544 THE SENSES AND SENSORY ORGANS. 



the elements which belong to the dioptric apparatus, including 

 the great rods (Nervenstabe).' 



\Yith regard to the manner in which the bulbus comes into 

 relation with the eye disc Weismann's description is not very 

 clear, but he insists ' that in Sarcophaga the union between the 

 eye disc and the bulbus is not complete on the twelfth day, 

 and that it is easy to separate them with a hair pencil,' and he 

 says ' much intervening fat is still found ' ; and concludes by 

 saying, ' The morphological value of the different parts of the 

 eye is as follows : the cornea is the chitinous skeleton ; the 

 other parts of the eye-chamber (the crystalline cones, nerve- 

 rods, and their investments) are modified hypodermis ; all the 

 central structures (the ganglion layers and bulbus) are formed 

 as outgrowths from the nervous system.' 



Thus so far as Weismann's observations carried him he 

 entirely agrees with my own views, but it must be admitted that 

 he thought that the nervous elements penetrate the visual rod 

 (sehstab), and he supposed that the nerve fibres are developed 

 in the layer of fat between the outer part of the bulbus and the 

 disc from the remains of the stalk of the optic disc, a view 

 which is entirely disproved by the examination of sections 

 made at various periods of development. 



As has been already stated, I hold that the compound eye is 

 developed from two distinct sources, that the dioptron originates 

 from the dermal epiblast, and that the retina is an outgrowth 

 from the central nervous system or from the neural plate. The 

 intervening parablastic cells from which the tracheae (or blood- 

 vessels in the Crustacea) are derived, also play an important 

 part in the development of the dioptron similar to that which 

 the mesoblast takes in the development of the vertebrate eye. 



I shall now proceed to give an account of my own observa- 

 tions and conclusions in detail. 



DESCRIPTION OK I'LATK XXXVII. 



A lateral section through the cephalic nerve-centres and part of the dioptron of a 

 I'.low-lly nymph on the fourth day of the pupa state. c f, the sub-dioptric 

 space; </, dioptron; o .< r , optic ganglion; o n, the zone of the optic nerve; 

 o s, o s, optic stalk ; /, pyramidal ganglion ; ;/, retina ; ///, the lhalamon. 



