INFLUENCE OF ENVIRONMENT 331 



latter develops mammary glands similar to 

 those of a normal female; in short he is "femin- 

 ized" by the stimulus of substances from the 

 ovary. 



Another gland whose secretions exercise a 

 profound influence on development is the 

 thyroid, which is found in the neck near the 

 "Adam's apple." If this gland becomes en- 

 larged it gives rise to goitre, protruding eye- 

 balls, rapid heart beat ; on the other hand if the 

 thyroid is deficient in a young child it causes a 

 peculiar type of idiotic dwarf known as "cretin" 

 ( Fig. 68 ). If the gland which lies between the 

 roof of the mouth and the base of the brain 

 and which is known as the hypophysis is de- 

 ficient the child, or young animal, remains in- 

 fantile ; if the hypophysis is too large the indi- 

 vidual's hands, feet and face become enlarged 

 and he may grow to be a deformed giant, but 

 with weak body and mind. 



Many cases are known in which the develop- 

 ment of a part is dependent upon the presence 

 of another part; this is technically known as 

 "correlative differentiation." Thus it has been 

 found that the lens of the eye will develop 



