MUSEUM OF COMPARATIVE ZOOLOGY. 33 



Here a hypodermal involution takes place, and a vesicle is produced, 

 but the cavity of the vesicle is soon obliterated. The mass of cells 

 which results from the fusion of the walls of the vesicle now divides into 

 an outer and an inner layer. These two layers do not necessarily cor- 

 respond to the outer and inner walls of the original vesicle. The super- 

 ficial hypodermis and outer layer of the infolded mass fuse, and give 

 rise to the retina. The crystalline cones are developed in that part of 

 the retina which is derived from the superficial hypodermis ; the rhab- 

 domes probably originate in that part which is derived from the outer 

 layer of infolded cells ; the inner layer of cells becomes ganglionic. 



Bobretsky's ('73) account of the development of the compound eyes 

 in Astacus and Palsemon agrees in its essential features with the de- 

 scription which Reichenbach has given for the eyes in the crayfish. 

 Bobretsky, however, does not describe an involution in the optic disks. 

 As Reichenbach suggests, the fact that Bobretsky did not have the oppor- 

 tunity of studying very early stages may explain his failure to observe 

 the involution. In other respects, the accounts are essentially alike, and 

 there is little doubt that the plan of eye which Bobretsky's researches 

 indicate is the same as that suggested by Reichenbach's studies. 



Each of the three structural types which have thus far been described 

 are dependent upon the formation of a hypodermal vesicle. The fourth 

 type is simpler than any of the three preceding ones, in that a vesicle is 

 not necessarily produced, the retina being supposed to originate as a 

 simple thickening in the superficial hypodermis. This type has been 

 advocated by Herrick ('86, p. 43) in his account of the development 

 of Alpheus. The researches of Nusbaum ('87, pp. 171-186) on the 

 development of Mysis indicate the same type. Neither in Grobben's 

 ('79) account of the development of the eyes in Moina, nor in Claus's 

 ('86, pp. 307-324) description of those in Branchipus and Artemia is 

 any mention made of an involution. These authors might, therefore, 

 be cited as favoring the fourth type of eye, although it is to be observed 

 that the special question of the vesicular origin of the eye is not dis- 

 cussed by them. 



The advocates of the fourth type find support, not only in the embry- 

 ology of the Crustacea, but also in that of the Hexapods. According to 

 Carriere ('85, pp. 181-186), the compound eyes of some Hymenoptera 

 and Lepidoptera develop as simple thickenings of the hypodermis. 



Of the four types which have been mentioned, the one with which 

 the development of the lobster's eye accords will be seen from the 

 following description. 



vol. xx.— no. 1. 3 



