34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



is of varying thickness, the thickest areas being on each side of the 

 forebrain ; it is more compact than in the earlier stages, and, owing 

 to the low magnification under which it is drawn, it is represented 

 here bv a single heavy line. Under this magnification only the 

 nuclei of the mesoderm cells (mes) can be seen, so that this tissue is 

 best represented by dots, more closely set in some places than in 

 others. The forebrain is an elongated cavity (fb) with thick, dense 

 walls. Attached to each side of the forebrain is an optic vesicle 

 ( ov ). which is considerably larger than in the preceding stage. The 

 connection between the cavity of the forebrain and that of the optic 

 vesicle is not seen in this section ; it is a wide passage that may be 

 seen in several sections posterior to the one under discussion. The 

 beginning of the invagination of the optic vesicle to form the optic 

 cup may be seen on both sides, but more plainly on the right. On 

 the right side also is noticed a marked thickening of the ectoderm, 

 which is invaginated to form a small pit, the lens vesicle (lv) ; on 

 the left side the section is just behind the lens vesicle. Above the 

 optic stalk on each side, in the angle between the optic vesicle and 

 the side of the forebrain, is a small blood-vessel (bv) . Several 

 other blood-vessels may be seen at various places in the mesoblast, 

 four of them, near the pharynx being especially noticeable. The 

 hindbrain (hb) is wider than, but not so deep as, the forebrain; its 

 walls are very thick laterally, but are thin on the dorsal and ventral 

 sides. The dorsal wall is reduced to a mere membrane, which, with 

 the overlying ectoderm, has been pushed into the brain cavity, as is 

 generally the case with such embryos. Close to the ventral wall of 

 the hindbrain the notochord (///) is seen. The character of the 

 notochord has already begun to change; the cells are becoming 

 rounded and vacuolated, with but few visible nuclei except around 

 the periphery of the notochord. Near the center of the section, 

 close to the ventral end of the forebrain, is the pharynx (ph) , cut 

 near its anterior limit ; it is here a small, irregularly rectangular cav- 

 ity with a comparatively thin wall. On the left side of the pharynx 

 the first gill cleft (g) is indicated as a narrow diverticulum reach- 

 ing toward the ectoderm. A few sections posterior to this one the 

 first gill cleft is widely open to the exterior. As has been said, in 

 the surface view of this stage above described none of the gill clefts 

 showed ; so that in this respect at least the sectioned embryo was 

 more nearly of the state of development of the embryo represented 

 in figure 14, to be described later. 



Figure 13/', about forty sections posterior to figure 130, passes 

 through the hindbrain in the region of the ears. Being back of the 

 region affected by cranial flexure, this section is of course of much 



