We have seen that our trabeculæ are entirely epiblastic in origin, for we have shown that the entrance of the nerve-fibres along the ventral wall of the embryonic optic stalk produces a confluence and stretching of the protoplasmic fibrils of the epiblastic cells of the stalk, which result in a complex framework of supporting elements radiating in every direction from the border of each nucleus of the stalk, and that this complex framework afterwards becomes more or less differentiated into a transverse, oblique, and longitudinal trabeculæ with the multiplication of the nuclei of the stalk and without any admixture of mesoblastic cells, for we have also shown that the nerve-fibres lie, throughout the whole of their course, in the optic stalk, within the membrana limitans externa, on the outside of which we have followed the gradual formation of the connective-tissue layer of the pial sheath.
We have noticed the obliteration of the lumen of the stalk, aud have ascribed it to various causes operating within the stalk itself and outside it, though chiefly to the ingrowth of nerve-fibres.
We have seen that, in the development of the optic nerve of the frog there is a period of slow growth, followed by one of great activity, and we have felt justified in ascribing this sudden outburst of activity to a greatly increased flow of lymph into it by means of the elaborate system of minute channels that follow the course of each fibril of the epiblastic trabeculas, and consequent upon the formation of the arachnoid sheath and the enclosure of the subarachnoidal lymph We have therefore shown that the cells of the optic stalk perform the following three functions : 1st. They conduct the nerve-fibres, which, in their turn, resolve the constitution of the cells of tbe stalk, so that they--2nd. Provide the nerve-fibres with a supporting framework which--3rd. Provides the whole interior of the optic nerve with an elaborate system of minute lymph channels.