8 SMITHSONIAiN' MISCELLANEOUS COLLECTIONS VOL. 54 



length. This embryo is several times the length of the one repre- 

 sented by figure 5, and has practically the external form of the 

 adult, even the pigmentation of the skin being partially formed. 

 In spite of this great advance in general development, about the 

 only change in the paraphysis, p., is a slight increase in length. Its 

 walls are still practically smooth and unconvoluted ; they are com- 

 posed of a simple columnar epithelium. 



The velum, v., has, in this median plane, nearly the same appear- 

 ance as in figure 5 ; in it are seen four or five of the same small 

 vesicles, vs., lined with cuboidal cells, that were seen in figures 

 4 and 5. The prolongations of the velum into the lateral ventricles, 

 the choroid plexuses, as seen in more lateral sections, are now large 

 and much convoluted. 



The post-velar arch, v. a., is much reduced in extent, so that the 

 distance between the posterior commissure, p. c, and the velum, v., 

 even in this much larger embryo, is only about two-thirds of the 

 corresponding distance in figure 5. A greater part of the roof of 

 the post-velar arch is considerably thickened and is somewhat con- 

 voluted. 



The brain cavities are all relatively and actually smaller than in 

 the preceding stage, and their diminution in size is made more evident 

 by the increased thickness of the brain wall. 



The oldest embryo studied was one of about 13 cm. length, nearly 

 twice the length of the one shown in figure 6. This embryo has 

 practically the adult form, and is fully pigmented, so that it is 

 likely that the paraphysis has here approximated more or less closely 

 its adult condition. 



In spite of the great increase in the size of the brain over the 

 preceding stage, there is very little change in either the size or 

 form of the paraphysis ; it is slightly longer, but it curves back 

 over the post-velar arch in the same way as is shown in figure 6. 

 Its walls are of the same character and are neither more nor less 

 wrinkled or folded than in this figure. Owing either to its increase 

 in length or to a change in the relative positions of parts of the 

 head the tip of the paraphysis is somewhat nearer the dorsal surface 

 of the head ; it is, in fact, in contact with a dense layer of connective 

 tissue which will form the roof of the skull. 



The velum is somewhat longer than in the preceding stage and 

 has rather more numerous vesicles than were there seen. It is, 

 perhaps, partly to this increase in length of the velum that the in- 

 crease in the length of the paraphysis is due. 



The post-velar arch is of about the same size and outline as in 

 figure 6 ; its dorsal wall is, perhaps, slightly more deeply wrinkled. 



