INFLUENCE OF ENVIRONMENT 339 



male which has been previously castrated, the 

 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 

 the peculiar type of idiotic dwarf known as 

 "cretin." 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 



