4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



the mouth and lining the mandibular folds, is composed of two or 

 three layers of compactly arranged cells. 



Figure 4D is through a plane sixteen sections caudad to the last. 

 In this region, which is just caudad to the otic vesicles, the pharynx 

 has still its rectangular outline, and its walls are of the same 

 character as in the preceding figure. The posterior edges of the 

 hyomandibular clefts are seen projecting in a ventro-lateral direction, 

 g^ ; while dorsal to these are the wider, second pair of clefts, g'. 

 Where the mandibular folds come together posterior to the mouth, 

 they fuse first at their outer or ventral border, which leaves a deep, 

 narrow groove in the anterior floor of the mouth. As this groove is 

 followed caudad its ventral wall is seen to become much thickened, 

 tg, to form the aniage of the thyroid gland. In the present section 

 the walls of the groove are just fusing, to cut off the cavity of the 

 gland from the dorsal part of the groove. The next section caudad 

 to this shows the thyroid as a round, compact mass of cells, with a 

 very small lumen, still closely fused with the bottom of the oral 

 groove. The lumen may, in this embryo, be traced for only a few 

 sections, caudad to which the thyroid is seen as a small, solid mass 

 of cells unattached to the oral groove. Close to the sides of the 

 thyroid are seen two large blood vessels, ar, the mandibular arches, 

 which unite into the single ventral aorta just caudad to the posterior 

 end of the thyroid. High power drawings of the thyroid just 

 described are shown in figures 4E and 4F. 



if'igure 4G is about fifty-five sections caudad to the preceding fig- 

 ure, and passes through the middle region of the heart, lit. The 

 enteron, ent, is cut caudad to the last gill cleft, but it is nearly as 

 large as in the pharyngeal region described above ; its walls are of a 

 more even thickness than in the more anterior sections, though there 

 is an area, just below the aorta, where the wall is still but one cell 

 thick. In the ventral wall of this part of the enteron, and, to some 

 extent, in the lateral walls, there seems to be a tendency for the 

 nuclei to become collected toward the side of the wall away from the 

 digestive cavity ; this condition cannot be well seen in the figure 

 owing to the amount of reduction in reproduction. 



Figure 4H is seventy-nine sections posterior to the last, and passes 

 through the foregut, etit, just cephalad to the anterior intestinal portal 

 and caudad to the heart. The outline of the enteron is here almost a 

 vertical slit, and the lining entoderm consists, in its dorsal and lateral 

 regions, of a single layer of columnar epithelium, while in its ventral 

 region, where it adjoins the liver trabeculae, it is made up of several 

 layers of cuboidal or irregular cells. The nuclei in the dorsal and 

 lateral regions of the entoderm are arranged in a verv definite laver 



