ORGANS OF VISION OF THE VERTEBRATA. 405 



(1) A. membi-aiie, which does not however, as usually believed, 

 become the membrana liniitans 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-Kbres. 

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



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



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

 layer becomes divided into thi'ee strata : viz. an outer and inner layer of 

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

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

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

 to the inner molecular layer. 



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

 has become considerably i-ubdividcd. 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 mei.'ibrana 

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

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

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

 are established in the Rabbit. It is impoitant 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 first form a contiuuous layer, in which 

 separate elements cannot be recognised. 



At a very early period there appears a membi'ane on the side of the 

 retina adjoining the viti-eous humour. This membrane is the hyaloid mem- 

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

 it may bo formetl at a time when there is no trace of mesobiastic structures 

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

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

 Arnold, Lowe, and other authors regard it however as a mesoblatstic 

 product; and Keilliker believes that a. primitive membrane is developed 

 frcmi 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 ou 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 

 nerves 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 



