CHAPTER 11 



Abnormal Growth 



The basic problem of morphogenesis, as stated frequently in the preced- 

 ing chapters, is posed by the fact that every organism is an organized 

 system that in its development tends to produce forms and structures of 

 specific character. Within each organism there seems to be a norm toward 

 which its development conforms. The expression of this norm, however, 

 may vary greatly as the result of a wide range of environmental factors. 

 Such variation is familiar and to be expected. There are many cases, 

 however, where divergence from the norm is so great that we usually 

 speak of them as "abnormal" or "atypical." Just what do we mean by 

 these terms? 



Every organism, and doubtless every cell, has a far wider range of 

 development potencies than it generally displays. When conditions are 

 different from the usual ones, the expression of its norm is also different, 

 but the norm is just as specific as before and the organism still per- 

 sistently regulates its development in such a way that the characteristic 

 form for that environment is produced. Norms of the individuals in a 

 given species are much the same. The basis of each is the genetic con- 

 stitution of the organism. It is well understood by geneticists, however, 

 that what a particular gene determines is not a particular character 

 but a particular reaction to a particular environment, external or in- 

 ternal. In many cases it requires very special environmental factors, 

 such as wounding, irradiation, application of growth substances, or 

 deposition of an insect egg to bring to expression developmental poten- 

 cies which would otherwise remain latent. 



Under such unusual conditions, or from genotypes which are markedly 

 different from those of most members of the species, individuals some- 

 times develop which are so unlike ordinary ones that they are called 

 "abnormal." This does not mean that they are exceptions to the general 

 biological determination that controls the growth of all living things. 

 Neither do they constitute a sharply defined group set apart from all 

 others, for every gradation between normal and abnormal may be found. 

 One may be uncertain, for example, as to whether the occasional pro- 



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