1884.] NATURAL SCIENCES OF PHILADELPHIA. 309 



a very meagre deposit of pigment ; the anterior edge of this 

 double-walled cup, formed by the retina and pigmented layer, 

 come together, owing to the absence of the lens. It is stated ^ 

 that no corpus vitreum is present. This degenerate eye is of 

 little use to the animal, and, besides the loss of the lens, it is 

 covered by the general integument of the body. Now it may 

 be argued, upon my hypothesis, that the lens should be last to 

 disappear, being phylogenetically the first to appear ; but as the 

 secondary optic vesicle has taken up the principal function of the 

 eye, viz. : the developing of nerve-energy, we would naturally 

 expect that the accessory organs would be the first to disappear 

 in the process of degeneration ; hence, the lens modified to an 

 organ of refraction, although the most primitive part of the eye, 

 would disappear before the secondary optic vesicle, since it has 

 lost its function as an eye and acts merely as a refractive agent. 



Another objection may be raised, which may be well to insert 

 here, viz. : Why should the process from the proencephalon 

 start before the invagination, to form the lens, the former being 

 a secondary state in the phylogeny of the animal ? I would 

 explain this by the fact that as the optic vesicle, being now the 

 most important part of the e^^e, and so established for many gen- 

 erations, now appears Jirst and disappears last in degeneration. 



In Myxena glutinosa, as described by Wm. Miiller,^ we have an 

 eye consisting of the secondary optic vesicle, as in the case of 

 Proteus, but open in front and filled with a plug of mesodermal 

 tissue. The eye is entirely devoid of pigment and lies buried 

 beneath a layer of muscle underlying the skin. The optic nerve 

 passes into the vesicle, and terminates in the retina, there being 

 no layer of optic nerve-fibres present at all. This eye has pro- 

 ceeded a step further in its degeneration, than the eye of Proteus, 

 being entirely devoid of pigment, and having become more deeply 

 imbedded, is covered by a layer of mesodermal tissue, the mus- 

 cular stratum. 



Thus in degeneration, the eye proceeds, step by step, back- 

 wards towards the brain, after first losing its accessories, such 

 as the lens, cornea, sclerotica, etc. 



' Semper, Carl. Animal life as affected by the natural conditions of exist- 

 ence. Intern. Sci. Series, vol. xxx, New York (Appleton), 1881, p. 78. 



^ Miiller, Wm. Ueber die Stammesentwickelung des Sehorganes der 

 Wirbelthiere. Festgabe an Carl Ludw;g. Leipzig (Vogel), 1875, p. vii. 



