No. 2.] EMBRYOLOGY OF THE LIZARD. 339 



cleft, there is another intermediate neuromere. Therefore if we 

 consider the visceral arches as indicating the metameres of the 

 head, the latter do not correspond to the neuromeres of the 

 brain. 



The fibrous elements of the central nervous system appear 

 first in the stages succeeding that of series C. The fibres seem 

 to be formed from the contents of the cells, by the breaking up 

 of the cell-nucleus and absorption of the wall ; or in other cases 

 by the attenuated prolongation of the distal pole of the cell. 

 The first method seems to be the case with the lateral longitudi- 

 nal fibres ; but I have not been able to follow it with certainty, 

 and cannot say whether it is preceded by a polar attenuation. 

 My sections, however, show the breaking up of the nucleus, and 

 the gradual disappearance of the cell-walls. The second method 

 is the case with the fibres forming the continuous transverse ven- 

 tral commissure. The cells internal to the lateral longitudinal 

 band of fibres give ofif long filaments from their distal poles, 

 which run ventrally to formi the transverse commissure. 



The first fibres to appear are a thin, superficial band of longi- 

 tudinal fibres, in cross-section, extending around the ventral 

 lateral corner of the neural tube. From this corner springs the 

 ventral nerve-root, and the greater part of the fibrous band lies 

 dorsal to this nerve-root. Shortly after the appearance of the 

 ventral band a similar superficial band of longitudinal fibres ap- 

 pears on the dorsal lateral corner, with its lower edge at the 

 point of exit of the dorsal nerve-root. In the spinal cord these 

 two bands are at first very distinctly separate ; but in the head 

 they blend into a single band lying superficially, — lateral in the 

 hind-brain and ventro-lateral in the mid- and fore-brain. The 

 appearance of these bands in the spinal cord is represented at 

 vL and dL in Fig. 66, PI. XVI. The same figure shows how 

 the distal poles of the cells internal to the longitudinal bands 

 bend downward, sending out fibres which run around the ventral 

 surface of the tube to the opposite lateral wall. These fibres 

 form the transverse ventral commissure. They appear at the 

 same time with the dorsal longitudinal band. The floor of the 

 central canal is still a single layer of columnar epithelial cells. 

 These cells appear to take no part in the formation of the fibres 

 of the ventral commissure, which lies closely attached to the 

 floor of the central canal. The fibres of this commissure seem 



