32 BULLETIN OF THE 



development of the eye in a Crustacean, and, as I have previously ex- 

 plained, the species most available for such a study was the lobster. 



The results which have been presented in the more recent papers on 

 the embryology of the compound eyes require a brief notice before the 

 development of the eye in the lobster is described. What is said in this 

 connection is purely introductory ; no criticism of the views of different 

 authors will be made until after the development of the lobster's eye 

 has been described. 



The writers who have thus far published accounts of the development 

 of compound eyes can be grouped under four heads, depending upon the 

 type of eye which their researches indicate. The first type is that 

 represented by the eye of Peripatus. Patten ('86, p. 688, '87, p. 211) 

 is of the opinion that the compound eyes of Hexapods, as well as Crus- 

 taceans, are constructed upon this plan. Each eye should then consist 

 of a closed vesicle which was produced by an involution of the hypo- 

 dermis. The eye would be composed of three layers, which in the 

 order of their positions are as follows : first, the superficial hypodermis ; 

 second, the outer wall of the vesicle ; and third, the inner wall of the 

 vesicle. Patten is of opinion that in the eye of an adult individual 

 these three layers are modified in the following way : the superficial 

 hypodermis becomes the corneal hypodermis; the outer wall of the 

 vesicle is so far reduced as to be inconspicuous, and the inner wall gives 

 rise to the retina. The retina includes the crystalline cones and the 

 pedicels (rhabdomes). Patten supports these conclusions mainly from 

 theoretical grounds, but he believes that he has found evidence of the 

 existence of this type in the development of the compound eyes of 

 Vespa, Blatta, and the Phryganids. 



The second structural type which I shall mention has been advocated 

 by Kingsley ('87, p. 51.) in his description of the development of the 

 compound eye of Crangon. 1 In this type the eye results from a vesicu- 

 lar infolding, as in that which was proposed by Patten, but it differs 

 from the latter in the fate which is ascribed to the two walls of the 

 vesicle. According to Kingsley, the outer wall of the vesicle is not 

 reduced, but gives rise to the retina. In it are developed the crystal- 

 line cones and the pedicels (rhabdomes). The inner wall of the vesicle, 

 instead of forming the retina, as Patten believed, is converted into a part 

 of the optic ganglion. 



The third structural type is that which is presented by Reichenbach 

 ('86, pp. 85-96) in his account of the development of the crayfish. 



1 See the note on page 41. 



