254 Transactions. — Zoology. 



ture. It is displaced so as to lie behind and beneath the left 

 parietal eye, and the latter gradually shifts into the middle 

 line. Thus the left parietal eye appears finally as a median 

 unpaired organ, and the right parietal eye as a median elon- 

 gated vesicle behind and beneath it. This very degenerate 

 right parietal eye has been termed the " parietal stalk," while 

 the left parietal eye was formerly assumed to have been de- 

 veloped by a kind of nipping-off of the end of the stalk, 

 which in turn was more or less closely identified with the 

 " epiphysis " or " pineal gland." 



In the Tuatara, as already stated, the left parietal eye is 

 very highly developed. It originates as a simj)le hollow 

 outgrowth of the fore-brain. This soon separates from the 

 brain, and forms a closed sac, lying beneath the integument 

 of the top of the head. The upper part of the wall of this sac, 

 next to the skin, thickens, and forms a very well developed 

 biconvex lens, composed of elongated cells. The lower part 

 of the wall forms the retina, which early becomes divided 

 into two primary layers. In connection with the outer layer 

 of the retina a special optic nerve is developed, and in the 

 inner layer pigment is deposited. The large cavity of the 

 eye, between the lens and the retina, becomes filled with a 

 coagulable humour. 



In lizards (Lacertilia) the development of the parietal eye 

 appears to follow much the same course as in the Tuatara, 

 although perhaps its paired origin has not been so clearly 

 recognised, owing to the greater extent to which the parts 

 concerned have undergone degeneration. 



In the Tuatara, and in lizards, and perhaps in all the 

 higher vertebrates, the so-called "epiphysis" or "pineal 

 gland " is a composite structure formed by various outgrowths 

 of the brain, of which the parietal eyes, or their degenerate 

 representatives, form only a very small part. 



Since the Tuatara is recognised as being the oldest surviving 

 type of terrestrial vertebrate, belonging to a family — the Rkyii- 

 cliocephalia — which dates back to the Palgeozoic (Permian) 

 epoch, and which is now on the verge of extinction,* we might 

 expect that its development would exhibit primitive features, 

 and throw light upon the ancestral history of the higher ver- 

 tebrates in general. To a certain extent, no doubt, this is the 

 case ; but we may expect more light to be thrown upon this 

 subject by the comparison of the different stages in the de- 

 velopment of the skeleton with the fossil remains of extinct 

 vertebrate types, and until the development of the skeleton is 

 worked out it would be rash to make any wide generalisations. 



* The Tuatara is the only surviving member of this family, and it is 

 now confined to certain small islands oS the coast of New Zealand. 



