THE VERTEBRATE EYE 239 



are suggestive rather than convincing. Moreover, in the case of a soft 

 organ any help from fossil types is lacking. We are therefore driven to 

 seek what evidence we can from ontogeny. 



Froriep. Hb. d. vergl. u. exper. Entwick- Romer. Vertebrate Paleontology, Chicago 

 lungslehre d. Wirbeltiere, Jena, 2 (1947). 



(1906). 



Steven. J. exp. Biol., 27, 350 (1950). 

 Quart. J. micr. Sci., 92, 233 (1951). 



Anat. Anz. (buppL), 29, 145 (1906). Trewavas, White, Marshall and Tucker. 



Lamarck. Zoological Philosophy (1809). Nature (Lond.), 176, 126 (1955). 



THE ONTOGENY OF THE VERTEBEATE EYE 



Ontogenetically, the central nervous system first appears as a 

 superficial groove along the mid-dorsal line of the embryo which 

 eventually invaginates,i the anterior part to form the anlage of the 

 brain, the remaining and greater part to form the spinal cord. At an 

 early stage before the closing-in process occurs, the anterior cephalic 

 end grows more rapidly than the rest and forms three primary vesicles, ^ 

 and at the cephalic end of the rudiment of the forebrain, tucked into a 

 recess at each corner, a paired lateral depression appears, known as the 

 optic pits {foveolce opticce). These paired pits, lying on the surface of 

 the open cephalic plate, have been seen on the surface of many types of 

 embryos in some of which they are pigmented (Froriep, 1906 ; Lange, 

 1908 ; Franz, 1934 ; and others) (Figs. 242 to 247). As the neural 

 groove invaginates to become the neural tube, the optic pits become 

 invaginated with it to form the primary optic vesicles, which, reaching 

 the surface as lateral out-pouchings of the cerebral vesicles, again 

 invaginate to form the secondary optic vesicles (or optic cups). 



In all Vertebrates the retina participates in the high degree of 

 differentiation which characterizes the central nervous system. The 

 proximal wall of the optic cup remains as a unicellular layer and 

 acquires pigment to form the pigmentary epithelium, but its inherent 

 plasticity is seen in the capacity of the amphibian epithelium to regener- 

 ate an entirely new functional retina if the inner layer is removed 

 (Stone, 1950). The neuro-epithelium which forms the distal layer 

 of the cup, like that which determines the cerebral and cerebellar 

 cortex, differentiates into three strata — a marginal zone of ganglion 

 cells, an intermediate mantle zone (bipolar, amacrine, horizontal and 



^ The fact that the nerve-cord in Amphioxus first appears as a solid rod which 

 canalizes at a later stage has suggested to some authorities that this sequence 

 represents a phylogenetic step in the evolution of the central nervous system of Verte- 

 brates ; but it is to be remembered that the evidence indicates that the Lancelets are 

 an off-shoot of the main vertebrate stock rather than a primitive type. It is also to 

 be noted that Graham Kerr (1919) described the forebrain of Lepidosiren and other 

 fishes as developing in the form of a solid rod from which the optic vesicles grew as 

 solid buds to become canalized later. This, however, is probably merely a question 

 of the timing of various stages of development ; and no dogmatic judgment on this 

 question can vet be given. 



2 p. 532.' 



