PARIETAL EYE AND ADJACENT OEGANS IN SPHENODON. 137 



of the development, in precisely the same manner as the left 

 parietal eye, of which it is quite independent. In the second 

 place it has for some time a very similar structure, though it 

 never acquires the same degree of perfection. 



The similarity in structure of the parietal eye and the 

 parietal stalk is a point on which I am inclined to lay 

 considerable stress as an argument in favour of their paired 

 origin. Both have the form of hollow vesicles, which become 

 completely shut off from the brain cavity. Except for the 

 fact that no lens thickening is formed in the case of the 

 ''stalk," the histological cliaracters of the two are, up to a 

 certain stage at any rate, identical. Fig. 21 represents a 

 transverse section through the " stalk '' at Stage R, taken 

 near its distal extremity, just at the place where pigment is 

 deposited in its ventral wall as already mentioned. It will be 

 seen that the wall is divided into two layers, as in the left 

 parietal eye, — an outer thin one composed of a single tier of 

 short columnar cells, and an inner very much thicker one 

 containing many large oval nuclei not regularly arranged in 

 tiers, and with a layer of columnar cells next to the central 

 cavity. The pigment is deposited between the cells of the 

 inner half of the inner layer of the ventral wall, exactly as in 

 the left parietal eye. On comparing this figure with the 

 sections of the left parietal eye represented in figs. 14, 16, and 

 17, and bearing in mind its developmental history, the con- 

 clusion that the so-called " parietal stalk " or " epiphysis " in 

 Sphenodon represents a right parietal eye appears to me 

 inevitable. 



The probability of this conclusion is greatly strengthened 

 when we come to compare certain observations of other 

 writers dealing with other types, by far the most important of 

 which are those of Hill (16, 17) on certain Teleostean fishes, 

 and on Ami a. 



In 1891 Hill described (16) in Coregonus albus two 

 " epiphysial outgrowths " from the roof of the primary fore- 

 brain. He says, " On the roof of the brain — in the median 

 line, and in a plane passing through the middle of the optic 



