ORGANS OF VISION OF THE VERTERRATA. 405 



(1) A membrane, which does not however, as usually believed, 

 become the membrana limitaus externa. 



(2) A layer of clear elements, derived from metamorphosed cells, 

 constituting the outer limbs of the rods and cones. 



(3) A layer of dark rounded elements. 



(4) An indistinctly striated layer, the future layer of nerve-fibres. 



The third of these layers gives rise to all the eventual strata of the 

 retina proper, except the outer limbs of the rods and cones. 



In the next stage, when the embryo has reached a length of 2 cm., this 

 layer becomes divided into three strata : viz. an outer and inner layer of 

 dark elements and a middle one of clearer elements. The two inner of thesw 

 layers become respectively the inner molecular layer and the layer of gan- 

 glion cells, while the outer layer gives rise to the parts of the retina external 

 to the inner molecular layer. 



In the newly born animal the outer darker layer of the previous stage 

 has become considerably Mibdivided. Its outermost part forms a stratum of 

 darkly coloured elements, which develop into the inner limbs of the rods 

 and cones. It is bounded internally by a membrane the true meissbraiia 

 elastica externa. The part of the layer within this is soon divided into the 

 outer and inner granular layers, separated from each other by the delicate 

 outer molecular layer. Thus, shortly after birth, all the layers of the retina 

 are established in the Rabbit. It is important to notice that, according to 

 Lowe's views, the outer and inner limbs of the rods and cones are metamor- 

 phosed cells. The outer limbs at rirst form a continuous layer, in which 

 separate elements cannot be recognised. 



At a very early period there appears a membrane on the side of the 

 retina adjoining the vitreous humour. This membrane is the hyaloid mem- 

 brane. The investigations of Kessler and myself lead to the conclusion that 

 it may be formed at a time when there is no trace of mesoblastic structures 

 in the cavity of the vitreous humour, and that it is therefore necessarily 

 developed as a cuticular deposit of the cells of the optic cup. Lieberkiihn, 

 Arnold, LiJwe, and other authors regard it howe\er as a mesobhu-tic 

 product; and Kijlliker believes that a primitive membrane is developed 

 from the cells of the optic cup, and that a true hyaloid membrane is deve- 

 loped much later as a product of the mesoblast. 



For fuller information on this subject the reader is referred to the 

 authors quoted above. 



The optic nerve. The optic nerves are derived, as we have said, 

 from the at first hollow stalks of the optic vesicles. Their cavities 

 gradually become obliterated by a thickening of the walls, the 

 obliteration proceeding from the retinal end inwards towards the 

 brain. While the proximal ends of the optic stalks are still hollow 

 the rudiments of the optic chiasma are formed from fibres at the roots 

 of the stalks, the fibres of the one stalk growing over into the 

 attachment of the other. The decussation of the fibres would 

 appear to be complete. The fibres arise in the remainder of the 

 uerves somewhat later. At first the optic nerve is equally con- 

 tinuous with both walls of the optic cup ; as must of necessity be 

 the case, since the interval which primarily exists between the two 



