2 88 AN INTRODUCTION TO ZOOLOGY 



the germ cells as well as the body cells are affected by the exter- 

 nal stimuli, and the body cells have no effect upon the germ cells. 

 The theory is also apparently valueless in explaining inheritance 

 in the Protozoa (p. 79). 



THE CONTINUITY OF THE GERM PLASM. The present wide- 

 spread belief in the theory of the continuity of the germ plasm is 

 largely due to the efforts of Weismann. We have already illus- 

 trated the theory in a general way, by the life history of Volwx 

 (p. 98, Fig. 47), and in another place (p. 32) have pointed out 

 that the chromosomes are thought by many to be the bearers of 

 hereditary characters. 



According to Weismann, there are " two great categories of 

 living substance hereditary substance or idioplasm, and ' nu- 

 tritive substance ' or trophoplasm " (278, Vol. I, p. 341). He 

 recognizes the chromatin as the idioplasm, and calls the idioplasm 

 of the germ cells, germ plasm. This germ plasm is " never 

 formed de now, but it grows and increases ceaselessly; it is handed 

 on from one generation to another like a long root creeping 

 through the earth, from which at regular distances shoots grow up 

 and become plants, the individuals of the successive generations " 

 (278, Vol. I, p. 416). 



The details of Weismann's theory, and the ingenious explana- 

 tions of such phenomena as cellular differentiation and regenera- 

 tion, are extremely complicated. Briefly stated, the mechanism 

 is conceived by Weismann as follows: Each large chromosome 

 of a germ cell is composed of chromatin granules, called " ids." 

 Each id is either male or female, and contains all of the " com- 

 plexes of primary constituents necessary to the production of a 

 complete individual " (278, p. 349). The production of a par- 

 ticular part of an organism is the role of the primary constituents 

 of the id, and, since these determine the nature of the part pro- 

 duced, they are named " determinants." Finally the determi- 

 nants are composed of one or more particles, called " biophors." 

 These are " far below the limits of visibility," but are " larger 

 than any chemical molecules because they themselves consist of 



