484 J. T. GRADON. 



As there are no blood-vessels inside the optic nerve of the 

 frog, and very few capillaries on the pial sheath, it will be 

 evident that, up to this stage, the nutrition of the stalk is at 

 a very low ebb ; there are, however, no indications of degene- 

 ration 3 in fact, it is possible to show a solitary instance of 

 cell-division now and again where the fibrils of the trabeculse 

 are in contact with the delicate capillaries that have crept up 

 the pial sheath from the pia mater of the brain (fig. 9). 



But when we turn to a 27 mm. tadpole {Qg. 7) it is evident 

 that a remarkable change has taken place ; mitosis is every- 

 where abundant, the number of cells has already greatly 

 increased, and the total diameter of the stalk is considerably 

 greater than it was in the preceding stage. 



This sudden change coincides with the more complete 

 enclosure of the subarachnoidal lymph space, which has come 

 about through the formation of the arachnoid sheath. Before 

 the dural sheath has had time to form, the arachnoid itself is 

 directly connected with the ophthalmic artery by a band of 

 connective tissue, by means of which transudation of lymph 

 from the artery doubtless takes place. 



The subarachnoidal space, which is evidently not suffi- 

 ciently enclosed until this stage has been reached, is now 

 filled with nutritive material, which passes through the pial 

 covering, and then reaches every nucleus of the stalk by 

 means of the elaborate system of tiny channels that follow 

 the course of each fibril of the epiblastic trabeculaj (figs. 8 

 and 13, and ;pc. fig. 17). 



Moreover, we may now find, among the meshes of the 

 connective-tissue cells that join the ophthalmic artery and 

 the optic nerve, numbers of lymph-corpuscles, all in various 

 stages of cell-division, though I have only shown them in 

 outline in fig. 7. 



Development now proceeds very rapidly, but I have not 

 thought it necessary to publish any drawings of stages 

 between that shown in fig. 8 and the adult stage shown in 

 fig. 10. 



In the latter figure we can see the final arrangement of th 



