MUSEUM OF COMPARATIVE ZOOLOGY. 41 



d action of the ganglion. This is fundamentally different from the con- 

 dition found in the lobster. The difference, however, is due, I believe, to 

 the fact, that in describing the later stages of development Kingsley has 

 pointed out a cavity which he believed to be the cavity of the invagi- 

 nation, but which in reality is not. The cavity which he has marked oc 

 in his Figures 3, 4, and 5, is unquestionably the cavity of involution, 

 but the space marked oc in his other figures is, in my opinion, a part of 

 the body cavity. 



My reason for this belief is as follows. The cavity of an involution 

 such as is found in the anterior median eyes of spiders or the median eyes 

 of scorpions is, when it has lost its connection with the exterior, a closed 

 ectodermic sac. That such a cavity should be occupied by migrating 

 mesodermic cells seems to me extremely questionable, for in order to 

 enter the cavity it would be necessary for the cells to penetrate one 

 wall of the vesicle. This of course is not impossible, but it is not borne 

 out by analogy with the Arachnoids. The cavity which Kingsley has 

 marked oc in Figure 7 is occupied by several mesodermic nuclei, and 

 what is more important, perhaps, is that it is apparently connected with 

 other cavities in the embryo. These cavities also contain mesodermic 

 tissue. Excepting Figures 3, 4, and 5, the cavities marked oc I believe 

 to be homologous, and I further believe that they represent, not the 

 cavity of involution, but the space which intervenes between the infolded 

 pocket and the superficial ectoderm. This is a part of the embryonic 

 body cavity, and is of course readily accessible to mesodermic cells. 

 The fate of the real cavity of involution is not so easily discovered. 

 Probably, as in the case of the crayfish, it is obliterated in the mass of 

 tissue from which the retinal ganglia arise. 1 



If the interpretation which I have suggested in the preceding para- 

 graph be admitted, the development of the eye in Crangon is essentially 

 the same as in the lobster and crayfish. The proliferation in the optic 

 disks of the lobster is represented by an involution in the disks of 

 Crangon. The cavity of the involution disappears in Crangon, and its 

 walls give rise to ganglionic tissue. The part of the superficial ecto- 



1 Since this paper was written, I have received a copy of the third part of 

 Kingsley's studies on the development of Crangon. As the following quotation 

 will show, Kingsley ('89, p. 20) has materially changed his views as to the forma- 

 tion of the retina : " I may say here that I am inclined to believe that I fell into 

 error in my account of the development of the Compound Eye of Crangon, and 

 that the invagination or inpushing which I there described as giving rise to the 

 ommatidial layer of the eye, in reality gives rise to the ganglion of the eye which 

 in the adult is contained within the ophthalmic stalk." 



