No. 2.] EMBRYOLOGY OF THE LIZARD. 341 



canal has a small round lumen, and is entirely surrounded by a 

 columnar epithelium. It looks as if the compression of the 

 walls had caused the edges of the original epithelium of the 

 floor of the canal to curve upward and unite dorsally, thus en- 

 closing a small lumen. 



Fig. 6^ also shows a few elongated cells isolated in the fibres 

 of the middle lateral region of the ventral commissure. The 

 fate of these cells I have not traced. The bundle of cells 

 from which spring the ventral roots («r) has increased in size 

 and is more definitely marked. This bundle of cells, the ven- 

 tral (anterior) gray column, gradually disappears on entering 

 the hind-brain, and is not found in any region of the brain an- 

 teriorly. In no part of the brain is there a ventral extension of 

 the lateral bands of fibres, like that which forms the ventral fis- 

 sure of the cord. The transverse and longitudinal fibres, how- 

 ever, are conspicuous in the brain. The ventral edge of the 

 longitudinal band is very distinctly marked. Between the ven- 

 tral edges of the two longitudinal bands extend the transverse 

 fibres, which run upwards laterally at right angles to, and appar- 

 ently interlacing with, the longitudinal fibres. The definite end- 

 ings of the transverse fibres of the brain I have not been able 

 to find. Fig. 65, PI. XVI., shows a part of a transverse section 

 through the mid-brain, cutting the roots of the third pair of 

 nerves. This section shows the ventral transverse fibres {TF), 

 and, as has been before intimated, is just a little behind their 

 anterior limit of extension. The section also shows the ventral 

 edge of the longitudinal band (LF). Just on the edge of this 

 band, fibres from the internal ganglion-cells pass outward, con- 

 stituting the root of the third nerve. This method of origin, 

 though not exactly similar, is at least homologous with that of 

 the ventral spinal nerve-roots. The sixth nerve originates in 

 exactly the same relations to the transverse and longitudinal 

 fibres as does the third nerve. In the case of the sixth nerve, 

 however, the fibres of the nerve-root pass out in two or three 

 separate bundles, which unite first external to the brain surface 

 in the cellular nerve rudiment. 



Although the lateral longitudinal bands of fibres in the fore- 

 brain possess the same fundamental features as in the posterior 

 parts, yet their relations to certain commissures make it more 

 convenient to treat of the fore-brain separately. As has been 



