ANATOMICAL SOCIETY OF GREAT BRITAIN AND IRELAND. V 



retina the optic stalk, the so-called oj^tic nerve in Vertebrates. Owing 

 to this amalgamation of the retinal ganglion with the epidermal 

 retina, it is in this second group that the retina proper has been sup- 

 posed to have a cerebral origin, although in the simple retina such 

 origin is clearly out of the question. The Vertebrata, like the 

 Arthropoda, possess median eyes and lateral eyes, and in all cases 

 the retina of the former, like that of the pineal eyes, is simple, while 

 the retina of the latter is either simple, as in the Arachnoids, or com- 

 pound, as in the Crustaceans. 



He pointed out further that the retina of the lateral eyes of the 

 spiders is inverted, while that of the median eyes is always upright, 

 so also the lateral eyes of Vertebrates possess an inverted retina, but 

 the pineal eyes an upright one. In every respect the pineal eyes agree 

 with the median eyes of the Arthropoda, but the lateral eyes are not 

 of the type found in the Arachnida, but possess a compound retina 

 like that found in the lateral eyes of the Crustaceans. 



He then compared the compound retina of the Vertebrata with 

 that found in the Arthropoda, and showed how closely the layers 

 corresponded, the correspondence being most marked when the retina 

 of Ammocoetes was made use of for the comparison ; not only are the 

 nervous elements remarkably similar, but also both retinas possess 

 supporting neuroglial elements, and in both the fibres of the optic 

 stalk or nerve form a true chiasma on their way to the optic lobes of 

 the opposite sides. 



In the vertebrate as in the arthropod, the withdrawal from the 

 surface of the whole brain, with the exception of the retinal ganglion, 

 explains the formation of the optic nerve, and accounts for the so-called 

 cerebral origin of the retina. 



The only essential difference between these two compound retinae 

 consists in the fact that a separate system of supporting fibres, the 

 Miillerian fibres, are found in the Vertebrate retina, but not in that 

 of the Crustacean. 



The author then pointed out that the Miillerian fibres are recog- 

 nised as modified columnar epithelial cells continuous with those of 

 the pars ciliaris retina', and, therefore, with the single-layered pig- 

 ment epithelium and the epithelial cells which form the core of the 

 optic nerve in Ammocoetes ; these latter cells are continuous with 

 the cells lining the central cavities of the brain, so that the whole 

 forms a single-layered epithelial tube — viz., that of the optic diver- 

 ticulum. In other words, the optic diverticulum, just like the rest 

 of the tube of the central nervous system, is naturally divisible into 

 two parts — 1, a non-nervous epithelial tubular part, consisting of a 

 tube of single-layered cells, which forms the cellular core of the optic 

 nerve, the pigment epithelium of the retina, the j^arx ciliaris retince, 

 and the fibres of Midler ; and 2, a nervous and retinal part, composed 

 of a retina, retinal ganglion, and optic stalk, directly comparable with 

 the compound retina and optic stalk of a crustacean lateral eye. 



The conclusion is inevitable ; the ancestor of the vertebrates must 

 have had a pair of anterior diverticula of the gut with which the 

 retinal ganglia and optic stalks of the two lateral eyes were closely in 



