MUSEUM OF COMPARATIVE ZOOLOGY. 199 
thicker around the whole circumference of the cell, and take on a lus- 
trous appearance. With Kleinenberg’s hematoxylin the substance of 
the rhabdomeres can be colored, and the line of demarcation between 
products of the separate cells can be distinguished. This structural con- 
dition can be traced to the deeper part of the retina, where the cell 
outlines become indistinct, the rhabdomeres incasing each retinal cell 
for a half or two thirds of its length. 
Pigment (Pl. III. fig. 19, gra. pig.) is uniformly distributed through 
the retinal cells, as well as the perineural cells to be described later. This 
is best seen in sections perpendicular to the axis of the eye. Phao- 
spheres, although present in the median eyes, do not occur in the lateral 
eyes. The optic nerve (PI. III. fig. 18, 2. opt.) emerges from the deep 
end of the retina, and its course is so oblique to the axis of the eye that 
a section which shows the retina well seldom shows much of the optic 
nerve. 
The perineural cells surround the depressed retinal area, and their 
attenuated ends, especially on the ventral side of the eye, often reach 
out, even in the adult condition, in front of the retinal cells themselves 
(Pl. III. fig. 18). The positions that the nuclei occupy in the ventral 
portion of the perineural ring suggest that these cells may at one time 
have extended far enough to have completely covered the retina, and the 
fact that in young individuals (PI. III. fig. 21) the retina is largely cov- 
ered by the perineural cells indicates that in all probability the lens is the 
product of these cells. In that event the perineural cells are the physio- 
logical equivalent of the lentigen. The peripheral margin of this lentige- 
nous ring passes by insensible gradations into the surrounding hypodermis. 
The development of the lateral eyes is referred to by Kowalevsky and 
Schulgin (86, p. 531) as follows: “ Die Seitenaugen entwickeln sich 
unabhangig von den Mittelaugen, und bei ihrer Ausbildung nimmt die 
Vertiefung der obern Schicht der Kopfplatte Antheil. Die Einzelheiten 
dieses Vorganges sind von uns noch nicht bearbeitet.” This is the only 
reference which they or other students have made to the development of 
the lateral eyes. 
The “ocular areas,” as Lankester and Bourne designate the regions 
occupied by the lateral eyes, appear in Centrurus as pigmented tracts of 
hypodermis on either side of the head and a little below and behind the 
median optic sacs. Horizontal sections of the embryo cut these areas in 
the most advantageous way for a general study; they show that the 
whole ocular area is produced by a thickening of the hypodermis. 
