80 The Unity of the Organism 



large number of adult tissues, as the blood, brain and nerves. 

 The nucleic acid of the spermatozoa is said by Cramer to be 

 verysimilar to that of the somatic cells and "probably iden- 

 tical with the nucleic acid prepared from the thymus." " 

 But more interesting than the substances of identical struc- 

 ture which may be extracted from both germ and adult are 

 the series of mixtures of phosphorized fats more complex 

 than lecithin, which are present in the yolk of various eggs, 

 some portions of which disappear as development progresses, 

 seemingly being consumed as a part of the energy of develop- 

 ment.^ Other portions are transformed (?) substances of 

 the same general nature in the tissues.^ 



So far then as concerns substances that are identical in 

 germ-cells and cells of the mature organism, development 

 consists merely or primarily in increasing their quantity. 

 Such substances may consequently be looked upon as an 

 actual realization of the doctrine of preformation which 

 formerly played so great a role in speculative embryology. 

 But the number of substances remaining exactly the same 

 from' the earliest to the latest stages of development is very 

 small in comparison with the number wholly or partly new 

 in the later stages. So that from the chemical standpoint 

 development is for the most part strictly epigenetic ; that 

 is, it is a process not merely of increasing the mass, the 

 quantity, of what previously existed, but as well of coming 

 into existence of new kinds of substance. It is a qualitative 

 as well as a quantitative process. The living, growing or- 

 ganism is creative in the strictest sense and that on a vast 

 scale. "The lout," writes Oliver Wendell Holmes, "who lies 

 stretched on the tavern bench, with just mental activity 

 enough to keep his pipe from going out, is the unconscious 

 tenant of a laboratory where such combinations are con- 

 stantly being made as never Wohler or Berthelot could put 

 together : where such fabrics are woven, such colors dyed, 

 such a commerce carried on ^vith the elements and forces 



