352 V. THE NATURE OF EVOLUTION 



cules arranged in parallel alignment, the cell itself should have a 

 structural polarity. Harrison (25) believes that the polarity is based 

 upon the dipole character of protein molecules. 



Such a polarity, due to the asymmetrical character of protein 

 molecules and numerous environmental factors as well, may control 

 the manner of distribution of some components, such as yolk and nu- 

 cleus, causing a dissimilarity of the cell mass, resulting in different 

 structures according to different parts of the cell. The differentiation 

 of a fertilized egg cell may thus start. At the beginning, however, 

 the differentiation appears to be insignificant and easily reversible. 

 As described already a newly formed structure is at first reversible 

 and labile, but if the structure remains long under the same environ- 

 ment favouring the structure, it will "forget" the former structure 

 becoming irreversible and stable with the establishment of the "adap- 

 tation" to the environment. 



For example, at an early stage of development, when a piece of 

 the tissue which is to develop into A-organ is transplanted to another 

 part where B-organ is to be developed, the piece will develop into B 

 not into A. Thus specific differentiation seems not developed at an 

 early stage, every cell of the embryo appearing capable of producing 

 every organ according to the region into which it is planted. That 

 is to say, each piece of tissue develops into the kind suited to the new 

 region, and not in accordance with the place of its origin. The lack 

 of specific differentiation is well shown by the fact that, if the first 

 two cells of the frog embryo are separated, each gives rise to a whole 

 embryo, although in some other animals, each of the first two cells 

 will develop only half an embryo if separated. 



However, as development progresses, certain cell masses are local- 

 ized and differentiated into specific organs, and after localization is com- 

 pletely established in a region, differentiation is apparently independent 

 of the influence of another region onto which it is grafted. At such 

 a stage, many embryonic tissues, once started on their course of 

 specialization, will continue the specialization as self-differentiating 

 structures even when removed from the body and grown in artificJal 

 cultures. 



Such a fixation of structure should be compared to the evolution 

 of viruses. Newly generated viruses are weak and labile, but gradu- 

 ally become stronger and stable, acquiring a faculty not only to retain 

 their pattern firmly, but to transfer their structural pattern to the 

 surrounding protoplasm. 



A group of cells, such as those of dorsal lip of the blastopore, 

 known as an organizer, can be regarded as part of embryo which m.ay 

 first acquire a highly stable structure comparable to a strong \irus. 



