48 THE EVOLUTION OF MAN 



tural change in the brain that can be correlated with the educa- 

 tional process. 



The Eye. The eye develops from three sources. The 

 retina, as noted above, is a direct outgrowth from the fore- 

 brain on the side of the head. The lens is an ingrowth from 

 the ectoderm, and the sclerotic and choroid membranes, which 

 form protective coverings enclosing the retina, are mostly dif- 

 ferentiated from the mesoderm. Up to the sixth month of 

 prenatal life a membrane stretches across the pupil of the eye 

 which sometimes fails to disappear and thus causes blindness. 

 Also the eyelids are grown together until about the same 

 period, but in man, unlike the condition in certain other 

 animals, such as kittens, a separation of the lids occurs before 

 birth. 



The Ear. The internal ear forms as a saccular ingrowth 

 from the ectoderm on the side of the head. This invaginated 

 sac becomes greatly modified but eventually forms the semicir- 

 cular canals, which preside over the function of equilibrium, 

 and the highly developed cochlea which is concerned with hear- 

 ing. The internal ear is regarded as a specialization of a part 

 of the lateral line sensory organs of the lower vertebrates, 

 which in the fish is concerned with balancing and movement 

 only. The added cochlea, which is present in terrestrial 

 animals, forms the organ of hearing. 



Alimentary Canal. The alimentary canal, as stated above, 

 is early folded off from the yolk sac as a tube lying under the 

 notochord of the embryo. Both ends of the tube are closed for 

 a time, but long before birth an anterior and posterior connec- 

 tion with the external surface of the embryo has been made. 

 At the anterior end of the body on each side of the neck, four 

 crevices (Fig. 14) appear in the early embryo which, in the 

 lower vertebrates such as the fish, open directly into the 

 pharyngeal region of the alimentary canal and form the gill 

 clefts. In man, however, these crevices never go on to the 



