ON PHYSIOLOGICAL MORPHOLOGY. 45 



III. THE MECHANICS OF GROWTH IN ANIMALS. 



In order to get an explanation of the phenomena of organiza- 

 tion we must ask, What are the physical forces that determine 

 the formation of a new organ ? We know that the ultimate 

 sources of energy for all the functions of living bodies are 

 chemical processes. The question is, How can these chemical 

 forces be brought into relation with the visible changes which 

 take place in the formation of a new organ ? The answer to 

 this question is to be obtained by a knowledge of the mechanics 

 of growth. It is very remarkable that the mechanics of growth 

 forms almost an empty page in the history of animal morphol- 

 ogy and physiology. I can refer here only to the few experi- 

 ments that I have made on this subject ; but fortunately the 

 subject has been worked out very carefully in plants, and as 

 my experiments show that the conditions for growth in animals 

 are, to a certain extent at least, the same as the conditions for 

 growth in plants, we have the beginning of a basis for work. 



A brief outline of the manner of growth in plants is as fol- 

 lows : Before the cell grows it forms substances which attract 

 water from the surroundings, or, as the physicist expresses it, 

 it forms substances which determine a higher osmotic pressure 

 within the cell than did the substances from which they origi- 

 nate. The walls of the cell, or rather the protoplasmic layer 

 that lines the cell wall, possesses peculiar osmotic properties, 

 in consequence of which it allows molecules of water to pass 

 through freely while remaining resistant to the passing through 

 of the molecules of many salts dissolved in the water. The 

 result is that when substances of higher osmotic pressure are 

 formed inside the cell, water from the outside passes in 

 until the pressure within again equals the pressure without. 

 The cell-wall becomes stretched and, according to Traube, 

 new material is precipitated in the enlarged interstices, 

 thus rendering growth permanent. This method of growth is 

 most conspicuous, perhaps, in the germinating seed. The rising 

 temperature in spring produces in the seed substances of higher 

 osmotic pressure (with greater attraction for water) than the 

 substances from which they originate. The result is that 



