THE DEVELOPMENT OF THE COMPOUND EVE. 547 
appears to be faceted, each column of cells forming one facet. 
This led Viallanes [27] to describe the disc in this stage as 
consisting of a layer of large cells, which he termed optogenic 
cells. 
During these changes the optic stalk becomes hollow and 
exhibits a distinct central cavity which extends to the surface 
of the retinal disc (Fig. 46). 
Third Stage of Development.—On the fourth day of the pupa 
state, or immediately after the evolution of the head, the optic 
stalk has become greatly elongated, and the neural layer of the 
disc is already very thin; the sub-dioptric cavity has greatly 
increased in size and contains many granule cells. It is seen 
to be lined by a cellular membrane, and the processes of the 
cells, which form a reticulum connecting the neural and epi- 
dermal discs, can be traced into the integumental disc 
(Pl. XXXVIII., Fig. 6). At this period the eye disc measures 
‘7mm. in its short and about 1°4 mm. in its long diameter, 
but it is only or5 mm. thick, whilst its neural layer is separated 
from it by a space ‘o5 mm. deep in the centre of the eye, and 
is itself not more than ‘oo5 mm. thick. 
Each ommateum is now seen to contain an ovoid body 
which does not stain with logwood, and which has a gelatinous 
appearance similar to the bulb-like so-called nerve terminals 
of the King-crabs (Limulus). This body is undoubtedly con- 
nected with a fibre from one of the cells of the sub-dioptric 
space, and is, therefore, of mesoblastic origin. In some of 
my sections the cells of the rhabdome, which subsequently 
become pigmented, are seen forming a distinct layer on the 
surface of these ovoid bodies (P]. XXXVIII., Fig. 6). 
The Eye of Limulus.—The eye of Limulus has been described 
by Grenacher [222], and by Lankester and Bourne [229]. 
The former speaks of the rhabdome as resembling an orange, 
the great cells representing the quarters. Lankester and 
Bourne figure this body as a hollow spheroid, and state that 
it is surrounded by intrusive connective tissue. Both authors 
agree in describing this body as a retina. My contention is 
that it is the homologue of the rhabdome of an insect’s eye, 
