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. Tliese 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 lentigen, as Grenacher (79, p. 40) first clearly demonstrated in 

 spiders, is distiuct from the retina, and is directly continuous v/ith 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. \b,pr v.). Tins 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 (PI. 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 (PI. 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, pi. X. fig. 8). In a longitudinal section of an adult head (PI. 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 



