the greater portion of the cell- 

 body. — just as one would expect if it were caused by muscle- 

 biophors which had migrated into the cell-substance and there 

 multiplied. 



The assumption of a ' ripening ' of the determinants, which 

 though not simultaneous, is yet exactly regulated, nevertheless 

 remains indispensable : or, to express it differently, we must 

 assume that tlie determinants pass througli a strictly regulated 

 period of inactivity, at the close of which the disintegration into 

 biophors sets in. The determinants certainlv continue to grow 



