MUSEUM OF COMPARATIVE ZOOLOGY. 39 



It will be remembered that the superficial layer of nuclei in the optic 

 disk of the lobster was divided into two regions. The one farther from 

 the median plane has been called the distal region ; the one nearer, the 

 proximal region. The broadening of the distal region produced the 

 retina. By a proliferation of its cells the proximal region resulted in 

 the formation of the optic ganglion. It is my opinion that this prolifer- 

 ation of cells represents what is produced in the case of some Crusta- 

 ceans by an involution, and that either an involution or proliferation, or 

 possibly a combination of both processes, occurs in the eyes of all Crus- 

 taceans. Whichever process characterizes the development of a given 

 eye, it must be borne in mind that the involution or proliferation is 

 connected with the formation of the ganglion only, and takes no part in 

 the production of the retina. The latter is a simple thickening in the 

 ectoderm ; the optic ganglion is developed either as a proliferation or 

 involution of the ectoderm which lies close beside the retina. The 

 results at which various investigators have arrived, different as they 

 may at first appear, can be harmonized, I believe, by this interpreta- 

 tion of the origin of the retina and optic ganglion. 



In his account of the development of the eye in the crayfish, Reichen- 

 bach ('86, p. 85) has described an ectodermic involution, which occurs 

 nearly in the centre of the optic disk. That portion of the disk which 

 is farther from the median plane than the region of involution gives 

 rise to the retina, so that the region of involution in the crayfish 

 occupies a position which corresponds to the region of proliferation in 

 the lobster. Not only do the two regions correspond, but the masses 

 of tissue which are developed from each have certain peculiarities com- 

 mon to both animals. In the case of the crayfish the mass of tissue 

 which results from the involution becomes divided into an outer and an 

 inner wall. These two walls are separated from each other by a band 

 of nuclei, which are larger and lighter in color than the surrounding 

 nuclei. In the lobster the ganglionic tissue which arises by proliferation 

 is divided into an outer and an inner part. The separation is effected 

 by a band of nuclei, which in position and structure resemble the band 

 figured by Reichenbach. (Compare Reichenbach, '86, Plate XII. Fig. 

 174, and Plate III. Fig. 41 of this paper.) The similarity presented by 

 the bands of nuclei in the lobster and crayfish supports the conclusion 

 that the involution in the crayfish and the proliferation in the lobster 

 are homologous structures. 



An objection to this comparison might be raised on the ground that, 

 according to Reichenbach's statement ('86, p. 93), the involution in the 



