ILLUSTRATIONS 



FIGURE PAGE 



The arrows show the varying directions of the light. 

 In each case only a particular part of each retina is 

 stimulated, the rest being in shadow. 



94. — Eye of Squid (Horizontal Median Section) 179 



(From Plate, after Hensen, Allgem. ZooL, Gustav 

 Fischer.) 



95. — Development of the Eye in Cephalopod 



Molluscs ...... 181 



(After Plate, Allgem. ZooL, Gustav Fischer.) 



A. Early embryonic stage, showing the spherical 

 retina (represented in the Pearly Nautilus). 



B. Snaring off of the eyeball and beginning of the 

 iris-folds and of a primary cornea. 



C. Development of the lens on either side of the 

 primary cornea, or transparent septum. 



D. Development of a secondary or outer cornea. 



96. — Light Cells of Amphioxus .... 183 



A. Forepart of a young Amphioxus, enlarged, show- 

 ing light cells (Becheraugen). (From Plate, after 

 Joseph, Allgem. ZooL, Gustav Fischer.) 



B. Cross-section of the spinal cord of Amphioxus, 

 showing the light cells, which are essentially like the 

 goblet eyes (Becheraugen) of invertebrates. 



(From Plate, after Hesse, Allgem. ZooL, Gustav 

 Fischer.) 



97. — Evolution of the Vertebrate Eye as Con- 

 ceived BY Studnicka .... 185 



(From Plate, after Studnicka, Allgem. ZooL, Gustav 

 Fischer.) 



In still earlier stages it is supposed the vertebrate 

 eyes arose, as in invertebrates, through a down- 

 pocketing of the light cells (Fig. 91) on the surface of 

 the embryonic nerve furrow, or medullary fold. When 

 the fold closed over, as it does in the developing verte- 

 brate embryo, the future eye spots found themselves 



xxxvi 



