6 SMITPISONIAN MISCELLANEOUS COLLECTIONS VOL. 54 



very sudden, so that the latter structure is more distinct than in the 

 preceding figure. 



Figure 4 represents an embryo 2 or 3 mm. longer than the one 

 just described. The section, which was drawn under somewhat lower 

 magnification than was figure 3, is almost exactly median in posi- 

 tion, and hence does not pass through the lateral ventricles. 



The cerebellum, cb., and posterior commissure, p. c, are more 

 distinct in outline, the distinctness of the latter being due to the 

 upward curvature of its anterior region, and to its now sharp differ- 

 entiation from the thin roof of the post-velar arch. 



The roof of the post-velar arch is thinner than in the preceding 

 stage, and its posterior portion, where it becomes continuous with 

 the posterior commissure, is, by the above-mentioned upbending of 

 the anterior edge of the commissure, carried suddenly upward as 

 a transverse furrow, seen in this figure just anterior to the posterior 

 commissure. 



The velum, v., is thinner in an antero-posterior direction than be- 

 fore, and contains, in this region, two or three small vesicles, lined 

 with distinct cuboidal epithelium. Each cell of this epithelium con- 

 tains a large, spherical nucleus. 



The paraphysis, p., is seen as a large, thin-walled sac, connected 

 by a very wide opening with the fore-brain. 



A plane twenty-four section laterad to the one shown in figure 4 

 is shown in figure 4a. Here the posterior commissure, p. c, and post- 

 velar arch, V. a., have about the same appearance as in figure 4 ; 

 but the paraphysis, p., which is cut laterad to its opening, is seen 

 as a small circular cavity with thicker walls than were shown in the 

 median section. The velum, v., shows the most marked change over 

 the earlier stages, and projects forward as an irregular mass into the 

 lateral ventricle. 



The paraphysis, at this stage, is a large, thin-walled vesicle, open- 

 ing by a wide mouth into the fore-brain just in front of the velum. 

 It is somewhat compressed, laterally, and, in some cases, lies more 

 on one side of the median line than the other. The width of its 

 opening is usually about one-third of the width of the entire vesicle, 

 while the antero-posterior diameter is at least one-half the antero- 

 posterior diameter of the entire vesicle. 



Figure 5 represents a sagittal section of a much older embryo than 

 the one shown in figures 4 and 4a, though the condition of the para- 

 physis is not very different from the description just given. The 

 greatest diameter of the head, from the tip of the snout to the roof 

 of the mid-brain, is about 9 mm. 



The outline of the head is beginning to assume the reptilian form, 



