BRAIN OF THE AMERICAN ALLIGATOR 5 



The posterior commissure, p. c, is somewhat more distinct than 

 in the preceding stage, and the post-velar arch, v. a., is distinctly 

 differentiated, partly because of the increasing thinness of its wall 

 and partly because of the marked increase in the size of the velum, 

 v., which now projects into the cavity of the fore-brain as a heavy 

 transverse ridge. As seen in this plane, the velum has two lower 

 angles, a more acute one projecting ventrad and caudad, and a 

 thicker, more obtuse angle, projecting forward under the paraphysis. 



Instead of the wide, flat arch, seen in the preceding stage, the 

 paraphysis is here a distinct, saccular diverticulum of the fore-brain, 

 with whose cavity it connects by a wide opening. 



A section cut laterad to the paraphysal opening is shown in figure 

 2a. Here the paraphysis is seen as a separate, circular cavity, p., 

 lying in the mesoblast between the wall of the fore-brain and the 

 superficial ectoderm. It is not strange that, when seen in such a 

 section, the paraphysis should have been taken for the epiphysis, 

 because the marked forward projection of the post-velar arch, -o. a., 

 gives the impression that it and not the arch anterior to the velum 

 is connected with this paraphysal vesicle. 



In this figure the paraphysis has thick walls, with a denser layer 

 of nuclei towards .the central cavity. The velum, v., has here but 

 one angle, that projecting caudad, the other angle is now continuous 

 with the roof of the fore-brain, beneath the paraphysis. 



The infundibulum is seen as a wide, shallow depression, in., in 

 the floor of the fore-brain. The plane of the section, being more 

 nearly median in the posterior region, cuts the side of the noto- 

 chord, ch., at its extreme anterior end. 



Figure 3 represents an embryo somewhat older than the one rep- 

 resented in figures 2 and 2a. The brain is considerably more elon- 

 gated than in the preceding stage, and the cranial flexure is more 

 marked. 



In the region of the hind-brain the section is exactly median, so 

 that the notochord, ch., and hypophysis, h., are cut through the 

 median plane. In the region of the paraphysis, p., the section is to 

 one side of the median plane, and the opening of that structure into 

 the fore-brain is not seen. 



The velum and paraphysis are about the same as in the preceding 

 figure, except that the latter, in its median region, has thinner walls 

 than in the earlier stage. The greatest change is in the post-velar 

 arch, V. a., which is now much more sharply defined, mainly because 

 its dorsal wall is much reduced in thickness. This thin roof of the 

 post-velar arch gradually increases in thickness as it passes into the 

 velum, but its transition into the posterior commissure, p. c, is now 



