54 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



fissure also enters a vascular tuft of mesoblast (pt) which may be 

 seen projecting into the optic cup after the disappearance of the 

 fissure. This loop of blood-vessels is doubtless the pecten. 



Figure 2od represents a section through the hindbrain (hb), 

 pharynx (ph), and tip of the snout. On either side of the hindbrain 

 are a convoluted auditory vesicle (0), and several blood-vessels and 

 nerves, while ventral to it is seen the anterior end of the notochord 

 (nt), around which the mesoblast is somewhat more dense than 

 elsewhere. The pharynx (ph) sends out toward the surface a nar- 

 row gill cleft (g f ) in the neighborhood of each auditory vesicle. 

 These clefts connect with the exterior by very narrow slits, not seen 

 in the plane of this section. The opposite end of the pharynx, as 

 seen in this figure, opens on the left (pn) into the nasal chamber. 

 The nasal cavity on the right is cut in such a plane that it shows 

 neither its external nor its pharyngeal opening. The nasal passages 

 are here fairly long and nearly straight chambers ; their lining 

 epithelium is quite thick in the middle region, but becomes thinner 

 where it merges into the epithelium of the pharynx at one end, and 

 into the superficial epithelium at the other end. The unusual appear- 

 ance of the eye (e), on the right side of the figure, is due to the fact 

 that the plane of the section cut tangentially through the extreme 

 edge of the eye in the region of the choroid fissure. 



Figure 20c is only a short distance posterior to the preceding. On 

 the left side the pharynx ( ph) is connected with the exterior through 

 the stomodeaum, and on the right the hyomandibular cleft (g f ) is 

 cut almost through its opening to the exterior. The auditory ves- 

 icle (o) on the right is cut near its middle region, while that on the 

 left is barely touched by the plane of the section. The notochord 

 (nt), with its condensed area of mesoblast, is somewhat larger than 

 in the preceding section. The nasal canal on the right (n) is cut 

 through neither anterior nor posterior opening, while on the left 

 side the canal shows the anterior opening (an). 



Figure 2of, which is in the region of the posterior part of the 

 pharynx and the anterior part of the heart, shows some rather un- 

 usual conditions. 



The spinal cord (sc) and notochord (nt), with the faintly out- 

 lined condensations of mesoblast in their region, need no special 

 description. The pharynx (ph) is here reduced to an irregular, 

 transversely elongated cavity, the lateral angles of which are con- 

 nected on each side with the exterior through a tortuous and almost 

 closed gill cleft (g), which must be followed through many sections 

 before its inner and outer openings may be determined. Dorsal to 

 the pharynx numerous blood-vessels (bv), both large and small, mav 



