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THE DEVELOPMENT OF THE COMPOUND EVE. 553 
During the third stage of the development of the dioptron, 
the optic stalk and neural portion of the disc expand and their 
tissue becomes spongy. The fibrous structure of these parts 
becomes indistinct, and its elements widely separated from 
each other. They eventually disappear entirely and the space 
which they occupied is seen to be filled with delicate retiform 
tissue. 
In the meantime the sub-dioptric space becomes larger, and 
the mesoblastic cells which occupy it become more numerous 
and increase rapidly in size. These give off processes which 
grow into the dioptron, and are gradually converted into 
tracheal vessels. 
In the fourth stage of development the retina has extended 
into the cavity of the neural portion of the disc, taking the 
place of the retiform tissue and eventually filling the whole 
space bounded by the peritoneal coat of the expanded optic 
stalk and its neural disc. 
During the last three or four days of the pupa stage the sub- 
dioptric space gradually disappears, and the pre-retinal mem- 
brane comes into immediate relation with the basilar mem- 
brane. 
The Development of the Retinal End Organs.—These appear to 
be developed from the epithelial elements of the retinal disc, 
but I have been unable to follow the whole process. It ap- 
pears to me that the cells from which they originate undergo 
considerable elongation, and that their nuclei are displaced 
inwards so that they lie beneath the end organs, between them 
and the optic nerve fibres. 
The non-inversion of the terminal elements in the sense in 
which they are said to be inverted in the Vertebrate eye, has 
already been used as an objection to the views which I have 
expressed ; but there is no involution of the neural epiblast 
from the commencement, and if the roof of the vesicle of the 
cerebron originates by delamination from the hypodermis, as I 
have suggested, no inversion of the retinal elements would 
occur. Nor do I think that the importance of the position of 
the rodlets is of so much importance as has been ascribed to 
