360 I EARLES I:. STOCKARD AND A. L. JOHNSON 



ly localized through the influence of genetic factors which do 

 not disturb other cartilage growths even in near-by regions. 

 It would seem as though there are separate genetic influences 

 for the direction of development in each limited place through- 

 out the general cartilage matrix of the entire skeleton. These 

 are the discreet influences which make possible abnormal 

 and distorted growth of sharply localized regions, such as 

 the upper jaw or the lower jaw, while all other parts of the 

 individual are developing in perfectly normal fashion. Both 

 the normal growth and the modified growth reactions are 

 taking place while in contact with the same body fluids, that 

 is, within the same internal chemical environment. The ele- 

 ments responsible for the local discrepancies in growth are 

 contained within the constitution of the individual tissues 

 themselves. 



The appreciation of these definitely inherited and sharply 

 localized modifications of structural form and quality iu other- 

 wise normal bodies is highly important in comparing the 

 results of these genetic experiments with the ordinary growth 

 disturbances which follow the operative removal of endocrine 

 glands or the administration of glandular extracts. The modi- 

 fications in skull form toward the bulldog type as obtained 

 by Dye and Maughan ('29) following the removal of the 

 thyroid gland in puppies, and the more recent study by Todd 

 and AVharton ( '34) of similar modifications in the skulls of 

 sheep after removal of the thyroid gland from the lamb, are 

 definitely concerned with the general inhibition of skeletal 

 growths resulting from induced alterations of the internal 

 environment. These experimental operations with consequent 

 deficiencies in the internal bodily environment furnish little 

 evidence for analyzing the nature of developmental dishar- 

 monies and sharply localized distortions of form as met with 

 under normally vigorous conditions in the bodies of man, 

 the dogs and many other mammals. The prognathous man- 

 dible or undershot jaw of the thyroidless dog and sheep is 

 not of the same nature as the undershot jaw of the bulldog. 



