381 



ends and where the ommatidium starts; and whether the 

 nerve terminates on a rhabdome or on the sheath cells. The 

 fact, however, that the latter have undergone pigment 

 degeneration, and the close resemblance of the sensory oells 

 of the ocelli (which are innervated) to the rhabdome cell, 

 seem to indicate that it is in the rhabdome of the ommatidium 

 that the nerves terminate. 



At this stage the fenestrate membrane shows a curious 

 appearance which may easily be misinterpreted; the ends of 

 the pigment cells which have secreted it become dilated into 

 small cones, and give the membrane the appearance of cellular 

 tissue; in reality, however, it is entirely non-cellular (cf. 

 fig. 62). 



The development of the great mass of fibrillae from the 

 side of the perioptic membrane towards the brain, as above 

 described, is confined to the middle third of that membrane; 

 consequently the complete optic "nerve" never occupies an 

 area greater than this. In the thirty-six hour pupa the fibrillae 

 have become so massed together as to form a thick layer 

 of fibres running longitudinally to and beneath the disc, the 

 individuals of which are no longer visible. These, then, pass 

 down the optic nerve and enter the brain. 



In the pupa of the third-da,y pigment granules begin to form 

 in the optic "nerve," and become very prominent a day 

 later. Changes are now taking place which involve the 

 ommatidia as well; these consist of a gradual alteration 

 of the shape of these structures. In the pupa at about the 

 end of the third day that portion of the perioptic membrane 

 which has not been concerned in the formation of the optic 

 nerve begins to undergo chitinisation, and since this mem- 

 brane was produced from, and still is continuous with, the 

 ectoderm immediately surrounding the eye, it follows that 

 this chitin layer will be similarly continuous with the chitin 

 which now begins to form on the head of the wasp ; this 

 chitinisation of the back of the eye appears to push the 

 nervous part of the optic "nerve" out of position, by pressing 

 on its periphery; at any rate, it now assumes an outwardly 

 convex form. This effect is really produced by a ''shear-like" 

 action of the chitinising perioptic membrane, the lower por- 

 tion pressing outwards and upwards, the upper down and 

 inwards. This process is complete in the four and a half-day 

 pupa (fig. 80). The "optic nerve" from the brain at this 

 stage has also assumed the appearance of a solid projection 

 from the lower side of the brain in its more ventral portion 

 and is crowded with nerve oells; the detection, however, of 

 individual nerve fibres in this region is quite impossible owing 

 to the close coherence of these. 



