38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



the superficial ectoderm. Under the low magnification used the 

 superficial ectoderm cannot be distinguished from the ectoderm of 

 the nervous system. The plane of the section being in the anterior 

 end almost exactly median, this part of the central nervous system 

 is seen as the usual retort-shaped cavity, while in the region back 

 of the brain, where the neural canal is narrow, the section passes 

 through the wall of the spinal cord (sc) and does not show the 

 neural canal at all. The wall of the forebrain (fb) is quite thick, 

 especially at the extreme anterior end ; the wall of the midbrain 

 {nib), where the marked cranial flexure takes place, is somewhat 

 thinner, and it gradually becomes still thinner as it is followed 

 posteriorly over the hindbrain (lib). Between the floors of the fore- 

 and hindbrains, in the acute angle caused by the cranial flexure, is 

 the anterior end of the notochord (///), the only part of that struc- 

 ture that lies in the plane of the section. Ventral and posterior to 

 the notochord is a large cavity, the pharynx (/>//'), whose ento- 

 blastic lining can scarcely be distinguished under this magnification 

 from the surrounding tissues. The stomodeal opening being as yet 

 unformed, the pharynx is closed anteriorly ; posteriorly also, owing 

 to the plane of the section, the pharynx appears to be closed, since 

 its connection with the yolk stalk is not shown. In the floor of the 

 pharynx, almost under the reference line ph, a slight depression 

 marks the position of the first gill cleft. In the mesoblast ventral 

 to the pharynx and near the gill cleft just mentioned, a couple of 

 irregular openings represent the anterior end of the bulbils arteri- 

 osus, posterior and ventral to which is the heart (lit), a large, 

 irregular cavity. The dorsal aorta (ao) may be seen as an enlon- 

 gated opening in the mesoblast, extending in this section from the 

 middle region of the pharynx to the posterior end of the figure 

 where it is somewhat torn. Two of the eighteen or twenty pairs 

 of mesoblastic somites possessed by this embryo are shown at the 

 posterior end of the figure (s), where the plane of the section was 

 far enough from the median line to cut them. 



Stack XI 



Figure 14 (Plate XVI) 



Only the anterior region of this embryo is shown in the figure, 

 which is a ventro-lateral view. While there is some change in the 

 general shape and in parts of the head, the reason for figuring this 

 stage is to show the first gill cleft (g'), which lies at an acute angle 

 to the long axis of the neck behind the eye (e). The cleft is narrow 



