Influence of Environment 225 



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 this gland becomes enlarged it gives rise to 

 goitre, protruding eyeballs, 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 deficient the child, or young animal, 

 remains infantile; if the hypophysis is too large the individual's 

 hands, feet and face become enlarged and he may grow to be a 

 deformed giant, but with weak body and mind. 



Correlative-Differentiation and Self -Differentiation. Many 

 cases are known in which the development 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 from any portion of the ectoderm, or outer 

 layer of the skin, if only the primitive retina, or optic cup, is 

 brought near to this layer; if the optic cup is transplanted from 

 the head to the thorax or abdomen a lens will form wherever the 

 cup comes in contact with the ectoderm. If an embryonic limb is 

 transplanted from its normal position to the middle of the back or 

 belly, it will develop, and nerves and blood vessels will grow into 

 it which would have had very different positions and distributions 

 if the limb had not been there. If one of the first four cleavage 

 cells is separated from the others it may develop into an entire ani- 

 mal though it would have formed only a quarter of an animal if 

 it had remained in contact with the other three-quarters of the 

 egg. All such cases are known as "correlative differentiation," 

 implying that differentiation is dependent upon the stimuli which 

 come from surrounding parts. On the other hand if the differen- 



