232 Heredity and Environment 



toplasmic movements within cells and these substances are then 

 isolated by cell divisions and by the formation of partition walls 

 between cells; these protoplasmic movements occur in response 

 to stimuli and if these movements are stopped cleavage and dif- 

 ferentiation are arrested. In later stages the infolding of the 

 gastrula, or neural tube, or alimentary canal, and the foldings of 

 layers in general, which play so important a part in development, 

 are due to the movements of substances within cells and to the 

 movements of cells in the layers in which they lie, and if these 

 movements are inhibited normal development is prevented. 



Nutrition and Development. Another type of functional ac- 

 tivity which is a potent factor in development is found in the 

 trophic or nutritive relations which exist between different parts 

 of an organism. Organs long unused undergo regressive changes 

 and may become rudimentary, for example the muscles of a limb, 

 which has been paralyzed or placed in a cast, shrivel; on the 

 other hand use increases the size and strength of any organ. In- 

 activity or atrophy of one part usually leads to imperfect nourish- 

 ment and development of related parts; for example, the optic 

 nerve atrophies when the eye is lost, and muscles atrophy when 

 the nerves leading to them are destroyed or paralyzed. In gen- 

 eral the normal development of any part is dependent upon its 

 proper nutrition and this is dependent upon the functional activity 

 of this and other related parts. 



Internal Secretions; Hormones. Still another phase of func- 

 tional activity is found in the effects of certain secretions and 

 chemical substances which are formed by different glands and 

 poured into the blood. In many cases the secondary sexual char- 

 acters which distinguish the male or the female are due to chemi- 

 cal substances from the interstitial cells of the gonads (testes or 

 ovaries), which stimulate or exhibit the formation of these 

 characters. If the ovary is removed from a young hen she de- 

 velops the larger size, more brilliant plumage and the pecu- 

 liar comb, wattles and spurs of the cock. These secondary sexual 



