CHAPTER X 



WHEN DEVELOPMENT GOES WRONG 



Differentiation with development • Underdevelopment, or dwarfing • Over- 

 development, or giants • Arrested development of a single character or 

 part • Overdevelopment of a single part • Doubling of parts • Fusing of 

 parts • When unit characters get misplaced • Abnormal growths 



Differentiation with development. The greatest marvel of 

 development is differentiation. That two nuclei not only from 

 different cells but from different individuals should fuse, absorb 

 food, and divide and subdivide into not hundreds but thousands 

 of others, — all this is wonderful enough, particularly when we 

 remember that without this union neither would be capable of 

 dividing at all.^ 



After all, however, the marvel is that with development comes 

 differentiation ; that is, that the result of growth is not a lump 

 of formless matter. On the contrary, here a leg, there an arm 

 " buds " out ; here an ear and there an eye or a tooth appears ; 

 here a lung forms to take in air and there a heart develops to 

 pump over the body the stream of digested food that wc call the 

 blood, — and so on, bit by bit, the whole complicated structure 

 of the body arises, each part in its proper place ; and not only 

 that, but in general an exceedingly striking resemblance to the 

 particular parentage results, so that the being w^hich develops 

 from a fertilized ovum of the horse, for example, is not only 

 another horse, instead of a cow or a pig, but it is a particular 

 kind of horse, depending upon the special individuals from 

 which he was born. 



1 There are a few exceptions to this statement, but they are concerned with 

 parthenogenesis, which is not involved in the subject matter here under 

 discussion. 



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