418 MARGARET ROBINSON. 



Unfortunately this stage is not sufficiently advanced to 

 enable one to write definitely about tlie future of the different 

 parts of the eye mentioned here ; still it seems almost certain 

 that the outer cells of the optic thickening first seen in Stage 

 B, seen also in Stages C and D, to lie in front and outside of 

 (i. e. more lateral than) the optic invagination and the optic 

 ganglion, and to be traced in the later stages, do really fur- 

 nish the cone cells and the corneal hypodermis, while the 

 optic invagination furnishes the retinulae, and in the early 

 stages certainly gives off some cells for the increase of the 

 optic ganglion. 



I have not been able so far to find any mesoderm cells 

 between the optic ganglion and the future retina. 



Since Reichenbach (1886) a great deal has been written 

 about the development of the crustacean eye as a whole, and 

 the optic invagination in particular. Kingsley (1887) seems 

 to derive the whole eye, corneal hypodermis excepted, from 

 the invagination which he says never becomes solid. Pai'ker 

 (1891) was of opinion that when an invagination occurs it is 

 concerned with the optic ganglion only. In a later paper (1895), 

 however, he seems to have veered round to Reichenbach's 

 view as to the Crayfish eye, viz. that the outer wall caused 

 by a division of the primary invagination forms the retina, 

 and the inner furnishes cells to the ganglion. There can be 

 little doubt as to the homology of the proliferation in 

 Branchipus, the Lobster and Mysis, with the invagina- 

 tion in the Crayfish and Nebalia. Parker (1895) says that 

 eventually in the Lobster, and probably in the Crayfish 

 the ganglion loses its connection with this centre of growth, 

 which continues as a growing area for the retina only. But 

 Claus (1889), on the other hand, considered that the prolifera- 

 tion in the Branchipus larva was continued as the zone of 

 growth in the adult, and that this zone furnished ganglion 

 cells in the direction of the ganglion, and retina cells in the 

 direction of the retina. He believed the zone of growth in 

 the eye of the adult Nebalia to be homologous with that in 

 the eye of Branchipus, and that in the Crayfish. In the 



