162 



BLIND VERTEBRATES AND THEIR EYES. 



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in 

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FIG. 6 S . Exit ot optic Ner from Eye of 



showing Ganglionic Cells and (s) cells at Entrance of Optic Nerve. 



fact that the optic nerve does 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 ex 't f the optic nerve, which should, 

 according to this view, be at the proxi- 

 rnal end of the choroid fissure. The 

 second objection is found in the phylo- 

 genetic stages of degeneration indicated 

 in different eyes, notably that of Typh- 

 lomolge. Furthermore, it would not 

 account for the groove that is un- 

 doubtedlv found alons; the ventral side 



- 



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- 

 portion 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 layer of thin cells without pigment. At the end of the period it is composed 

 of cylindrical cells 12 ju, 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 any dif- 

 ferentiation into layers. The inner reticular layer first appears as a number of 

 irregular spaces separating the ganglionic from the nuclear layer when the em- 

 bryos are 5 mm. long. These spaces soon unite into a single layer, but this does 

 not occur till the very latest stages of the period when the choroid fissure has been 

 closed for some time, and in fact they may never form a layer entirely around the 

 central ganglionic cells. In earlier stages the layer extends between the dorsal and 

 lateral parts of the ganglionic and nuclear layers. The nuclear layers never 

 become separated into outer and inner ones, nor is an outer reticular layer ever 

 formed. There is no indication of cones such as are seen in some adult eyes. 

 Miillcrian fibers are well formed in older individuals at this period. 



The development of the scleral cartilages described under another head also 

 takes place toward the close of this period. No dividing cells have been found in 

 tin- eves of specimens more than 7 mm. long. The nuclei of the retina in the 

 lo-mm. stage are all granular and measure 4 to 5 p. in diameter. 



Tin- Third Period. -- This extends from the time the fish has reached a length 

 of 10 mm. till marked senescent changes begin, which take place when the fish 

 approaches 100 mm. in length. 



The nuclei of the retina, when the fish has reached a length of 25 mm., are 

 no longer alike. There are two types of cells in all layers : cells with larger granu- 

 lar nuclei, and cells with smaller compact or dense nuclei. The difference is per- 

 haps due less to histogencsis than to the process of degeneration which has already 



