Chapter XV 



EVIDENCE FROM SOMATIC HISTOGENESIS IN 

 MULTICELLULAR ORGANISMS 



Ayl/ E must now give the greatest possible concreteness 

 ^ '^ to the general truths stated bj Conklin that growth 

 is the result of the interaction between nucleus and cyto- 

 plasm and that heredity includes such fundamental vital 

 processes as assimilation, grow^th, division, and differentia- 

 tion. But the one and only way, I again insist, to attain 

 concreteness and certainty in the matter is through a maxi- 

 mum of observation coupled with a minimum of inference. 

 That is, the goal must be reached mainly by direct study of 

 the anatomical, histological and physiological transforma- 

 tions through which hereditary attributes are produced. The 

 issue can be met squarely only by a still further considera- 

 tion of what we actually know about the participation of 

 all sorts of elements of relatively undifferentiated cells in 

 the production of obviously hereditary parts. The study 

 of interactions between nucleus and cytoplasm and of as- 

 similation growth, etc., in germ-cells is not enough. What 

 we have seen in preceding pages about the development of 

 organs in the protozoa and in spermatozoa is that much 

 toward the end sought. Our task now is to consider the 

 local transformations by which structures are produced in 

 multicellular organisms, especially in those which develop 

 from eggs. 



The Mitochondrial Theory of Heredity 



This task may well begin by examining the recent efforts 

 to locate the "hereditary units" in the mitochondria of the 



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