Influence of Environment .225 



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

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

 iously castrated, the latter develops mammary glands similar to 

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

 lus of substances from the ovary. 



Another gland whose secretions exercise a profound influence 

 n development is the thyroid, which is found in the neck near the 

 Adam's apple." If this gland becomes enlarged it gives rise to 

 oitre, protruding eyeballs, rapid heart beat; on the other hand 

 f the thyroid is deficient in a young child it causes the peculiar 

 of idiotic dwarf known as "cretin." If the gland which lies 

 etween the roof of the mouth and the base of the brain and which 

 ; known as the hypophysis is deficient the child, or young animal, 

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

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

 eformed giant, but with weak body and mind. 



Cor relative -Differentiation and Self-Differentiation. Many 

 ises are known in which the development of a part is dependent 

 pon the presence of another part; this is technically known as 

 correlative differentiation." Thus it has been found that the lens 

 f the eye will develop from any portion of the ectoderm, or outer 

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

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

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

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

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

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



which would have had very different positions and distributions 



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

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

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



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

 All such cases are known as "correlative differentiation," 

 lying that differentiation is dependent upon the stimuli which 

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



