THE EYE OE I'ECTEN. 75 



Avas known previously about this part of the eye. I believe, 

 also, that most of Patten's good work was due to the great 

 use of maceration preparations, though perhaps owing to the 

 more primitive methods of section Avork he did not check his 

 results as much as lie possibly could by this means. It is a 

 great pity, therefore, that his description should have been 

 couched in terms which, accentuated by his theories, did 

 much to bring tlie whole paper into some disrepute. 



The retina covers almost exactly half of the interior of the 

 optic vesicle, and since it is of considerable thickness com- 

 pared with the size of the eye there is not much space left 

 in the pi-oximal hemisphere. The retina and underlying- 

 layers will be considered together. They are separated from 

 that part of the eye previously considered by a membrane, 

 the septum, first discovered by Krohn (5). 



This septum is a homogeneous sheet of connective tissue 

 which is slightly thicker in the middle than at the sides, and 

 at the periphery it appears to become continuous with the 

 inner wall of the proximal half of the optic vesicle, that part 

 termed the ''sclerotica" by Patten (PI. 6, fig. 1, iSV.). This 

 author described it as cellular, but no traces of cells or nuclei 

 are to be seen, though the corresponding structure in the 

 eye of Spondjlus is formed of distinct cells. Patten also 

 stated that it was double. This has not been alluded to by 

 other observers, but I thought I had detected this double 

 nature (44). I have since found out my error and I believe 

 also the cause of Patten's mistake. He writes that the distal 

 branch of the optic nerve, which lies across the septum, has 

 no sheath, since the latter terminates where the nerve enters 

 the optic vesicle. The nerve, however, has a distinct sheath, 

 and this accompanies it to the middle oE the retinal surface, 

 where just as the nerve branches (PI. 7, fig. 18) and spreads 

 out over the centre, the nerve-sheath spreads out too, covering 

 all the diverging nerve-fibres which lie therefore between 

 two sheets of connective tissue, the nerve-sheath above and 

 the septum below (PI. 6, fig. 1). This nerve-sheath fuses 

 with the septum, and I think the two sheets of tissue were 



