106 THE ORIGIN OF VERTEBRATES 



Petromyzon of such a character as to make this animal one of the 

 most valuable objects for deciding the various controversial questions 

 connected with the genesis of its elements. The lumen of the stalk 

 of the primary optic vesicle is obliterated quite early by a prolife- 

 ration of its lining epithelium. Also the original continuity of this 

 epithelium with that of the pigment-layer is at an early period 

 interrupted at the point of attachment of the optic stalk. This 

 interruption occurs at the time when the fibres of the optic nerve 

 first become visible." 



Further on he says — 



" The epithelium of the optic stalk develops entirely into sup- 

 porting cells, which in Petromyzon fill up the original lumen and so 

 form an axial core (Axenstrang) to the nerve-fibres which are formed 

 entirely outside them ; the projections of these supporting cells are 

 directed towards the periphery, and so separate the bundles of the 

 optic nerve- fibres. The mesodermal coat of the optic stalk takes no 

 part in this separation ; it simply forms the connective tissue sheath 

 of the optic nerve. The development of the optic nerve in the 

 higher vertebrates also obeys the same law, as I am bound to conclude 

 from my own observations." 



The evidence, then, of Ammococtes is very conclusive. Originally 

 a tube composed of a single layer of epithelial cells became expanded 

 at the anterior end to form a bulb. On the outside of this tube or 

 stalk the fibres of the optic nerve make their appearance, arising from 

 the ganglion-cell layer of the retina, and, passing over the surface of 

 the epithelial tube at the choroidal fissure, proceed to the brain by 

 way of the optic chiasma. Owing to the large number of fibres, their 

 crossing at the junction of the stalk with the bulb, and the narrow- 

 ness at this neck, the obliteration of the lumen of the tube which 

 takes place in the stalk is carried out to a still greater extent at this 

 narrow part. The result of this is that all continuity of the cell- 

 layers of the original tube of the optic stalk with those of both the 

 inner and outer walls of the bulb is interrupted, and all that remains 

 in this spot of the original continuous line of cells which connected the 

 tube of the stalk with that of the bulb are possibly some of the groups 

 of cells which are found scattered among the fibres of the optic nerve 

 at their entrance into the retina. Such separation of the originally 

 continuous elements of the epithelial wall of the optic stalk, which 

 is apparent only at this neck of the nerve in Petromyzon, takes place 



