No. 2.] EMBRYOLOGY OF THE LIZARD. 347 



Whether the fibres of these dorsal commissures are continua- 

 tions of the lateral longitudinal fibres, I have not been able to 

 determine with certainty. The commissural fibres spread wider 

 apart on entering the region of the lateral bands, so that their 

 appearance is easily deceptive ; nevertheless, many of them, per- 

 haps the majority, continue downwards, crossing for a distance 

 the lateral longitudinal fibres nearly at right angles, but not ap- 

 pearing ventral to the lateral bands. Their ultimate endings I 

 could not find. The superficial position of these three commis- 

 sures, — anterior, superior, and posterior, — their similar connec- 

 tions with the lateral bands, and their relation to the constrictions 

 of the brain, suggest at this period a striking homology between 

 them. 



Before hatching, the brain of the lizard undergoes great histo- 

 logical changes, as well as changes in the relative size and posi- 

 tion of its parts. The most remarkable of these changes is the 

 great development of white matter, and the change in the char- 

 acter of the comparatively few remaining ganglion-cells. The 

 tissues assume the complex appearance of the adult brain-tissues. 

 Through these changes I have not followed the fate of the lateral 

 and anterior bands. Their position remains up to about the 

 time of hatching, still occupied by white matter, but this is not 

 distinguishable from adjacent parts; and the courses of the 

 fibres appear very complicated, — an appearance which is perhaps 

 increased by the greater irregularity in the surface of the brain. 

 The fibres of each optic nerve can be traced along the opposite 

 wall of the thalamencephalon, where they gradually become in- 

 distinguishable, a large bundle of them apparently passing to 

 a ganglionic kernel in the lateral wall of the thalamencephalon. 

 The anterior and posterior commissures are at this time well 

 marked, for they constitute the largest bundles of parallel fibres. 

 The fate of the above-described rudiment of the superior com- 

 missure I have not discovered. 



Although it is not my intention at present to follow out the 

 later stages in the development of the brain, yet I wish to call 

 attention to a feature of the secondary cranial flexure, which 

 falls partly in the embryological period with which I have dealt. 

 As seen in Fig. 2, C, PI. XII., the hind-brain has a slight dor- 

 sally convex curve. The future roof of the mouth from the 

 hypophysis to the nasal tip lies nearly at a right angle to the 



