40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



cranial nerves, which is connected with the hindbrain a few sections 

 anterior to the one under consideration. The pharynx (ph), which 

 is cut near its extreme anterior end, is represented by three irregular 

 cavities near the base of the forebrain. Scattered throughout the 

 mesoblast, which makes up the greater part of the section, are 

 numerous blood-vessels (bv). 



Figure 15& is twenty sections posterior to figure 15a and passes 

 through the tip of the bent-under forebrain (fb). On the left the 

 section is anterior to the optic vesicle and barely touches the side 

 of the optic stalk, which is seen as a small lump on the ventro- 

 lateral wall of the brain. On the right the connection of the optic 

 vesicle (ov) with the forebrain is shown. Dorsal to the optic vesicle 

 just mentioned is a markedly thickened and slightly invaginated re- 

 gion of the ectoderm ( 11) ; this is the nasal pit ; on the left side of the 

 figure the thickening is shown, but the section did not pass through 

 the invagination. The hindbrain (lib) is somewhat narrower than 

 in the preceding figure, but is otherwise about the same ; the origin 

 of a cranial nerve is seen on its left side (en). The notochord (nt) 

 has the same appearance as in the preceding section. A number of 

 blood-vessels may be seen, the pair lying nearest the notochord 

 being the aortoe (ao), while the two other pairs, on either side of 

 the fore- and hindbrains, are the anterior cardinals (ac). The first 

 aortic arches are shown at ar. On the left the section passes through 

 the exterior opening of the first gill cleft (g'), so that the mandibu- 

 lar fold (md) on that side is a distinct circular structure, made of a 

 dense mass of mesoderm surrounded by a rather thick ectoderm. 

 The mesoderm of this fold is especially dense near the center, prob- 

 ably the beginning of the visceral bar. Near the center is also seen 

 the aortic arch that has already been mentioned. On the right the 

 section does not pass through the external opening of the first gill 

 cleft (g f ) so that the tissue of the mandibular fold is continuous 

 with the rest of the head. It is of course the slight obliquity of the 

 section that causes the pharynx (ph) to be completely enclosed on 

 the right, while on the left it is open to the exterior both through the 

 gill cleft and between the mandibular fold and the tip of the head. 

 The superficial ectoderm shown here as a heavy black line varies 

 considerably in thickness, being thickest in the region of the nasal 

 pit already mentioned and thinnest over the roof of the hindbrain. 

 The amnion (a) in this, as in the other sections of the series, has 

 the appearance of a thin, very irregular line. 



Figure 15c is posterior to the region affected by cranial flexure 

 and so shows only one region of the embryo, that of the hindbrain 

 (hb), which is here of essentially the same structure as above de- 



