Heredity and Environment 



A. 



FIG. 19. GEOTROPISM OF SEEDLING OAK. After starting to grow with the 

 axis A-A in vertical position the seedling was gradually turned through 

 90 until the axis B-B was .vertical; during this change of position the stem 

 continues to grow upward (negative geotropism), the root downward 

 (positive geotropism) as indicated by dotted outlines. 



quently the case in animals, they are known as "instincts.'' Re- 

 flexes and tropisms occur in the simplest organisms, such as bac- 

 teria, protozoa, and single cells, as well as in higher plants and 

 animals (Figs. 15-19), but instincts are limited to animals with 

 a nervous system. 



Reflexes and Tropisms of Germ Cells and Embryos are seen in 

 movements of spermatozoa, movements of the protoplasm in egg 

 cells and embryonic cells, movements of cells and cell masses in 

 the formation of the gastrula, alimentary canal, nervous system 

 and other organs. Indeed the entire process of development, 

 whether accompanied by visible movements or not, may be re- 

 garded as a series of automatic responses to stimuli. When the 

 embryo becomes differentiated to such an extent as to have spe- 

 cialized organs for producing movement its capacity for making 

 responsive movements to stimuli becomes much increased. In the 

 embryo the rhythmic contractions of heart, amnion and intestine 

 are early manifestations of reflex motions. These appear chiefly 

 in the involuntary muscles before nervous connections are 



