STRUCTURE OF THE VERTEBRATES 321 



and differentiation, for the same result is often obtained by the 

 operative removal of different glands. Infantilism, for example, 

 results from the removal of either the thyroid or the pituitary; 

 and thyroidless animals may be brought to sexual maturity by 

 the injection of the extract of the pituitary. In like manner 

 the removal of the sex glands affects the structure and function 

 of the pituitary, and the excision of the pituitary prevents the 

 full development of the gonads. 



So frequently and so intimately is the pituitary linked with 

 the glands causing morphological changes that it is often called 

 the "master gland". It affects sex development and bodily size, 

 the effects appearing together in one individual, or independ- 

 ently of each other; and, conversely, other glands are necessary 

 for the proper function of the pituitary. 



From our present knowledge it seems justified to speak of the 

 balance of glandular function; and any imbalance existing in 

 the group will cause widespread morphological and physio- 

 logical upsets. The subject is still in its infancy, and far-reach- 

 ing conclusions are not warranted by the evidence. 



Conclusion. The three mentioned regulators of development 

 are not independent or mutually exclusive. The original pattern 

 within the egg may be modified in its expression by the organ- 

 izators, and both may be shifted by the function of the ductless 

 glands; but under normal circumstances all work interdepend- 

 ently and synchronously. The endocrines are, perhaps, most 

 readily affected by external, environmental causes; but in the 

 final analysis all are but the expression of the inherited tenden- 

 cies, the genetic pattern. The frequent statements regarding the 

 ''effects of the endocrines upon evolution" are but another way 

 of saying, "the effects of heredity upon the glands, and thus 

 upon evolution". 



Gland complexes are as definitely inherited as are e^'e color or 

 hair conditions. A recent discovery (MacDowell) proves that a 

 single point mutation may affect the pituitary so that infantil- 

 ism results; and although the individual may be brought to 

 normality by gland injections, the genetic mutation remains 

 unaffected and is passed on in a Mendelian manner. Therefore 

 the endocrines, like all other known characters, are the expres- 

 sion of heredity modified by environment. 



