LINNEAN SOCIETY OF LOXDOX, 



33 



Dr. GiiskeH's discussion of the miiaule structure of the retiua. 

 If the comparison is to be valid the retina of the pineal eye must 

 be a simple retina, that is to sa}^ it must not contain an optic 

 ganglion. Dr. Gaskell savs " neither I myself nor Studnicka 

 have been able to see any detinite groups of cells between the 

 nerve end-cells and the optic nerve sucli as a compound retina 

 necessitates." It is difficult to reconcile this statement with 

 wliat Studnicka himself says. According to this author,* the 

 retina of a developed Ammoccete consists of the following cell- 

 layers : — 



(1) At the bottom, a layer of nerve-fibres, \Ahich are in 



direct connection with those of the pineal nerve. 



(2) A layer of basal cells ; large, very clear cells with lightly 

 staining protoplasm and large nuclei, with a number of 

 nei've-libres running between them. 



(3) A laj^er of nuclei belonging to small cells. 



(4) A layer of cylindrical cells which correspond to the rods 



of older authors and which consist of sense-cells and 

 supporting cells. 



This does not sound very much like a simple retina. Dr. Gaskell 

 quotes Studnicka as saying that the nerve end-cells pass directly 

 into the nerve, which, Dr. Gaskell observes, "points dii'ectly to 

 the conclusion that this retina is a simple, not a compound retina, 

 and that it therefore in this respect agrees Avith the retina of all 

 median eyes." I do not know where Studnicka makes the state- 

 ment upon which Dr. Gaskell bases this conclusion. What I do 

 find hiui saying (oji. clt. p. 25) is that tlie lower extremity of the 

 sense-cell is produced into a nerve-fibre which loses itself in the 

 nerve-fibre layer of the retina (1). He further expressly states 

 thxat in the adult Petromyzon there are amongst the round basal 

 ce Is many which undoubtedly have the character of ganglion cells, 

 and that the processes of these cells may be followed into the 

 layer of nerve-fibres, while they also send processes into the layer 

 of cylindrical cells. 



In short the retina of the pineal eye of Ammocceies is iin- 

 doubtedly a compound retina and not, as Dr. Gaskell would have 

 it, a simple one. My own observations on the pineal eye of 

 Geotria fully confirm this view. In this animal also a well- 

 developed retinal ganglion is pi'esent. Dr. Gaskell endeavoui's to 

 harmonize my observations with his theory by supposing that the 

 cells of which this retinal ganglion is composed "do not represent 

 the original optic ganglion of a compound retina, but rather the 

 subsequent invasion, by way of the pineal nerve, of ganglion cells 

 belonging to a portion of the brain." When undoubted facts 

 have to be ignored or explained away in this manner in order to 



* "Die Parietalorgaiie " (in Oppel's ' Lelirbuch der vergleiclieiuleii uiikro- 

 skopischen Anatomie der Wirbelthiere ')> P- 24. 



LINX. SOC. PROCEEUINGS. — SESSION 1909-1910. d 



