544 THE SENSES AND SENSORY ORGANS. 
the elements which belong to the dioptric apparatus, including 
the great rods (Nervenstabe).’ 
With 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 trachez (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 OF PLATE XXXVII. 
A lateral section through the cephalic nerve-centres and part of the dioptron of a 
Blow-fly nymph on the fourth day of the pupa state. c¢ c, the sub-dioptric 
space; d, dioptron; 0, optic ganglion; 0 m, the zone of the optic nerve ; 
0s, 0S, optic stalk ; #, pyramidal ganglion ; 7¢, retina; ¢, the thalamon. 
