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 Carri^re ('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 Grenadier, '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 Grenadier has not fig- 

 ured anytliing which may be fairly taken to represent Graber's pre-retinal 

 membrane, it is evident from Carriere'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, Carriere ('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. Carriere 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 sufHcient 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. 



