4 CHARLES E. STOCKARD 



reports by Mohr ('26, '29) and Mohr and Wriedt ('30) on 

 hereditary defects in cattle are among the few contributions 

 most closely related to the physical and morphologic side of 

 our present problem. 



In the dwarf mice of Smith and MacDowell we have a 

 simple hereditary deficiency of the pituitary giving an in- 

 ternal environment unfavorable for normal growth and de- 

 velopment. These dwarf individuals, when supplied with 

 pituitary secretion, respond and grow to normal size and 

 adult function, and this reaction to pituitary treatment is 

 exactly comparable to that of the dwarf cretin with deficient 

 thyroid secretion to the administration of thyroid extracts. 

 These cases are examples of the developmental failure of an 

 essential organ through an hereditary defect, just as albinism 

 is due to an hereditary absence of melanin pigment, hemo- 

 philia to the absence of fibrinogen in the blood plasma, and 

 color blindness to a retinal deficiency; all other tissue con- 

 stituents in such individuals may be normally developed. 



An insufficient amount, or the complete absence of thyroid 

 or pituitary secretions, results in a deficient chemical environ- 

 ment within the body of the developing mammal, either before 

 or after birth, and the tissues are unable to grow and develop 

 in the usual manner in this defective milieu. However, if the 

 missing substances are added to the environment, the animal 

 may recover its normal development. 



The growth reaction of the dwarf mice in response to 

 pituitary treatment has not been interpreted by all investi- 

 gators in so general a way. Some writers have claimed this 

 reaction to indicate that the pituitary secretion contains a 

 growth stimulating substance; yet such a growth stimulating 

 hormone has not been postulated for the thyroid even though 

 it brings about an equally remarkable growth response when 

 administered to the cretin. It could be claimed, however, 

 that the growth of the cretin results from a stimulating effect 

 upon the pituitary caused by the thyroid administration, but 

 there are many facts to contradict such a claim. A more 

 acceptable explanation for both cases is that the addition 



