1884.] NATURAL SCIENCES OF PHILADELPHIA. 301 



(coloboma retinae) ; in the latter case, on examining the eye 

 with an ophthalmoscope, we can see a wedge-shaped white patch, 

 the base downwards, at the inferior part of the background 

 of the eye, the white is the sclerotica shining through. This is 

 merely a reversion to a primitive state — a failure in the union, 

 posteriorly, of the two lateral walls of the secondary optic vesicle. 



The optic nerve is formed by the bending over, and union 

 below, of the connecting portion of the secondary optic vesicle 

 with the fore-brain. The portion of mesoderm included by this 

 process is later termed the arteria centralis retinas. The sclerotica 

 and the vascular laj'er of the choro'idea are formed from the 

 mesoderm, and are merely organs for the nourishment and the 

 protection of the nerve-elements within. 



It seems to me that the steps taken in ontogenic development 

 of the eye, point out to us that the course which has been 

 pursued in its phylogenesis from a simple epithelial pigmentary 

 deposit, is as follows : The first visual organ primarily consisted 

 of a deposit of pigment, centralized at that portion of the animal 

 where it will be of the most use, viz. : at the oral pole. Since 

 animals as a rule proceed with this extremity forward, they are 

 developed in this situation ; but in some cases, as in the Lamelli- 

 branchiata, as pointed out above, they are developed at that 

 portion of the body which needs their protection. The next step 

 in advance is to protect these important organs, and as a conse- 

 quence invaginated grooves result, which graduall}^ shorten to 

 form a sphere. The refracting media of separate cells soon 

 coalesce, to produce a cuticular lens. The nerves of general 

 sensibility, connecting this eye with the brain, soon become 

 specialized, and form a distinct (primitive) optic nerve. As the 

 eye increased in importance and usefulness to its possessor, a 

 corresponding stimulation took place in the brain, where sight is 

 without doubt seated. Increased activity in any organ causes a 

 corresponding increase in blood-supply — or better, nutriment- 

 supply — and an increase of development took place all along the 

 tract, from the eye to the seat of vision in the brain. As this 

 increased, that part of the brain nearest the e^'e enlarged, and 

 proceeded by steps toivard the eye, similar to the process nowtaking 

 place in the development of the eyes of the Verlebrata, the primi- 

 tive optic nerve still connecting the two. We then have a stage 

 in which a part of the brain closes over the superior part of the 



