378 



this cell complex. But the chief agents in the formation of 

 the lens are the lens cells, which gradually transform the 

 rather thin concave-convex cuticle of the early pupa into 

 the thick biconvex mass as we see it in the late pupa and 

 adult (fig. 64). 



By this remarkable series of changes, the originally 

 three-layered condition of the imaginal disc of the first larval 

 instar, which has itself doubtless been produced from a pre- 

 viously single-layered ectoderm, gradually transforms itself 

 into the wonderfully specialized state in which we see it in 

 the adult — a state in which it does not depart essentially 

 from the three-layered condition, and in which the function 

 of secreting cuticle is retained, though modified, in such a 

 way as to aid it in performing its new function. 



A feature of the compound eye of Nasonia is the entire 

 absence of tracheae between the ommatidia, structures which 

 are so prominent in the eye of Callijjhora (Lowne, Hickson). 



The description of the compound eye has been confined, 

 so far, to a consideration of the development and differentia- 

 tion of the simple optic disc of the newly hatched larva. But 

 immediately below the eye there is formed during the late 

 larval and pupal life an important structure whose function 

 it is to support the nervous elements of the eye. This struc- 

 ture is developed directly from the ectoderm surrounding the 

 optic imaginal disc, and it will be necessary to describe its 

 development here; it will also be convenient, though not 

 perhaps strictly logical, to give an account at this stage of 

 the development of the innervation of the compound eye, 

 since these two processes are intimately connected with one 

 another. 



An examination of a medium-sized larva shows that 

 immediately below the ectoderm there is a delicate membrane 

 with distinct nuclear swellings, the mesodermal somatopleure; 

 it is clearly visible below the optic disc, and no other tissues 

 underlie it. But when the larva is about to defaecate it is 

 seen that two areas of proliferation have arisen in the head 

 ectoderm above and below, and in close contact with the optic 

 disc. From these areas the proliferating ectodermal cells 

 grow towards each other, and finally meet, forming a very 

 prominent bridge across the back of the imaginal optic disc. 

 The cells soon spread out laterally, and form a membrane 

 completely covering the back of the developing eye (figs. 77, 

 78). Internal to this disc, of course, the somatopleure must 

 lie. In the larva which is about to pupate the membrane has 

 extended completely behind the eye. When the membrane 

 is examined at this stage a very remarkable thing is seen. 

 The cells stand off a considerable distance from the adjacent 



