xv DEVELOPMENT 315 



the latent manifoldness of the liberated fragment, or 

 sample, or germ-cell. It is the explicating of what is 

 implicit in the fertilised egg-cell, and it seems to most 

 reflective minds one of the major wonders of the world. 

 There are two great processes involved : differentiation 

 or the growing complexity of parts, and integration or the 

 unification and harmonisation of the parts. Differentia- 

 tion is like the extension of a great empire, integration 

 means federation. Integration follows on differentia- 

 tion, and is established in great part by the nervous system, 

 aided by the common medium of the blood and the dis- 

 tribution of the regulative secretions or hormones of 

 ductless glands such as the thyroid and the adrenal. 



How differentiation comes about we do not as yet know. 

 Localised differences of structure appear in the developing 

 embryo, affecting the constitution of the colloidal physical 

 substratum. According to Prof. Child the organism 

 is fundamentally a specific reaction system ("a proto- 

 plasm of specific constitution with a corresponding meta- 

 bolic specificity "), and differences in the rate of meta- 

 bolism initiate the process of organisation, somewhat 

 in the same way as differences in the bed of a river are 



M 



determined by the rate of flow. 



Weismann's view of development was that qualita- 

 tively diverse particles in the germ-plasm are sorted out 

 in proper sequence to the various areas of the embryo, 

 the chromosomes being the carriers and distributors of 

 these materials. The germ-plasm was supposed to dis- 

 integrate in different ways in different body-cells. If 

 we picture the whole inheritance as a collection of many 

 different kinds of seeds, we may think of certain seeds 

 being planted in certain cells and others elsewhere. But 

 the behaviour of the chromosomes does not hint at any 

 sorting-out procedure, and the way in which a fragment 

 may regrow a whole is difficult to explain (without subsidi- 

 ary hypotheses) on Weismann's theory of differentiation. 



Prof. De Vries, Prof. T. H. Morgan, and others have 

 argued from experimental results and microscopical 

 observations that all the hereditarv constituents or 



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factors are present in every cell in the body. Each gets 



