482 J. T. G RADON. 



takes up the position lately occupied by those that have been 

 assimilated. 



An inspection of fig. 2 will show that the entrance of the 

 nerve-fibres along the ventral wall of the stalk produces a 

 confluence and stretching of these delicate protoplasmic 

 fibrils, and, at the same time, brings into prominence the con- 

 nections subsisting between nucleus and nucleus. 



Further ingrowth of the nerve-fibres resolves these fibrils 

 into a complex framework of supporting elements which, 

 from transverse, longitudinal, and horizontal sections, may be 

 seen to radiate in every direction from the border of each 

 nucleus of the stalk. 



This intermediate arrangement of the condensed proto- 

 plasmic fibrils finally becomes differentiated with the multi- 

 plication of the nuclei of the stalk into a transverse, oblique, 

 and longitudinal framework which, as we shall afterwards 

 see, also provides a complete system of Ij'^mph channels 

 throughout the interior of the optic nerve. 



III. Obliteration op the Lumen op the Optic Stalk. 



The obliteration of the lumen of the stalk has received con- 

 siderable attention from previous observers. Assheton (1) 

 ascribes it to pressure from the cartilaginous walls of the 

 cranium, whilst Robinson (7) considers that this cannot be 

 looked upon as an important agent, and concludes that the 

 obliteration " is brought about by developmental changes in 

 growth and relationship of the constituent parts of the stalk,'' 

 and that " with these is associated the invasion of the optic 

 nerve-fibres." 



In fig. 12 we can see that pressure is exerted on the stalk 

 by the cartilaginous walls of the cranium, and it is also cer- 

 tain that pressure produced by contact with the back of the 

 eye is the cause of the very decided oval shape of the stalk 

 at this point J in an 11 mm. tadpole (fig. 4), its shape, when 

 free, is almost round. But there are probably several causes at 



