THE COMPOUND EYE OF THE BLOW- FLY. 535 



Blow-fly and in those insects in which the chaplet cells are 

 most conspicuous ; or they may be true nervous elements. 



My own opinion inclines entirely to the first of these views. 

 I regard them as mesoblastic elements which form the outer 

 surface of the neuroblast from which the retina is developed, 

 and which grow in between the bacillary elements, and form a 

 kind of sustentacular framework. They are possibly analogous 

 to the pigment epithelium of Vertebrates, although, like the 

 choroid of the latter, they have a mesoblastic origin. 



In support of this view, I may mention that they first appear 

 as an invagination (Fig. 63, />), and that their behaviour with 

 stains is somewhat different to that of the nerve cells of the 

 ganglia. They colour more feebly. In specimens prepared with 

 gold chloride they exhibit the form of a continuous network 

 (PI. XXXVIII. , Fig. 5,/), and in their deeper layers this is easily 

 seen in tangental sections ; they are entirely replaced by fringed 

 pigment cells in noctuid Moths and in many other insects. 



It must be admitted, however, that, tempting as this explana- 

 tion of their nature is, the observations of Viallanes and of 

 Hickson lead to a different conclusion. Viallanes states that 

 they form chaplets and are not connected with each other 

 transversely, and Hickson gives some remarkable figures from 

 specimens fixed with gold chloride and subsequently teased out. 

 I confess I have been unable to obtain specimens such as he 

 figures. His figures are, however, very remarkable [237, Figs. 

 16 and 17). What he terms a neurospongium is evidently, I 

 think, a tracheal capillary network, but he represents fine fibres, 

 which he regards as nerve fibres, connecting these cells on the 

 one hand with the optic nerve, and on the other with the so- 

 called palisade cells of Carriere, my proximal segments of the 

 bacilli. 



The Pigmented Tapetum. One of the principal objections 

 advanced against the views here advocated is that in most 

 insects a thick layer of opaque pigment intervenes between 

 the palisade layer of Carriere and the membrana basilaris. 

 This is especially the case in noctuid Moths. This pigmented 

 layer takes the place of the chaplet cells, and, according to my 



