128 OUTLINES OF CHORDATE DEVELOPMENT 



mately 6-7 mm. in total length. Another frequent reference 

 point is the time of the opening of the mouth, which usually 

 occurs in tadpoles of 9-10 mm., only a few days after hatching. 

 The limbs appear as small buds in tadpoles of 11-12 mm.; the 

 fore-limbs are of course concealed underneath the operculum, 

 but they develop at about the same time and rate as the hind- 

 limbs. 



I. THE NERVOUS SYSTEM 



1. The Central Nervous System 



In the preceding chapter we described the formation of the 

 neural tube and noted the differentiation between the narrow 

 spinal cord and the dilated brain region. Posteriorly the cord 

 is bent downward toward the blastopore, and the cavity of the 

 cord is continuous with the archenteron by way of the neuren- 

 teric canal. Anteriorly the brain is strongly flexed around the 

 tip of the notochord. The neuropore, which is located ante- 

 riorly from the tip of the chorda, has just closed and remains 

 connected with the surface ectoderm by a broad cone of pig- 

 mented cells (Fig. 37, A). 



The development of the brain from the stage described is 

 comparatively simple. In its early history it differs from most 

 other forms in two important respects; no neuromeres or brain 

 segments are indicated, and the division of the primitive brain 

 into its primary fore-, mid-, and hind-brain regions is incom- 

 pletely indicated and appears relatively late. The chief mor- 

 phological characteristics of the brain result largely from two 

 groups of processes, (a) thickenings and thinnings, (6) out- 

 growths and ingrowths of the wall. In describing these proc- 

 esses it is convenient to distinguish the roof, floor, and sides 

 of the brain tube. 



One of the chief features of the brain is its well-marked 

 ventral flexure, the large anterior part of the brain lying below 

 the level of the notochord (Figs. 37, 40); this flexure remains 

 a permanent characteristic of the brain, although as we shall 

 see, it soon becomes masked by the unequal growth of the neigh- 



