278 The Allirator and Its Allies 



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 men- 

 tioned 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 iic, bv. The ectoderm surrounding the 

 lower side of the embryo was so thin and indistinct 

 that it could scarcely be distinguished from the 

 mesoderm of that region. The amnion (a) is still 

 a continuous envelope entirely surrounding the 

 embryo. 



Figure I2d, about twenty sections posterior to 

 Figure 12c, is in the posterior heart region. The 

 spinal cord (sc), as might be expected, is smaller 

 than in the more anterior region, but is otherwise 

 not markedly different from what was there seen. 

 The notochord {iit) also has the same appearance 

 as before. The enteron (ent) shows of course in 

 this region no gill clefts; it is a small, irregular 

 cavity with thicker walls than in the figure just 

 described. The ventro-lateral depression is entirely 

 distinct from the depression that was called the 

 thyroid rudiment in the preceding figure. Dorsal 

 to the enteron are the two dorsal aortae {00), 

 now smaller and more ventral to the notochord 



