42 BULLETIN OF THE 



derm which is immediately external to the ganglionic involution thickens 

 and produces the retina. 



The mode of development which Patten has suggested for the com- 

 pound eye of Arthropods has not received any support, so far as I am 

 aware, from the embryology of the Crustacean eye. The only observa- 

 tions which go to confirm Patten's opinion are those of his own on 

 Vespa, Blatta,. and the Phryganids. Possibly the compound eyes of 

 Crustaceans may be developed upon a different plan from those of Hexa- 

 pods. Certainly the evidence which Patten has given for the Peripatus- 

 like type of the compound eye of Hexapods has not been found in any 

 of the Crustacea. According to Patten's view of the origin of the com- 

 pound eyes, the corneal hypodermis should arise on the sides of the 

 optic area, and spread over the retina until the latter is entirely cov- 

 ered. This constitutes the closing of the shallow vesicle. When I 

 come to describe the differentiation of the ommatidia, I believe it can be 

 shown beyond a doubt that in the lobster the corneal hypodermis arises, 

 not by any lateral growth from the edge of the optic area, but by a 

 simple process of delammation. The cells of the corneal hypodermis are 

 the differentiated superficial cells of that thickening in the hypodermis 

 which produces the retina. Thus the plan which Patten has suggested 

 for the compound eyes in Arthropods is not supported by the evidence 

 derived from the development of the lobster. Patten himself ('87, 

 p. 202) admits that in Vespa he did not see the closure of the hypo- 

 dermis over the retina, and that in Blatta and the Phryganids ('87, 

 pp. 208 and 211) the process is very obscure. 



The researches of Carriere point to a type of compound eyes for the 

 Hexapods which is similar to that exhibited by the lobster. It is possi- 

 ble that the vesicular origin of the compound eyes of Hexapods, owing 

 to their obscure method of formation, may have been overlooked by 

 Carriere, but I am inclined to believe, after a consideration of both sides 

 of the question, that the evidence favors the simpler method of origin, 

 and therefore that the compound eyes of Hexapods as well as Crusta- 

 ceans arise as simple hypodermal thickenings. 



In Mysis, according to Nusbaum ('87, pp. 171-185), the develop- 

 ment of the eye follows essentially the same course as that which I have 

 described in the lobster. The optic disks are formed and the ganglionic 

 and retinal portions are differentiated in the same manner in both cases. 

 The development of the retina is slightly complicated in Mysis by the 

 fact that, instead of lying in one plane, or very nearly so, the retina is 

 strongly folded on itself, so as to give the appearance of two lamellae, an 



