430 W. H. GASKELL. 



the inner surface of the portion of the wall which has been 

 removed. In Ahlborn's fig. 5, PI. xiii (3), this hollow appear- 

 ance is perhaps represented. 



In horizontal sections the manner in which the eye is 

 attached to the brain-case is clearly visible, and in some cases, 

 as in fig. 26, PI. XXVIII, the projecting knob of the tissue of 

 the brain-case into the pineal eye so as to form its most anterior 

 wall is very conspicuous. We see how the tissue of the cranial 

 wall is thickened at this one spot, and dips down to meet the 

 pineal eye which is lying beneath it. We see how easy it is 

 to understand that the removal of the brain-case must leave a 

 shallow depression in the anterior surface of the eye. Further, 

 the structure of this tissue is very peculiar. When dissecting it 

 out it feels more like cutting through cartilage than through 

 fibrous tissue. Upon section it presents a curiously homo- 

 geneous appearance, with an evident tendency to split into 

 fibrous-looking laminae. It is remarkably free from any sign 

 of nuclei in it, and is very apt to contain fine globules of 

 yellowish refractile substance, somewhat like those which appear 

 also in the sheath of the notochord and other places. It stains 

 very deeply with haematoxyliu, similarly to the bodies in the 

 eye already mentioned, just as the cuticular lens and rhab- 

 dites of the eye in Arthropoda are known to stain deeply with 

 hjematoxylin. 



In its reaction to staining fluids and in its general appear- 

 ance it resembles that curious laminated layer of tissue which 

 lies just below the epithelium layer of the skin in the Ammo- 

 coetes. 



If, then, as I believe, this thickened portion of the tissue of 

 the cranial wall represents the position of the cuticular lens of 

 the pineal eye, it follows that the brain-case itself is partly a 

 modified portion of the integument of the Crustacean-like 

 ancestor — a conclusion which is entirely in harmony with my 

 theory, and which will be considered by me when I come to 

 treat of the manner in which the skeletal tissues of the Verte- 

 brates have arisen. 



5. The Optic Nerve and Optic Ganglion. — From the 



