V. THE NATURE OF EVOLUTION 



4. The Significanca of Fertilization 



The germ cell with primitve structure returns to its differentiated 

 state through the development and this developmental process is 

 generally initiated by fertilization. Fertilization is comparable to the 

 application of a stimulus to super-cooled water, which will be frozen 

 by the stimulus ; super-cooled water corresponds to the egg cell and 

 the stimulus to the sperm. 



Somatic cells or mother germ cells are reduced to their primi- 

 tiveness to produce egg cells, but when factors are removed which 

 caused the reduction and the eggs are brought under the condition 

 which favours the differentiated structure, the egg cells may be in a 

 metastable state, tending to return to the original, differentiated state. 

 This is the reason why germ cells are compared to super-cooled 

 water. 



The sperm cell may act as a stimulus to initiate the recovering 

 change because of its structure somewhat different from that of the 

 €gg. Similar effect can be provided by numerous forms of artificial 

 means, such as subjection of the egg to certain chemicals, to changes 

 in temperature or in density of the surrounding water, or even to 

 mild shaking, or to the prick of a needle; all these can initiate fertili- 

 zation as does the sperm. Natural parthenogenesis, which is common 

 among invertebrates, must be caused by a proper stimulus provided 

 by a natural process. 



The development occurs in general only from the egg cell because 

 the egg, in contrast to the sperm, contains a plenty of cytoplasmic 

 substances, which will provide the energy and substances required for 

 the development. Sperm cells like eggs must be in a metastable state, 

 tending to return to the original specialized state, so that they can 

 develop like eggs if cytoplasmic substances are provided. It is actual- 

 ly known that if a sperm is inserted into an egg cell from which the 

 nucleus was preliminarily removed, the development Avill take place 

 from the sperm, a phenomenon known as merogeny. 



If the returning process of a germ cell to its original state is 

 once inititated by a stimulus of a sperm or of another factor, the gene 

 in the germ cell will start on a gradual change towards the developed 

 structure, and the protoplasm or the embr^'o will be compelled to take 

 one form after another in succession, in response to the structure ap- 

 pearing successively in the returning process, since the form of an 

 organism is determined by the "crystal shape" of protoplasm, which 

 in turn is governed by the structure of the gene. The cell division 

 may be required for the acquisition of the compelled form. 



