CHAPTER XXXV 

 DEVELOPMENT 



"... I compared the cell-growth, by which Nature builds up a plant or an 

 animal, to the glass-blower's similar mode of beginning, always with a hollow 

 sphere, or vesicle, whatever he is going to make." 



OLIVER WENDELL HOLMES. 



OCCURRING simultaneously with increase in size, are changes 

 in external form and internal structure the organism develops. 

 Mainly through the brilliant researches of J. Loeb and his school, 

 some light has been thrown on this seemingly mysterious and 

 apparently inexplicable process. The changes taking place are 

 most readily perceived when the transparent eggs of the echino- 

 derms are used as the material on which to experiment, and 

 consequently, our ideas of the processes involved in mammalian 

 development are largely derived from the study of processes in 

 the lower aquatic animals , which may or may not be quite 

 analogous. 



The unfertilised ovum is a moribund body which disintegrates 

 more or less rapidly. If, before disintegrative processes have 

 become apparent, the egg undergoes fertilisation, destruction is 

 stayed. The fertilised egg develops, grows and becomes differ- 

 entiated into various structures. This process of differentiation 

 of protoplasm is an orderly one, taking place always in the same 

 manner and being modified always by the same conditions. 



On the entry of the spermatozoon, some change in the free 

 energy of the egg must take place. The egg is no longer static 

 but becomes endowed with dynamic force. In order to discover 

 the underlying physico-chemical change, Loeb attempted to induce 

 development of unfertilised eggs by alteration of the environ- 

 mental conditions. No change in a system in equilibrium can 

 take place unless the relative amount or incidence of the free 

 energy of the environment is first altered. The two series of 



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