xxxvi Introduction. 



the central canal of their spinal cord, and has an aggregation 

 of pigment granules^ the rudimentary eye, placed anteriorly to 

 it. The cerebro-spinal system of other Vertebrata resembles this 

 axial nervous cord of the AmpJdoxiis, in being developed from the 

 uppermost part of the three layers into which the germinal mem- 

 brane divides itself in the embryo, but differs from it in the great 

 size and complete differentiation which its anterior segments 

 attain to in the brain and organs of special sense. The brain 

 consists of three primary vesicles, the anterior one of which is 

 subsequently differentiated into a ' prosencephalon ' and ' dien- 

 cephalon/ the latter division corresponding to the parts sur- 

 rounding the 'third ventricle^ of anthropotomy ; the middle one 

 of which, or ' mesencephalon/ remains undivided ; whilst the 

 posterior^ like the anterior, is ultimately distinguishable into two 

 portions, an anterior corresponding to the cerebellum, and a pos- 

 terior corresponding to the parts bounding the posterior parts of 

 the 'fourth ventricle'' of anthroj)otomy, or to the single ventricle 

 of the Amphioxus already mentioned. As in the higher Mollusca, 

 the organs of smell, taste, sight, and hearing are always limited 

 to the head ; and with the exceptions of the Amphioxus and 

 Cyclostomi, in which there is but a single nasal opening, and of 

 the asymmetrical Pleuronectidae, these organs always consist of 

 single bilaterally symmetrical pairs. The essential elements of 

 the peripherally-placed portions of the organs of special sense, are 

 mainly, though not exclusively, developed from the epidermic 

 portion of the same uppermost layer of the trifid germinal mem- 

 brane, whence the cerebro-spinal nerve-centres are themselves 

 developed. The so-called olfactory and optic ' nerves '' are direct 

 oiitgrowths of the anterior cerebral vesicle ; but in all other cases 

 the central and perij)heral factors of the sensory organs are brought 

 into connection through the intermediation of nerves, strictly 

 so called, and developed in the middle one of the three layers of 

 the germinal membrane. In the peripheral apparatus also, certain 

 enveloping and protecting structures, such as the sclerotic coat of 

 the eye, and the skeletal elements in the auditory and olfactory 

 organs are also productions of this intermediate layer ; and in the 

 eye the cornea, and in part the lens, are also formed from it. The 

 peripheral apparatus retains its typical character as an involu- 

 tion of the integument in the olfactory, but loses it in the optic 



