Development of Sense Organs. 137 



external structures, therefore epiblastic. But in the vertebrate 

 (and in some other animals too) the sense-organs are concealed 

 within the body for protection. Nevertheless, their develop- 

 ment shows that in all cases the sense-organs are epiblastic 

 structures — that is, of course, the actual sensory parts of the 

 organs ; for, in the complicated sense-organs of the vertebrates, 

 the eyes and ears, various accessory structures, which have 

 merely a subsidiary function, also exist. 



The ear first appears as an invagination of epiblast, which 

 ultimately comes to form a cjosed sac. From this sac are 

 formed the three semicircular canals and the rudimentary 

 cochlea — in fact,- all the essential parts of the ear. The mass 

 of bone in which it is imbedded, and the small bones which 

 serve as a conduit to the sound waves, are mesoblastic 

 structures. 



The eye is formed from three sources. There is, first of 

 all, a pair of outgrowths from the brain in the shape of hollow 

 vesicles. These become curved flattened plates, hollow within 

 and connected with the brain by a hollow stalk. The plates, 

 into which the ends of the primitive outgrowths of the brain 

 expand, form the retina of the eye, while the stalk ends as 

 the optic nerve. The lens of the eye is produced by a direct 

 invagination from the exterior of the body. The rest of the 

 eye structures are mesoblastic. It will thus ' be seen that the 

 retina, which is the essential part of the eye, is epiblastic ; not 

 so directly, it is true, as in the case of the sensitive part of the 

 ear. But, as the brain itself is formed by an involution of the 

 epiblast, all structures derived from it must be also epiblastic. 



The olfactory organs are also developed as epiblastic in- 

 vaginations, but they have retained, to a greater degree, 

 their primitive position. They still communicate with the 

 exterior. 



From the hypoblast is developed: the epithelium of the 

 digestive tract, apart from that of the stomodfeum and procto- 

 dseum just referred to; the epithelium of all the glands 

 appended to the alimentary canal; the liver (including the 

 gallbladder); the pancreas; the lining of the lungs, which 

 are developed as outgrowths of the pharynx, and of the 



