ORGANS OF VISION. 483 



Into this space some mesoblast cells penetrate at a slightly later period. 

 Both the epidermic and neural layers next become divided into two strata. 

 The outer stratum of the epidermic layer gives rise to the crystalline cones 

 and Semper's nuclei ; each crystalline cone being formed from four coalesced 

 rods, developed as cuticular differentiations of four cells, the nuclei of which 

 may be seen in the embryo on its outer side. The lower ends of the cones 

 pass through the inner stratum of the epidermic disc, the cells of which 

 become pigmented, and constitute the pigment cells surrounding the lower 

 part of the crystalline cones in the adult. The outer end of each of the 

 crystalline cones is surrounded by four cells, believed by Bobretzky to be 

 identical with Semper's nuclei 1 . These cells give rise in a later stage (not 

 worked out in Astacus) to the cuticular corneal lenses. 



Of the two strata of the neural layer the outer is several cells deep, while 

 the inner is formed of elongated rod-like cells. Unfortunately however the 

 fate of the two neural layers has not been worked out, though there can be 

 but little doubt that the retinulae originate from the outer layer. 



The mesoblast which grows in between the neural and epidermic layers 

 becomes a pigment layer, and probably also forms the perforated membrane 

 between the crystalline cones and the retinulae. 



The above observations of Bobretzky would appear to 

 indicate that the paired compound eyes of Crustacea belong to 

 the type of cerebral eyes. How far this is also the case with the 

 compound eyes of Insects is uncertain, in that it is quite possible 

 that the latter eyes may have had an independent origin. 



The relation between the paired and median eye of the 

 Crustacea is also uncertain. 



In the genus Euphausia amongst the Schizopods there is present a series 

 of eyes placed on the sides of some of the thoracic legs and on the sides of 

 the abdomen. The structure of these eyes, though not as yet satisfactorily 

 made out, would appear to be very different from that of other Arthropodan 

 visual organs. 



The Eye of the Vertebrata. In view of the various 

 structures which unite to form it, the eye is undoubtedly the 

 most complicated organ of the Vertebrata ; and though its 

 mode of development is fairly constant throughout the group, 

 it will be convenient shortly to describe what may be regarded 

 as its typical development, and then to proceed to a comparative 

 view of the origin of its various parts, and to enter into greater 

 detail with reference to some of them. At the end of the section 



1 There would appear to be some confusion as to the nomenclature of these parts 

 in Bobretzky's account, 



312 



