36 



BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



I have no corresponding illustration of the condition of the optic 

 nerves in Stage II., but a model of the anterior part of the brain and 

 the optic nerves of a specimen in Stage III. a is shown in Figure E {\\\q 

 anterior portion of the left optic cup has been omitted in the model ; the 

 cut surface being indicated by horizontal lines). The left eye is higher 

 than the forebrain ; its ventral edge is at the same level as the dorsal 



tcl. opt. s. 



oc. m%g. 



cb, dx. 



.. cb. s. 



n. opt. s. 



efts. opt. 



^ 



Fig. F. 



Front view of the fore part of the brain, the optic nerves and portions of the 

 optic cups in Stage IV. From a model (Born's method). X 50. 

 Compare Fifj. E. 



For meaning of lettering, see Abbreviations under Explanation of Plates. 



side of the right eye, and the transverse plane tangent to its posterior 

 surface would cut the right eye about midway between its anterior and 

 posterior faces. The right eye may have moved slightly ventrad from 

 the position which it occupied in Stage I", The slackness of the nerves 

 is shown by the curve that they take as "they ])ass forward and out- 

 ward. The whole of the midbrain and most of the forebrain have lost 

 their earlier position between the eyes, owing to the growth in length 

 of the facial cartilages. Figure 9 (Plate 2), a side view of the brain of 

 a fish three inches long, shows this antero-j)OSterior separation between 



