254 



DR WALTER H. GASKELL. 



tinuity 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 supporting 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 



On 



Fig. 8.— Diagram of the relation of the optic nerve to the optic cup. The upper 

 diagram represents a stage before the formation of the choroidal slit, the 

 lower one the stage of closure of the choroidal slit. R., retina ; O.n., optic 

 nerve ; j>, pigment epithelium. 



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 must conclude from my 

 own observabions." 



The evidence, then, of the Ammoecetes is very clear. Originally 

 a tube composed of a single layer of epithelial cells became 

 expanded at the anterior end to form a bull). On the outside of 



' 1'. xxxvii. 



