152 



CAUSES OF VARIATION 



is of rare occurrence, is never known in embryonic tissues, and 

 is characteristic of tissues "on the way towards degeneration." 1 



4. Quite generally unicellular organisms display extreme 

 irregularities in mitosis, some species omitting one and others 

 another of the processes typical in higher species. Prominent 

 among these deviations is the failure of chromatin granules to 

 unite to form definite chromosomes. In place of this the indi- 

 vidual granules themselves divide, suggesting that fission of 

 the granules is the elementary and essential feature of nuclear 

 division?" 



Other minor deviations are known, though much of the 

 field is yet unworked. These may account to some extent for 

 differentiation during cell multiplication, and yet so far as is at 

 present known all processes that do not accomplish an equitable 

 division of the chromatin granules through the splitting process 

 are looked upon as distinctly pathological. Normal cell division, 

 therefore, seems to be in the interest of constancy, not differentia- 

 tion, and what power it is that produces one sort of tissue from 

 another, as must happen in the developing embryo, is still a 

 mystery. 



SECTION V PHYSIOLOGICAL UNITS 



This difficulty has led to the assumption of some sort of phys- 

 iological units, some of which are active at certain stages of 

 development, others at other stages ; and the chromatin granules 

 whose qualitative division is in most cases carefully insured 

 are quite generally regarded as the repository of these units 

 and the common vehicle of hereditary transmission. Such were 

 the gemmules of Darwin, 3 the stirp of Galton, the idioplasm of 

 Nageli, and the determinants of Weismann. 4 



1 Wilson, The Cell, pp. 116-121. 2 ibid. p. 90. 



8 See Darwin, Animals and Plants, chap, xxvii. 



4 Weismann's elaborate theory of heredity regarded the germ plasm as the 

 original substance, of which the body is the natural expansion. This "ancestral 

 germ plasm " is unchanging, unchangeable, and, so long as the species endures, is 

 immortal. He regards this germ plasm as comprised ultimately of "biophors" 

 (life bearers), which may be spoken of as living molecules. These biophors, or 

 ultimate units, are combined in an orderly manner into " determinants," whose 

 activity at development determines what the particular part shall be. These 



