BY W. J. McKAY. 881 



the elements of this complex structure of the retina arise from 

 the breaking clown of original columnar cells. The eye is 

 marked off from the suxTOunding tissue very distinctly, but I do 

 not think, however, that there is a distinct boundary layer 

 As regards pigment it has not yet made its appearance, so that 

 this stage is especially suitable for the histology of the eye. 



Optic Vesicle. — The optic vesicle (Op. F.), as before mentioned, 

 is the space between the retina and the lens. It is filled with 

 minute strands, to which I shall presently allude. 



The epiphysis at this stage is composed of layers of columnar 

 cells. The nuclei for the most part stain deeply, but occasionally 

 a lai'ge round nucleus scaining but slightly can be detected in that 

 portion of the epiphysis which was directly continuous with the 

 retinal part of the eye. 



Spencer has drawn attention to Ahlborn's description of the 

 epiphysis in Petromyzon, and I think that it is very probable, as 

 he suggests, that the supposed nervous strands in the optic vesicle, 

 as described by Ablborn, are nothing more than the coagulable 

 remains of the fluid contents of the brain cavity. The strands that 

 can be seen in the optic vesicle of the pineal eye of Hinidia and 

 Grainmatopliora are identical with the fine strands in the cavities 

 of the brains of both lizards. 



A blood vessel which runs along the roof of the brain dips 

 below the eye. This vessel is very conspicuous when the embryo 

 is examined alive (fig. 7, B.V.). 



In the next stage of the development of the eye in Sinulia 

 tceniolata, we find that it has become separated off from the 

 epiphysis, and a considerable space intervenes between the two 

 (as shown in fig. 9). It is to be noted that no connection what- 

 ever now exists between the eye and the epiphysis. The eye (fig. 

 12) is seen embedded in connective tissue lying above the posterior 

 portion of the cerebral hemispheres. 



The epidermis above is composed of a layer of squames resting 

 on a layer of columnar cells {R. Mp.). The first signs of the 

 formation of the scales are now visible. The rete mucosum being 



