422 HISTORY OF THE HUMAN BODY 



by the growth of other parts. In man it lies so hidden that the 

 early anatomists, finding- it as it were in the innermost pene- 

 tralia of the organ of life and individuality, deemed it the seat 

 of the soul, a view from which the morphologists of the pres- 

 ent day have escaped only by substituting one mystery for 

 another. 



The sporadic occurrence of these vestigial sense organs, 

 paraphysis and epiphysis, which, save perhaps in the case of 

 the parietal (epiphysial) eye of the lizard, cannot be of the 

 slightest use, points definitely to the presence of similar organs 

 in a functional condition in some remote ancestor. That these 

 parts were organs of vision there can be but little doubt, and 

 there are certain indications which lead us to think that they 

 were once paired, although always close together. Beyond 

 this, investigation has as yet shown nothing, and the whole 

 subject remains at present one of those half-completed histo- 

 ries, of which the record consists of a few poorly preserved 

 fragments. 



Far more satisfactory is the history of the diverticula which 

 develop laterally from the sides of the part under consideration, 

 for, although we do not have adult animals which show the 

 steps in the development, they are yet traced in perfect agree- 

 ment during the embryological history of every vertebrate, a 

 procedure familiar to all students of embryology. These appear 

 at an extremely early age, often beginning before the com- 

 pletion of the telencephalic lobes, and soon assume the form of 

 spherical vesicles, connected with the brain by narrow stalks, 

 and almost in contact at their outer surface with the external 

 germ-layer, the surface ectoderm. By an invagination of this 

 outer surface the vesicle is transformed into a double-layered 

 cup, and in this one may recognize the fundamental elements of 

 the eye. The primary vesicle is hence called the optic vesicle, 

 the transformed cup-like figure, the optic cup. [See Fig. 136.] 



Of this the invaginated layer, now lining the cup, becomes 

 the retina, certain cells of which give rise to the rods and cones, 

 the essential nervous elements of the organ ; the other layer, 

 now forming the covering of the cup, develops pigment and 



