2O6 THE ASCIDIANS. 



The other pigment-cells of the dorsal wall of the cerebral 

 vesicle collect themselves together and form a slight pro- 

 tuberance in the right dorso-lateral corner of the vesicle, 

 while the pigment-granules, which were at first scattered 

 about in the interior of the cells, become concentrated at 

 their converging extremities towards the cavity of the 

 vesicle. And in this way is formed the single eye of the 

 Ascidian tadpole ; the original pigment-producing cells 

 constitute the retina, which retains its primitive position 

 as part of the epithelial wall of the brain.* 



Subsequently two or three cells from the adjoining wall 

 of the vesicle take up a position, one above the other, in 

 front of the mass of pigment and, having previously, by 

 an alteration in the character of their protoplasmic con- 

 tents, acquired a high refractive index, constitute the lens 

 of the eye, which projects obliquely downwards into the 

 cavity of the vesicle. (Cf. Fig. 105 A.} 



The cerebral vesicle of the Ascidian tadpole is the un- 

 doubted homologue of the corresponding, but less pro- 

 nounced, structure in Amphioxus. It differs from the 

 latter in lying wholly in front of the anterior extremity of 

 the notochord, in possessing a more highly organised eye, 

 provided with a cellular lens, and in the presence of an 

 otocyst, which, as we have seen, is evolved from the same 

 group of cells which gave rise to the eye. 



The eye of the Tunicate tadpole agrees fundamentally 

 with the type of eye peculiar to the Vertebrates, in that 

 the retina is derived from the wall of the brain. On this 



* The fact that the lens of the Tunicate eye as well as the retina and 

 the otocyst arise by differentiation of one and the same epithelial layer of 

 the primitive cerebral vesicle, has recently been described by SALENSKY for 

 the larva of Distaplia, magnilarva. (W. SALENSKY. Morphologische Studien 

 an Tunicaten : I. Ueber das Nervensystem der Larven u. Embryonen VOK 

 Distaplia magnilarva. Morph. Jahrb. XX. 1893. PP- 4^-74- ) 



