BRAIN OF THE AMERICAN ALLIGATOR 7 



largely because of the growth of the mandible, md. The nasal 

 cavity, n., opens to the pharynx through the posterior nares, p. n., 

 though this connection is not shown in the figure. The cerebral 

 hemispheres, c, are large, thick-walled structures, while the in- 

 fundibulum is now relatively smaller than in the preceding stage. 



The posterior commissure, cerebellum, etc., are not very different 

 from what has been described. 



The post-velar arch, v. a., has changed considerably ; it is now 

 higher and less wide, in an antero-posterior direction, than in figure 

 4. Although the length of the head has increased 50 per cent over 

 the preceding stage, the actual diameter of the arch, from the velum 

 to the posterior commissure, is less than is shown in figure 4. 



The paraphysis, p., also shows signs of having been compressed 

 in an antero-posterior direction. The lateral diameter of its opening 

 is still about one-third of the width of the entire structure, while 

 the antero-posterior diameter of the opening is only one-fifth of the 

 antero-posterior diameter. The walls of the paraphysis, as well as 

 the roof of the post-velar arch, are somewhat thicker than they 

 were in the preceding stage, and consist, apparently, of simple 

 columnar epithelium, though the exact character of the cells could 

 not be determined because of poor fixation. The paraphysis, which 

 is now more tubular than spherical in outline, is slightly inclined 

 caudad instead of having a slight inclination cephalad, as in the 

 preceding stage. As will be seen, this inclination of the paraphysis 

 away from the cerebral hemispheres becomes more marked in later 

 development. 



The velum, v., has undergone marked development. In the median 

 plane, as shown in figure 5, it is seen as a somewhat arched band 

 of tissue forming the posterior border of the paraphysis. In this 

 plane are seen in the velum the two or three vesicles, vs., noted in 

 the preceding figure. These vesicles are not connected with either 

 paraphysis or fore-brain. When followed laterally the velum is 

 seen to expand to form, on each side, a much branched structure, 

 the plexus of the lateral ventricle, figures 5a and 5b, c. p. 



Figure 5a represents a section laterad to the one just described. 

 It passes through the median edge of the choroid plexus, c. p., just 

 described, and through the lateral portion of the paraphysis, p., 

 whose walls appear thick merely because cut tangentially. 



Figure 5b is a section of a somewhat older embryo than the one 

 shown in figures 5 and 5a. It is about, though not exactly, in the 

 plane a-b of figure 5. It shows the laterally compressed paraphysis, 

 p., and in each lateral ventricle, c, the much convoluted plexus, c.p. 



Figure 6 shows the paraphysal region of an embryo of about 7 cm. 



