186 PATTEN. [VoL. II. 
EXPLANATION OF PLATE XI. 
Development and Structure of: the Fifth Eye. 
Fic. 61. Cross section of the head in Fig. 3; showing the large median nuclei 
and two of the sensory pits of which the eye is composed, etc. X 430. 
Fic. 62. Same, in a little older stage ; showing formation of a large ganglion cell. 
X 560. 
Fic. 63. Section of the eye in the stage shown in Fig. 6a. It shows a large clus- 
ter of newly formed tripolar ganglion cells, wandering from the retina to the optic 
ganglion. X 560. 
Fic. 64. Section of the eye before the rupture of the embryonic membranes. The 
pits and their cuticular thickenings have united to form a single sensory layer, and 
the large nucleus has sunk into the retina. The formation of ganglionic cells from 
the retina has ceased. The last one formed is now seen in the optic ganglion on one 
side of the optic nerve. X 560. 
Fic. 65. Section of the eye just after the rupture of the embryonic membranes. 
Fic. 66. Section of the eye just after the closure of the optic cup. The median 
row of giant cells, although small compared with those in the other eyes, is very 
conspicuous on account of the manner in which it protrudes above the level of the 
surrounding cells. X 560. 
Fic. 67. Section of the eye in a later stage, showing the first steps in the differen- 
tiation of the corneagen, and the outer wall of the optic vesicle. 
Fic. 68. Section from an embryo about ready to hatch. The outer wall of the 
optic vesicle is formed by two great masses of inverted rod-bearing cells, derived 
from the sense organs 5 and 6. See Fig. 62. We can now see the minute rods of 
which the cuticular thickening is composed. Most of the reddish brown pigment 
has been dissolved by the alcohol. X 560. 
Fic. 69. Section of the eye one or two days after hatching ; showing the distribu- 
tion of pigment, and the well-developed inverted rods of the outer wall of the optic 
vesicle. X 520. 
Fic. 70. The eye as shown in a longitudinal horizontal section of a fully devel- 
oped larva. X 220. 
