442 V. THE NATURE OF EVOLUTION 



The gene of a germ cell thus reduced to the primitive stage can 

 again recover its previous, advanced structure owing to the reversible 

 character, when factors or environmental conditions are provided which 

 favour the development of the advanced structure. The recovering 

 change is generally initiated by fertilization, but the fertilization can 

 be substituted by a proper physical or a chemical stimulus, which may 

 act like a mechanical stir in the case of the transformation of super- 

 cooled water into ice. 



Various characters are revealed in a developing individual with 

 the progress of the recovering change of the genes, because the proto- 

 plasm is looked upon as a kind of liquid crystal, whose crystal shape 

 is determined by the structure of protein components, which is in turn 

 controlled by the genes. The change is continued until the complete 

 recovery of the advanced structure is established, whereby various 

 morphological and functional characters are successively revealed with 

 the progress of the change. 



Regeneration, which is common to plants and some lower animals, 

 can be explained in the same way. The somatic cells in the site of 

 cutting are reduced in their structure to a primitive, undifferentiated 

 state, from which subsequently the differentiated structure is anew 

 recovered. Regeneration, therefore, is dependent upon the reversible 

 character of protoplasm protein just as ontogeny. 



The formation of organs or tissues in the various body portions 

 during the ontogeny is apparently due to the environmental effects 

 varying with the portion. Since, as above cited, protein structures are 

 determined by environmental factors, different environmental effects 

 produce different structures which in turn iuduce different organs or 

 tissues. 



Different structures thus produced in the different places of develop- 

 ing body are unstable at the start, and tend to change to other 

 structures if the environments are changed, but by a prolonged ex- 

 posure to the same environment the structures are gradually fixed and 

 become so stable that they can retain their specific structure even 

 when transplanted to another part. The so-called organizer is an 

 embryonal part whose structure has been thus fixed. 



The protein of organizer, if liberated as protoplasm particles, may 

 be able to transfer its specific structure to other unfixed parts of the 

 embryo as a "virus", but the protein molecules, which fail to form 

 virus-like particles, or other components, such as steroid, liberated 

 from the organizer may behave as hormones by exerting on the sur- 

 rounding embryonal parts their structural effect which is peculiar to 

 the organizer, though they are not so powerful as being capable of 

 transmitting the specific structure itself as a "vii-us". Thus, if a 



