560 CYCLOSTOMES 



cornea by the vitreous. The surface ('hyaloid') membrane of the latter 

 is conspicuous in microscopic sections, but it contains no blood vessels 

 where it contacts the retina. Indeed, apart from the iris and chorioid 

 there are no vascularized structures; and there is no canal of Schlemm. 



The Retina — The lamprey retina (Fig. 162) differs from all others in 

 that its ganglion cells are not separated from the inner nuclear layer. 

 As a consequence, the nerve fiber layer lies embedded high in the retina 

 instead of near its inner surface (cf. Fig. 19, p. 43). Though one cannot 

 be certain, it is not likely that this is a primitive arrangement (nor does 

 it smack of 'degeneracy') . True, in the histogenesis of any (other) retina 

 the bodies of the ganglion cells are at first contiguous with those of the 

 inner-nuclear elements; but it is stretching a point to suggest that this 

 is an ontogenetic recapitulation of the adult cyclostome arrangement. 



The lamprey optic nerve is, however, assuredly primitive in its organ- 

 ization. Running axially through it is a column of cell-bodies, appearing 

 in cross-sections of the nerve as a rosette of nuclei, each of whose single 

 processes radiates to the surface of the nerve. These cells are obviously 

 ependymal — not of a higher, glial, type (which they have usually been 

 called). If we think of the optic nerve as a cylinder, then its radius 

 represents morphologically the thickness of the neural tube of which the 

 retina is an evagination. The axis of the nerve — even though the nerve 

 is not tubular, but solid — thus stands for the inner surface of the brain 

 wall. Thus, each ependymal cell in the optic nerve maintains the orien- 

 tation of any ependymal cell in a primitive brain (see pp. 126-9). Verte- 

 brates above the lampreys all have at least neuroglial tissue, if not meso- 

 dermal connective tissue as well, forming the supporting framework of 

 their optic nerves. 



The visual cells of lampreys exhibit variations from genus to genus, 

 but within the Petromyzonidse these can be arranged in a fairly satis- 

 factory series with regard to taxonomy. In the primitive genus Ichthy- 

 omyzon the rod and cone differ but little in length, and the outer seg- 

 ments of both are tapered, and to this extent, 'cone-like'. The rods out- 

 number the cones by five-to-one in the parasitic lake species castaneiis 

 and unkuspis, by three-to-two in the brook form fossor. In Petromyzon 

 (Fig. 163b), the next higher genus in the scale, the cones have become 

 much longer than the rods; and the rods, which here outnumber the 

 cones three-to-one, have cylindrical outer segments of moderate length. 

 This differentiation in length and shape reaches a maximum in Ento- 

 sphenus, and the numerical predominance of the rods is greatest also in 



