No. I.] EYES OF ARTHROPODS. 21 1 



and in an obscure way, similar to that seen in Blatta and 

 Phryganids. 



I have made some observations on the development of the 

 compound eyes in Blatta and Phryganids, which will be of in- 

 terest here, since the closing-in of the ommateum is quite differ- 

 ent from that in Vespa. The process in these two forms is, very 

 likely, the more prevalent one ; and, as I have already sug- 

 gested, a similar process probably takes place in Astacus and 

 Crangon. 



There is a thickening of the hypodermis on one side of the 

 infolding of the optic ganglion. The cells on the periphery of 

 the thickening become flattened, and their nuclei are gradu- 

 ally pushed over the outer ends of the ommateal cells to form 

 a very thin, but continuous layer over them, but without the 

 formation of any fold, and with only a barely noticeable depres- 

 sion of the thickening. It is remarkable that the slight de- 

 pression is not deepest in the middle of the thickening, but on 

 its posterior ventral edge, as in Vespa. 



The formation in this way of a double layer of cells is un- 

 doubtedly a modification of the process seen in Vespa. In my 

 paper on the " Eyes of Molluscs and Arthropods" (27) I gave 

 reasons for supposing that the primitive Arthropod ocellus was a 

 closed sac, the whole eye consisting of three layers. I further 

 suggested that the compound eye was a modified ocellus, and 

 consisted of two layers, the middle one, the outer wall of the 

 optic vesicle, having disappeared. The development of the eyes 

 of Vespa, Blatta, and Phryganids, shows that surmise to be 

 very nearly correct; for the compound eye of Vespa is at one 

 period a closed but flattened vesicle, covered by a layer of 

 hypodermis, in other words, it consists of three layers similar to 

 to those we supposed to exist in a primitive ocellus. 



The ocelli of Vespa appear at the close of the larval period 

 diS four deep pits whose walls consist of a single layer of cells. 

 The two anterior pits are situated close together, and soon fuse 

 to form a single depression which develops into the anterior 

 unpaired ocellus. 



Just after the cessation of larval activity the lips of the pits 

 probably close ; for, at the beginning of the pupal stage, the 

 cavities have disappeared, and we have three depressed, oval 

 thickenings, composed of two layers of cells. I have, unfortu- 



