68 T. H. MORGAN. 



The evidence that the eyes of the Sea-Spiders have evolved 

 along some such line as this is found in the structure of the eye 

 as existing to-day. The median raphe of the middle layer indi- 

 cates the points of contact between the retinal elements from the 

 two sides. These, as they became inverted, came into contact in 

 the middle line. The bilateral arrangement of the nuclei follows 

 almost as a necessity of this two-sided inversion. These would 

 be crowded together when the retinal elements turn in, and few 

 or none along the middle line. Patten shows on the outer ends 

 of the retinal elements of Insect eyes a layer of what he calls 

 " secondary nuclei," which, so far as I can judge, are identical 

 with the round stainable bodies found at the inner ends of the 

 inverted elements of the Pycnogonid eye. One sees at once that 

 by the inversion of the lateral elements of the cup the bodies 

 must be found in exactly the same position as they are found in 

 the Pycnogonid eye. 



If further evidence were needed to prove the two-sided inver- 

 sion, it would, I think, be amply found in the conditions existing 

 in the pigment layer of the eye. The cells immediately at the 

 back (at the base of the cup) are cylindrical cells, such as would 

 result from a shortening of the retinal elements. If one draws a 

 diagram of the process I have sketched, it becomes evident that 

 as we progress around the sides of the eye the pigment cells 

 must pass insensibly into the retinal cells ; and such, indeed, we 

 found previously to be the case. Thus the retinal cells here 

 have not suffered so great a change as those at the back of the 

 eye; but, developing pigment, they furnish a dark background 

 for the retinal cells, but still retain largely their ancestral shape. 

 Finally, the large nerve process running from these pigment cells 

 is explained by their ancestral function as fibrils to the retinal 

 elements. 



To sum up, I believe all the layers of the Pycnogonid eye give 

 abundant evidence that the eye has developed by the turning in 

 of two sides of a primitive optic vesicle, and that the simple eyes 

 of Insects furnish all the intermediate stages, both in develop- 

 ment and adult structure, between a simple cup-like invagination 

 and the three-layered condition of the Pycnogonid eye. 1 



'This and several other facts (as multipolar delamination, etc.) point 

 perhaps to a genetic relationship of Insects to Arachnids, but I cannot go into 

 these comparisons at present. 



