OBSEEVED DEVELOPMENT. 379 



■wrinlding ; hence in these intermediate spaces no uniform re- 

 fraction of rays of light can take place in any one point, while 

 it can in the smooth rounded papillae. These vary gi'eatly in 

 size, and they constantly increase in number with the age of the 

 animal. The smallest have beneath the cuticle, or outer skin, 

 merely a simple cellular layer — the epithelium — like all uni- 

 valves. The next in size show, exactly in the centre of the 

 papillae, a celhilar mass growing inwards and downwards from 

 the epidermis (see fig. 101, the upper fig. to the left), in which 

 one or two cells may already be discerned as the basis of future 

 gland-cells ; in the next this group of gland-cells are pushed 

 aside by another cellular mass proceeding in the same way 

 from the epidermis at the summit of the papilla (see fig. 101, 

 right, top fig.) and of peculiar aspect. Subsequently the inner- 

 most cells of this last-named mass become conspicuously modi- 

 fied, they increase much in size, their contents become peculiarly 

 granular, and their circumference highly refractive, and then a 

 fine nerve may be seen proceeding from the interior of the skin 

 towards this cellular mass. In still larger papillae we find 

 roundish cell-bodies which are in direct communication with a 

 nerve, and which at the back are already partly surrounded by 

 pigment (see fig. 101, bottom, left fig.). At the spot where the 

 nerve enters, and where the pigment layer is not altogether 

 closed up, there are a few peculiar cells which are of precisely the 

 same size and aspect as the cells above mentioned in the largest 

 papillse without pigment. Finally, the pigment layer closes 

 round the central cellular mass, and the primitive rudimentai-y 

 eye is complete. 



The structure is not, of couise, thereby definitely completed. 

 Very striking modifications now take place in the central 

 bodies, composed of homogeneous cells and enclosed in pig- 

 ment ; one of them, lying nearest to the prominent surface of 

 the papilla — the cornea — grows more than its neighbours ; soon 

 others do the same, and a true lens, consisting of at least four 

 cells,' is thus formed. The slill unmodified cells, IjTng between 

 the lens thus formed and the pigment sheath, now are trans- 

 formed into a retina of which the structure has been al)ove 

 described. The process and conditions here described, and con- 



