378 



THE INFLUENCE OF LIVING SDRROUNDINGS. 



construction of the possible process of development of sncli an 

 eye from the simplest conditions. A short account of these 

 observations will here be of service. 



All the species of Onchidium observed by me, the blind as 



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ret. 



Fin. 101,— The development of the eye of Onchiilmm. Abore, to the left, the first stage; 

 small vesicular cells ((f) close nnrler ttie c-iiiik-nnis cells of the prominent point of 

 the papilla. Above (riglit) a larger mass of these vesicular cells, which gradually 

 increases and grows spherical. Below (left), this hudy of liomogcneous cells is 

 enclosed in i)ipni(.-iit layer. Below (right), tlie eyeball tluis formed coninuinicates 

 with the optic nerve (o/j/), and its cellular mass lias teen differentiated into a large 

 lens lying in front, and rctiiia-cells behind it. 



well as those tluit caii see, are covered with a great number of 

 tubercles of various sizes, of which the surface is everywhere 

 curved very regularly and is at the same time quite smooth. 

 The intervening portions of the skin of the back are, on the 

 contrary, distinguished by much roughness and granulation or 



