THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 187 



been retained in Amblyopsis out of all proportions 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 micra high which are markedly pigmented. Pigment granules first make 

 their appearance when the larva is about 5 millimetres long. The remainder of the 

 retina is at the beginning of the period several cells deep without any differentiation 

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

 separating the ganglionic from the nuclear layer when the embryos are 5 millimetres 

 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 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. Mullerian fibres 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 the eyes of specimens more than 7 milli- 

 metres long. 



The nuclei of the retina in the 10-miUimetre stage are all granular and measure 

 4 to 5 micra in diameter. 



3. The Third Period. — This extends from the time the fish has reached a length 

 of 10 millimetres till marked senescent changes begin, which take place when the 

 fish approaches 100 millimetres in length. 



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

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

 nuclei, and cells with smaller compact or dense nuclei. The difference is perhaps due 

 less to histogenesis than to the process of degeneration which has already set in. The 

 cells with smaller nuclei are probably degenerate. In the oldest fish only cells of the 

 second type are found. 



A number of changes take place during the third period, some of which can 

 be classed neither as progressive nor as retrogressive. As the fish grows the eyes are 

 farther and farther removed from the surface. In the fish 25 milhmetres long they 

 are nearly 1 millimetre below the skin, and in the largest specimen examined they 



