492 THE OPTIC NERVE. 



Lowe's account of the development of the retina in the Rabbit is in many 

 points different from the above. He finds that three stages in the differen- 

 tiation of the layers of the retina may be distinguished. 



In the first stage, in an embryo of four or five millimetres, the following 

 layers are present, commencing at the outer side, adjoining the external wall 

 of the secondary optic cup. 



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

 become the membrana limitans 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 these 

 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 subdivided. 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 membrana 

 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 first 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, Lowe, and other authors regard it however as a mesoblastic 

 product ; and Kolliker believes that a primitive membrane is developed 

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

 developed 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 



