30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



side only was cut. The wall of this part of the forebrain is of about 

 the same thickness and appearance as in the preceding stage. The 

 other cerebral cavity (mb) of this section is probably the hinder part 

 of the midbrain, though it may be the anterior part of the hind- 

 brain ; there is no sharp line of demarcation between these regions 

 of the brain. This cavity (mb) is much smaller in section than the 

 forebrain ; its walls are of about the same thickness. 



Ventral to the midbrain is the anterior end of the notochord (nt), 

 surrounded by the mesoblast. At various places throughout the 

 mesoblast irregular open spaces may be seen ; these are blood- 

 vessels. The ectoderm (cc) and amnion (a) have about the same 

 appearance as in the preceding figure, though the former seems 

 somewhat thinner. 



Figure 12c is just back of the bent-under forebrain represented in 

 the preceding figure and in front of the main body of the heart. 

 The plane of the section not being at right angles to the long axis 

 of the body (as was mentioned above), the figure is not bilaterally 

 symmetrical. The neural canal, since the section passes through 

 the auditory vesicles, may here be called the hindbrain (hb). It has 

 an almond-shaped cavity, surrounded by a wall of medium thickness. 

 In close contact with the wall of the hindbrain, on each side, is the 

 inner side of the auditor}- vesicle (0) , which is seen as a deep, wide- 

 mouthed pit in the superficial ectoderm. On the right side of the 

 section the auditory pit is cut through its middle region ; it is simply 

 a thickened and condensed area of the ectoderm which has been in- 

 vaginated in the usual way. Directly beneath the hindbrain is the 

 notochord (nt), on each side of which, in the mesoblast, is the dorsal 

 aorta (ao), or rather the continuation of the aorta into the head. 

 Beneath these structures and extending from one side of the section 

 to the other is the pharynx (ph) ; its lining wall is fused on each 

 side with the ectoderm, but there is no actual opening to the. ex- 

 terior. These points of contact (g) between entoderm and ecto- 

 derm are of course the gill clefts; they are not yet visible from the 

 outside. The roof of the pharynx is flat and comparatively thin, 

 while the floor is thickened and depressed to form a deep, wide pit, 

 traceable through six or eight sections. This pit may be the thyroid 

 gland already noticed in the preceding stage. Below the main 

 cavity of the pharynx and close to each side of the thyroid rudiment 

 just mentioned is a large blood-vessel (tr). These two vessels 

 when traced posteriorly are found to be continuous with the anterior 

 end of the heart, and hence may be called the truncus. They were 

 noticed in figure lie, bv. The ectoderm surrounding the lower side 



