THE DEVELOFMKNT OF LHI'IDOSIIIEN PARADOXA. 441 



Optic Nerve. 



Lepidosiren is not well suited for a study of the liis- 

 togeny of the optic nerve on account of its very slender 

 character. 



Structure of the Wall of the Optic Cup. 



It will be convenient, before describing the histogenesis of 

 the retina, to give a short account of its structure in a young 

 Lepidosiren 78 mm. in length. 



Near its centre the retina, including the pigment layer, 

 measures about 125 fx in thickness. 



The retina consists of three definite layers of cell elements 

 — a layer of rod elements or percipient cells, a middle mass 

 of nerve-cells, and a single layer of ganglion cells next the 

 vitreous humour. These cellular layers are separated as 

 usual by two " molecular " layers composed of the extremely 

 fine ultimate ramifications of the cell bodies. On its inner 

 surface the ganglion layer is covered by the thin layer of 

 fibres going to the optic nerve. 



Layer of Visual Cells. 



Drawings of rod elements highly magnified are given in 

 fig. 11, M and N. 



Each cell is cylindrical in shape; at its inner end it is 

 continued by irregular processes into the outer molecular 

 layer, while its outer end bears the rod. Each rod is 

 cylindrical, or rather very slightly conical. It measures from 

 •Oil to "015 mm. in length, i.e. it is much shorter than the 

 corresponding structures in Amphibia. At its outer end the 

 rod is rounded. The substance of the rod shows a division 

 into alternate dim and clear layers, the latter being the 

 narrower. I have not observed any longitudinal mark- 

 ings. 



The above measurements refer to a specimen killed during 

 daylight. Eods from the corresponding part of the retina in a 



