96 AV. J. DAKIN. 



fittaclied to the retina, and tlien will occur a stretcli where it 

 has evidently been torn off, and remains attached to the 

 underlying" pigment-layer (PI. 6, fig. 1, Pf/.). This layer is 

 also very often pulled away from the wall of the optic vesicle, 

 and, whilst remaining- attached to the tapetum, leaves 

 fragments adhering to the wall, indicating where it once has 

 been. In the normal eye, retina, tapetum, and pigment-layer 

 ai'e all in contact with one another, and no space occurs 

 between the latter and the wall of the eye. 



The tapetum. — This layer is very conspicuous both in 

 the living eye and in sections, and was very early discovered 

 by Krohn (5). Hensen stated that it consisted of polyhedral 

 cells. Patten called it 'Hhe ai-gentea " (a name which I 

 previously employed, but since "tapetum" is more correct 

 by order of priority I have gone back to it). It is unfortunate 

 that the term " tHpetum " has been used to designate two 

 different layers. 



Hickson and Carriere believed the structure was formed of 

 a number of fine fibres crossing at right angles. Patten 

 considered it to be a modification of two layers of cells into 

 refractive laminated membranes composed of minute square 

 plates. Hesse found the tapetum to contain always a single 

 nucleus surrounded by some residual protoplasm and there- 

 fore derives this layer from a single large cell. 



The tapetum is made up of several layers of minute square 

 plates (PI. 7, fig. 19), which are yellow by transmitted light 

 and reflect the light like silvery plates. 



This gives the diamond-like lustre to the living eye, and I 

 have even a series of transverse sections, mounted in canada- 

 balsam, which retain the same property. The layer is 

 thickest in the centre and shades off gradually to a very thin 

 peripheral region, which can be traced between the retina 

 and the pigment-layer to the wall of the optic vesicle. 



I have been unable to trace Hesse's nucleus, and in adult 

 eyes it is impossible to detect any remains of cells. I believe 

 rather that this layer is formed by the underlying pigment 

 containing cells or by other cells which disappear, but more 



