Influence of Environment 233 



characters of the male are potential in the female but are kept 

 from developing or are inhibited by the activity of the ovary. 

 On the other hand the castration of the young cock does not 

 prevent the development of most of the secondary sexual char- 

 acters of the male. In the case of mammals removal of the ovar- 

 ies of a young female or of the testes of a young male does not 

 lead to the development of the secondary sexual characters of 

 the other sex, but both sexes remain in a sexually undeveloped or 

 infantile condition, that is, the presence of ovaries or testes serves 

 as stimulus to call forth the development of the secondary sexual 

 characters in mammals, and not as inhibitors to prevent the de- 

 velopment of the secondary sexual characters of the opposite sex, 

 as in the female fowl. If bits of the ovary of a guinea-pig are 

 inserted under the skin of a young male which has been pre- 

 viously castrated, the latter develops mammary glands similar to 

 those of a normal female; in short he is "feminized" by the stim- 

 ulus 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 the secretion of this gland is over-abundant 

 it causes rapid heart-beat, higher temperature, increased activity, 

 and in general a higher rate of metabolism, and if the gland be- 

 comes much enlarged it gives rise in addition to goitre and pro- 

 truding eyeballs. Excess of the thyroid secretion hastens certain 

 processes of development; young tadpoles that have been fed on 

 thyroid gland or its extract quickly transform into frogs which 

 are sometimes "no larger than flies" (Gudernatsch). On the other 

 hand if the thyroid secretion is deficient the rate of metabolism is 

 decreased, body temperature is lowered, muscular movements be- 

 come slow and awkward, the skin may become swollen with mu- 

 cous (myxedema) and the mind becomes dull or stupid. Thyroid 

 deficiency in a young child prevents the normal development of 

 body and mind and in extreme cases ca.uses that peculiar type of 

 idiotic dwarf known as "cretin." If thyroid extract is admin- 



