No. 2.] EMBRYOLOGY OF THE LIZARD. 



337 



tricle are in connection with the first five roots of the vagus 

 and the glossopharyngeal root, and a swelHng is also inter- 

 calated between the first root of vagus and the glossopharyn- 

 geal root." In his figure (Fig. 5, PI. XVI., / <;) there are no 

 external marks of these divisions, and the " swellings " lie 

 opposite the nerve-roots, while in the region between the nerve- 

 roots there are internal depressions. In the lizard, on the con- 

 trary, in the region between the nerve-roots are internal ridges. 

 The two conditions are thus very different ; but possibly younger 

 elasmobranch embryos might show a connection between these 

 "swellings" and neuromeres. 



As has already been mentioned, the greater number of the 

 cranial nerves appear during the stages of development de- 

 scribed in the preceding paragraphs. By examining a transverse 

 section of the neural tube of the young embryo it will be seen 

 that the lateral outlines are for a distance almost straight, and 

 then meet dorsally and ventrally in rather sharp curves. In the 

 region of the dorsal curve, on each side, the cells point laterally 

 and ventrally from the inner to the outer surface, while the 

 straight part of the wall has its cells directly transverse. It is in 

 this region of the latero-dorsal curve that the cells which form 

 the nerve-ganglion are proliferated from the neural tube. With- 

 out observing the actual moving process, it is a difficult point to 

 decide ; but all my sections lead me to believe that the cells are 

 proliferated throughout the entire dorsal curve, retaining their 

 connection with the tube, however, only at the lower end of 

 the curve, which thus remains the permanent point of exit of 

 the dorsal nerve-root. I find nothing to support the view that 

 the permanent attachment of the dorsal nerve-root is secondary 

 to the formation of the ganglion. The origin of the dorsal nerves 

 in the brain seems not so simple as in the spinal cord, but is 

 apparently of the same nature. Their earliest appearance, as 

 observed by me, is figured at Fig. 5, PI. XII. ; a few cells of the 

 ganglion of the fifth nerve lie adjoining the dorso-lateral part of 

 the hind-brain, midway between two neuromeric constrictions. 

 Excepting the optic, the third, and the sixth, the first nine 

 pairs of nerves of the brain are exclusively of dorsal origin, i.e., 

 they first arise from the dorsal parts of the lateral brain-walls. 

 The optic nerve has been in part described, and I shall again 

 recur to it. The third and sixth pairs of nerves have a ventral 



