182 BULLETIN OF THE 
then have a vertical direction. The only planes which in both the adult 
and embryo cut the eyes similarly are those parallel with the sagittal 
plane. In a horizontal section of a young embryo the eye shows the 
same relation of parts as one sees in a transverse section of an adult. 
As previously stated, the eye in the embryo consists of three cell layers, 
lentigen, retina, and post-retina. These three layers are recognizable 
in the adult eye, and in considering the histology of this structure the 
three layers will be treated in the order named. 
The Jlentigen, as Grenacher (’79, p. 40) first clearly demonstrated in 
spiders, is distinct from the retina, and is directly continuous with the 
hypodermis. Graber (79, p. 61) established the existence of a similar 
condition in the median eyes of scorpions. 
The lentigen results from a modification of the hypodermis directly 
external to each optic sac. For some time after involution this hypo- 
dermis consists of undifferentiated cells, whose positions are indicated by 
their spherical nuclei. About the time when pigment is deposited in 
the retina, the hypodermis in front of each pigmented area thickens, and 
the outlines of its cells become visible (PI. III. fig. 15, pr 7.). This is the 
first modification in the formation of the lentigen. The thickening of 
the lentigen increases, and each cell assumes the form of a long pyramid, 
whose base rests upon a membrane between retina and lentigen, and 
whose slightly truncated apex reaches the forming lens (Pl. II. figs. 9 
and 10). In immature eyes the sides of the lentigenous cells are perpen- 
dicular to the surface on which they rest. In a transverse section of the 
head of an adult (Pl. I. fig. 2), the cells are curved. About three fourths 
of the lentigen, extending from the median toward the lateral margin of 
the eye, has its cells convex toward the sagittal plane; in the lateral 
fourth, the cells are concave toward the sagittal plane, and in the small 
intermediate region they are straight (compare Lankester and Bourne, 
’83, pl. X. fig. 8). In a longitudinal section of an adult head (Pl. I. 
fig. 1), the lentigenous cells all appear perpendicular to the surface on 
which they rest. 
The nuclei of the lentigen cells, at the first indications of a thickening 
in the lentigenous region, keep to its deeper parts, and form in the 
adult eye a continuous line close to the deeper face of the lentigen (PI. I. 
fig. 2, nl. pr r.). 
The lentigen as a whole is of glassy transparency. In young stages 
the hypodermis at the edge of the lens nearest the median plane shows a 
deposit of pigment. This pigmented region in time extends around the 
