THE THYROID GLAND AND 

 DEVELOPMENT 



I'^KKDKKICK S. IL\M.MKTT 



Tlic U'istar Institute of .hiatoiiix and f'loloc/y, Pliiladclphia 



DEVELOPMENT is l)()th qualita- 

 tive and quantitative. The type 

 and functional aspects of devel- 

 opment arc qualitative. The extent 

 and rate of development are quantita- 

 tive. Qualitative development is spe- 

 cific and normally fixed. Quantitative 

 development is specific as to its limita- 

 tions but is subject to variation due 

 to the environmental influences of diet, 

 exercise, toxemia, and so forth. The 

 fundamental type of structure and 

 function characteristic for a given 

 species is w^hat sets the species apart. 

 The differences in extent and rate of 

 development of individual organisms 

 within a species are causes of the 

 variations within the species. These 

 variations may be proportionate or 

 disproportionate. Proportionate varia- 

 tion occurs when inter-organic differ- 

 • ential development differs in absolute 

 but not in relative value. Such varia- 

 tion gives rise to a series of individ- 

 uals differing in absolute size, but not 

 in the relative sizes of the constituent 

 l)arts of the individual. Dispropor- 

 tionate variation occurs when the rate 

 or extent of development of one or 

 more parts of an individual with re- 

 spect to other parts of the same indiv- 

 ual. differ from the rate or extent of 

 development of the same i)arts in an- 

 other individual with respect to its 

 other parts. 



Disproportionate developmental var- 

 iation gives rise to groups of individ- 

 uals which for want of a better term 

 have been designated as "types" with- 

 in a species. These "types" differ 

 from each other in bodily proportions, 

 but the fundamental specific type is 

 the same in all. That is to say. the 

 qualitative development is the same, 



while the ((uantitative dift'erential de- 

 velojjment is different. An instructive 

 classification along these latter lines 

 has been given by Stockard in his 

 broad division of man into the "Lin- 

 ear" and the "Lateral" type of struc- 

 ture. 



Much interesting speculation has 

 been made of the role of the thyroid 

 gland in "type" development of man. 

 This, as should be clear from the fore- 

 going, is actually a question of the 

 part played by thyroid activity in 

 differential development, which in turn 

 is largely a phase of quantitative, not 

 of qualitative development. Although 

 it is possible that thyroid function is 

 associated with qualitative develop- 

 ment, there is no evidence at present 

 available which would justify the allo- 

 cation of any definite influence of that 

 gland on this phase of growth. 



On the other hand, it is possible 

 to show that thyroid activity is a 

 factor of great importance in quanti- 

 tative differential development. It 

 seems best, therefore, at the present 

 time, to confine ourselves to an inter- 

 pretation of known facts, rather than 

 to attempt the elaboration of an hy- 

 pothesis for which no adequate ex])er- 

 imental data are available. 



Other things being equal, quantita- 

 tive development depends on the kind 

 and amount of food ingested by the 

 individual, although there is. of course, 

 a specific limitation of the size to 

 which an organism can grow. Quan- 

 titative development is thus intimately 

 associated with metabolism. There is 

 a certain fundamental type of meta- 

 bolism common to all the living cells of 

 the organism. It is known that a 

 function of the thvroid gland is con- 



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