378 



THE INFLUENCE OF LIVING SURROUNDINGS. 



construction of the possible process of development of sucli an 

 eye from the simplest conditions. A short account of these 

 observations will here be of service. 



All the species of Onchidium ob^ei-ved by me, the blind as 



FIG. 101.— The development of the eye o£ Onchidium. Above, to the left, the first stage ; 

 small vesicular cells («) close under the epidermis cells of the prominent point of 

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

 increases and grows spherical. Below (left), this body of homogeneous cells is 

 enclosed in pigment layer. Below (right), the eyeball thus formed communicates 

 with tlie optic nerve (opt), and its cellular mass has teen diflferentiated into a large 

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



well as those that can 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 



