246 



DR WALTER II. GASKELL. 



connection vnth nerve fibres, justifies the conclusion, I believe, 

 that they are not nervous, but rather sustentative in function. 

 The layer that they form can be traced directly into a mass of 

 ectodermic tissue that lies on the anterior face of the ganglion, 

 and the resemblance between the nuclei in the distal nuclear 

 layer and those in the ectodermic mass is so striking that, though 

 these cells are most probably sustentative in function, I believe 

 them ectodermic in origin. The proximal nuclear layer (n.l.r.g.i. 



- (m 



-ul.o.g 



-ml 



'••g- 



_f:e>o« a!i8r^i'«'"V<»-- nl.r.sf.o 



;^V— f.br.r 



Fig. 5. — Retina of Astacus (after Berger) with the blood-vessels omitteil. 

 hr., brain; o.n., optic nerve; nJ.o.;/., nuclear layer of ganglion of optic 

 nerve ; m.7., molecular layer (Punktsubstanz) ; n.Lrjj.i. and iiJ.r.(j.o., inner 

 and outer nuclear layers of the ganglion of the retina ; f.hr.r., terminal fibre 

 layer of retina ; r. , layer of retinal end cell.s. 



fig. 5) is precisely similar to the distal one, except that it 

 contains, on the whole, fewer cells." 



If we use the same terminology as in Mliller's description of 

 the layers in Ammoccetes, then Parker's view of these layers in 

 Astacus is that they form a double layer of tangential fulcrum 

 cells, while Viallanes would describe them as a double layer of 

 ganglion cells, just as Langerhans and Kohl describe them in 

 Ammoccetes. Whatever the final interpretation of these cells 



