THE DEVELOPMENT OF THE COMPOUND EYE. 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 

 7 mm. in its short and about i'4 mm. in its long diameter, 

 but it is only "015 mm. thick, whilst its neural layer is separated 

 from it by a space '05 mm. deep in the centre of the eye, and 

 is itself not more than '005 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 (PI. 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, 



