ORIGIN OF THE TRABECULE OF THE OPTIC NERVE. 489 



the interior of the optic nerve of the chick with blood and 

 lymph descending into it from the pial sheath which is richly 

 supplied with blood-vessels, and the adjacent fibrils of the 

 epiblastic trabeculte may be seen in contact with it (fig. 18). 



In conclusion, I deeply regret to say that since this article 

 was written, the sudden death of the Linacre Professor of 

 Comparative Anatomy has rendered it impossible for me to 

 publicly express my thanks to him for allowing me to carry 

 on my researches in ocular embryology in the Department of 

 Comparative Anatomy at Oxford, and more especially for the 

 kind interest that he always took in my work. But I grate- 

 fully avail myself of this opportunity of thanking Dr. J. W. 

 Jenkinson, Assistant to the Linacre Professor, for kindly 

 providing me with unlimited material and preparations for 

 the purpose of the present article. 



Summary. 



We have seen that our trabeculae 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 

 trabecula3 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, 

 and have ascribed it to various causes operating within the 

 stalk itself and outside it, though chiefly to the ingrowth of 

 nerve-fibres. 



