66 BULLETIN OF THE 
tion of some of the early stages in the formation of the ocelli of Vespa, 
which Mr. F. A. Houghton is investigating, that a process of involution 
takes place ; and I believe that here also it will be shown that there is an 
inversion of the retinal area.* 
If the presence of a distinct and continuous layer of “ vitreous” cells in 
front of the retina possesses any weight in favor of an involution after the 
type of spiders’ eyes, then the simple ocelli of adult Hexapods are likely 
to have followed the same plan of development as the eyes of Arachnoids. 
That the cells of the vitreous layer are usually so flat and thin that they 
have sometimes been overlooked, does not in the least diminish their im- 
portance as an index to the manner in which the retina was produced. 
Indeed Carriére (’85, p. 178) has shown conclusively that the cells com- 
posing the thin layer which represents the “vitreous” in the completed eye 
of Vespa, are much reduced in size as compared with their condition dur- 
ing the formation of the lens. The figure which he has given (Fig. 142) 
of the eye of the wasp during this stage is very instructive, for it shows 
that, however obvious the continuity of hypodermis and retina may ap- 
pear in the finished state of the eye (compare Grenacher, ’79, Fig. 31), 
they are separated during this earlier condition by a wide interval, and 
that consequently the supposed continuity can have no such importance 
as might otherwise be attributed to it. Although Grenacher has not fig- 
ured anything which may be fairly taken to represent Graber’s pre-retinal 
membrane, it is evident from Carriére’s figure of the earlier condition that 
retina and ‘‘vitreous” are sharply separated by a line which seems to be a 
continuation of the inner cuticula of the hypodermis, much as in the eyes 
of spiders ; and Grenacher himself, criticising Leydig’s views, has insisted 
upon the sharp separation of the two cell-layers. 
* Since the above was written, Carritre (’86) has published an article in the 
Zoolog. Anzeiger (Jahrg. 9, no. 217, pp. 141-147), in which he has reverted to the 
histological conditions of the ocelli in the Diptera and Orthoptera ; but he has not 
given any further evidence concerning their development. _ 
Postscript. — Under date of June 1, Prof. Carriére writes me that he has arrived 
(independently) at the conclusion that the ocelli in Hymenoptera and Diptera are 
formed by a process of involution, but that the infolded region does not become 
detached from the hypodermis. 
It is possible that this difference of opinion is more formal than real, since there 
is probably no period in the formation of the ocellus, after the earliest stages of 
involution, during which the involuted portion is not in contact with the hypodermis 
in the region of the ‘vitreous ;” but the ultimate intervention of the pre-retinal 
membrane is to me sufficient evidence of an interruption in the original continuity 
of the cell-layers. That is all I should wish to claim by saying the infolded portion 
of the hypodermis became ‘‘ detached” from the permanent hypodermis. 
