10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 54 



called. These lateral branches are nearly solid, at this stage, though 

 the cavity of the hypophysal stalk may be traced, in sections nicdiad 

 to this, for a short distance into them, as a very narrow slit. 



Figure 2a is through a more nearly median region of the hypo- 

 physis, as may be seen by the section of the notochord, ch., that is 

 shown. The region of thickened epithelium is more extensive than 

 in figure i. The hypophysal stalk is at the end of the reference 

 line, h., caudad to which are seen three or four wrinkles in the 

 epithelium. The most posterior of these wrinkles, p. s., is much 

 deeper and more distinct than the others, and probably represents 

 the pharyngeal sac, though no connection whatever may be seen 

 between it and the notochord. 



In figure 3 is represented a section through the median region of 

 the hypophysis, Ji., now considerably more developed than in the 

 preceding stage. The actual depth of this median invagination or 

 stalk is about 0.3 mm., or about one-twelfth of the greatest length 

 of the head. The greatest width of the hypophysis is about equal 

 to the depth of the stalk, 0.3 mm. The lateral, inside diameter of 

 the stalk is about 0.2 mm. ; the antero-posterior diameter is about 

 0.12 mm. 



On each side of the stalk are two lateral diverticula, seen better 

 as a horizontal section, to be described later, figure 5b. Of these the 

 pair nearer the notochord are the larger and are directly continuous 

 with the inner end of the stalk ; the other and smaller pair open into 

 the outer, lateral angles of the stalk near the opening of the latter 

 structure. 



Just caudad to the stalk of the hypophysis is a distinct invagina- 

 tion of the epithelium, p. s., the pharyngeal sac, which shows no 

 connection whatever with the notochord. 



The walls of the hypophysis have become so much thicker, and the 

 basal wall of the infundibulum has become so much thinner that 

 now the former wall is, in places, thicker than the latter. 



Figure 4b is a sagittal section of an embryo of 13 mm., " crown- 

 rump " measurement, if such a term be here permissible. The 

 hypophysis has made considerable progress in development and has 

 increased somewhat in size over the last stage ; its greatest lateral 

 diameter is slightly less than 0.5 mm. ; its greatest antero-posterior 

 diameter is practically the same. 



The shape of the head has changed in several particulars. The 

 infundibulum, in., is a more definitely outlined depression in the 

 floor of the fore-brain, and the roof of the mouth or pharynx, which- 

 ever it may be called, extends back of the stalk of the hypophysis 

 instead of ending in the hypophysal invagination as in the last stage. 



