. Cave vertebrates of America; a study in degenerative evolution. Cave animals; Evolution. 162 BLIND VERTEBRATES AND THEIR , in. ,-^ ^ Kic. 65 showing fact Ihat the optic nerve docs not form a central strand through the funnel of ganglionic cells, but passes through it in several strands as it does through the mass of cells at the entrance of the optic nerve (fig. 65). The objection is that it would not account for the position of the e.\it of the optic nerve, which should, according to this view, be at the proxi- mal end
. Cave vertebrates of America; a study in degenerative evolution. Cave animals; Evolution. 162 BLIND VERTEBRATES AND THEIR , in. ,-^ ^ Kic. 65 showing fact Ihat the optic nerve docs not form a central strand through the funnel of ganglionic cells, but passes through it in several strands as it does through the mass of cells at the entrance of the optic nerve (fig. 65). The objection is that it would not account for the position of the e.\it of the optic nerve, which should, according to this view, be at the proxi- mal end of the choroid fissure. The second objection is found in the jjhylo- genetic stages of degeneration indicated in different eyes, notably that of Typh- lomolgc. Furthermore, it wf)uld not account for the groove that is un- Exit of opiic Nerve from Eye of c;w/-.«as/fr/|a;..7/,/.TH^^ doubtcdK' found along thc vcntral side Ganglionic Cells and (::) cells al tnlrante of Optic .Nerve. ^ *-• of the larval eye, nor would it account for the presence of the inner reticular layer around the optic nerve. It would, moreover, make it necessary to assume that the cells found about the entrance of the optic nerve in Chologaster have been retained in Amblyopsis out of all pro- ])ortion to the other structures of the eye. These objections seem to me fatal to this second supposition. During this period the differentiation of the several layers of the retina also takes place. At the beginning of the period the pigmented layer is represented by a la3'er of thin cells without pigment. At the end of the period it is com])osed of cylindrical cells 12 /a high which are markedly pigmented. Pigment granules first make their appearance when the larva is about 5 mm. long. The remainder of the retina is at the beginning of the period several cells deep without an\' dif- ferentiation into layers. The inner reticular layer first appears as a number of irregular spaces separating the ganglionic from the nu
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