390 H. M. BERNARD. 
former being the more important element, built up a layer of 
cells, the outermost of which typically harden into horny 
scales. We thus get the Vertebrate type of skin as shown to 
right and left of Diagram II. The cutis is richly provided with 
blood-vessels (shown diagrammatically as loops), the chroma- 
tophores are numerous, and those which have reached the 
epidermis are seen forcing their way up between the palisade 
cells. Lastly, the ultimate ramifications of the integumentary 
nerves no longer end, as in the primitive skin, in or among 
the palisade cells, but, penetrating that layer, terminate 
among the cells of the rete mucosa. 
In this change in the character of the skin we can perhaps 
find, on the one hand, a partial explanation of the degenera- 
tion of the pineal eye, and, on the other, a clue to the chief 
differences between that eye and the Vertebrate eye proper. 
The pineal eye, sunk below the palisade layer and no longer 
in organic connection with it, would probably suffer by any 
change such as that described in the character of the outer 
skin. The separation of the ocular vesicle from the palisade 
layer would hamper its control over the development of that 
layer, which might or might not run a course favourable to 
the eye as an organ of vision. As far as we can see, the 
secondary thickening of the outermost. layers by stratification 
from the palisade layer was not calculated to benefit the pineal 
eye. It would impede its function, and therefore cause it to 
degenerate. A still more important factor making for degene- 
ration is probably to be found in the rise and development of 
the skull. A review of the available facts relating to these 
eyes involuntarily suggests that they—there were, it seems, 
originally two!'—succumbed, first one and then the other, 
before the advancing edges of the bony plates which developed 
to protect the ever-enlarging brain. This degeneration would 
be hastened if new and perhaps more efficient eyes developed 
to replace and more than compensate for the loss of the old. 
The eyes which appear actually to have replaced the pineal 
1 Owsianikow, ‘Mem. Akad. St. Petersburg,’ 7, xxxvi, 1888; also Locy, 
‘ Anat. Anz.,’ 1894, p. 169. 
