The Development of the Alligator 295 



roof is reduced to the usual thin, wrinkled mem- 

 brane. Close to the floor of the hindbrain lies the 

 notochord (;//), which is large and is distinctly 

 vacuolated. To the right of the hindbrain a large 

 mass of darkly stained cells (en) is one of the 

 cranial nerves, which is connected with the hind- 

 brain 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 fore- 

 brain. Scattered throughout the mesoblast, which 

 makes up the greater part of the section, are nu- 

 merous blood-vessels (bv). 



Figure 1 5^ 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 region of the ectoderm (77) ; 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 (hb) is somewhat 

 narrower than in the preceding figure, but is other- 

 wise about the same; the origin of a cranial nerve 

 is seen on its left side (cii). The notochord (nt) 

 has the same appearance as in the preceding section. 



