546 THE SENSES AND SENSORY ORGANS. 
on the one hand with the epithelioid membrane covering the 
neural disc, and on the other penetrate the integumental layer 
or optic disc. These connecting fibres are at first exceedingly 
fine and are frequently torn through by the contraction of the 
tissues which accompanies their hardening, but a day or two 
later they are very distinct (Pl. XXXIX.). 
The cells of the integumental disc are seen to be arranged 
in columns, each column corresponding to a future ommateum. 
It is exceedingly difficult to determine the number of cells 
Fic. 71.—A section through the optic disc and ganglion of a Blow-fly nymph about 
the middle of the third day of the pupa stage. c, the sub-dioptric space ; d, the 
optic disc ; s, the optic stalk, spreading out into the neural disc; 77, the rudi- 
mentary retina of the imago. 
in each column, but it is quite evident that they are very 
numerous and form four more or less distinct layers, a corneal 
layer, a series of bundles of long fusiform cells beneath the 
corneal layer, a double layer of round cells, and an inner 
columnar layer. 
When seen from the surface the integumental disc already 
