40 BULLETIN OF THE 



case of the crayfish gave rise to a mass of tissue which formed on the 

 one hand the deeper part of the retina, and on the other a portion of 

 the optic ganglion, while the cells which arise by proliferation from this 

 region in the lobster produce only a part of the optic ganglion. But, 

 as Patten ('87, pp. 208, 209) has shown, Reichenbach himself was not 

 certain that any part of the infolded ectoderm contributed to the for- 

 mation of the retina. Reichenbach found it difficult to locate exactly 

 the position w T hich the developing rhabdomes occupied. On page 92 in 

 his account he describes a part of the outer wall (Ausserwand) of the in- 

 folded ectoderm, and states his belief that in it the rhabdomes develop ; 

 in fact, he describes certain red bodies which he says are without doubt 

 the rhabdomes themselves. On page 96 he admits that the layer in 

 which he supposed the rhabdomes originated may be a layer of nerve- 

 fibres. Granting this interpretation, it is no longer possible to consider 

 the previously described red bodies as rhabdomes. Reichenbach does not 

 make this last statement, but his description implies it when he states 

 that, although the region of the red bodies may not be the region of the 

 rhabdomes, yet the rhabdomes doubtless originate in a somewhat more 

 superficial part of the outer wall. Apparently he has not identified the 

 rhabdomes in their new position ; at least, he makes no such statement 

 in his text or description of plates. Since he has also admitted that the 

 red bodies may not be rhabdomes, I cannot see that he has positively 

 identified any structure as a rhabdome. Such being the case, it is diffi- 

 cult to understand on what grounds he can maintain the assertion that 

 the rhabdomes develop in the outer wall. If this assertion cannot be 

 defended, then it is possible that they may develop in the superficial 

 hypodermis. This would be analogous to the condition presented in the 

 lobster. 



If the rhabdome in the eye of the crayfish is developed, as I believe 

 it is, in the superficial hypodermis, and not in the outer wall of the in- 

 folded hypodermis, the objection which was suggested in homologizing 

 the involution in the eye of the crayfish with the proliferation of cells 

 in the eye of the lobster has no weight. 



In the development of the eye of Crangon, according to Kingsley's 

 ('86 a ) account, there is also an optic invagination. If what I have at- 

 tempted to show in regard to the eyes of the crayfish be true, then this 

 invagination in Crangon should be connected with the formation of the 

 ganglion only. This, as I have already stated, is not the view held by 

 Kingsley, for he maintains that the outer wall of the invaginated pocket 

 gives rise to the retina, and only the inner wall is concerned in the pro- 



