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 isshown in Figure £# (the 
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 
tet, opt. Ss. 
— 
f 
I 
Hie. 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). xX 50. 
Compare Fig. £. 
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 pass 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-posterior separation between 
