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 Jaw, as I am bound to conclude 
from my own observations.” 
The evidence, then, of Ammoccetes is very conclusive. Originally 
a tube composed of a single layer of epithelial cells became expanded 
at the anterior end to forma 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 
