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 



Fir.. 71. A section through the optic disc and ganglion of a Blo\v-fly nymph about 

 the middle of the third day of the pupa stage, c, the sub-dioptric space ; if, the 

 optic disc ; s, the optic stalk, spreading out into the neural disc; ;/, 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. 



\Yhen seen from the surface the integumental disc already 



