66 T. H. MORGAN. 



eye. Patten's suggestion was at the time, I think, little more 

 than a suggestion of a possibility, and no structures in the 

 Arachnid eye were given to support it. 



Mark, on the other hand, has complicated his hypothesis by 

 certain additions, both of which I believe one would, a priori, 

 rather reject, if possible. First, that the one-sided development 

 he supposes to have taken place to have been the result of a 

 change of position in the lens, so that part of the eye was placed 

 to better advantage than the rest, a supposition not founded on 

 any facts, and it seems to me highly improbable. Secondly, the 

 inverted elements of the embryo right themselves (as they are so 

 found in the adult) by a complete . change in the parts of each 

 element, so that the rods which are at first secreted at one end of 

 the cell — the inner — subsequently are formed at the opposite end 

 of the element — the outer — and that the nerve fibrils were also 

 transferred from one end of the cell to the other. To be sure we 

 do not yet know how the eye does right itself, yet it seems most 

 improbable it has done so by such an unusual method, and it 

 would be much easier to believe that the elements shift around 

 so that their inner ends become the outer. 



I have examined carefully the inverted eye of the Fycnogonids 

 to see if it would throw any light upon the early inversion of the 

 eyes of Arachnids. In the first place, I am inclined to believe 

 that this inversion of the Spider's eye cannot be explained as a 

 purely embryonic phenomenon. From what has been given in 

 the preceding sections, I am ready to believe that this inversion 

 of the embryo Spider's eye is to be explained as a phylogenetic 

 process, and that the adult Pycnogonid's eye represents approx- 

 imately the condition of the eye of the ancestral Arachnid (or, 

 at any rate, the ancestral Spider). 



Let us now see if there is anything in the Pycnogonid eye to 

 determine whether this primitive inversion was caused by a one- 

 sided development of the eye, as Mark has assumed (for Spiders), 

 or whether we have 'any evidence pointing in other directions. 

 I have been much impressed by the bilateral arrangement of the 

 eye in the Pycnogonids, and it has seemed to me probable that 

 this might give a clue to the solution we are in need of. The 

 corneal hypodermal cells show a marked bilateral structure in 

 the arrangement of the cells on each side of the middle line. 



