Figure 12_d_, about twenty sections posterior to Figure 12_c_, 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 (_nt_) 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 aortæ (_ao_), now smaller and more ventral to the notochord than in the preceding figure. Ventral to the enteron is the large heart (_ht_), projecting below the body cavity, which is no longer enclosed. The mesodermic wall (_mes′_) of the heart is still comparatively thin and is separated by a considerable space from the membranous endocardium (_en′_). The extent and shape of the heart are shown in the surface view of this stage. On the right side of the section the body cavity extends to a point nearly opposite the middle of the spinal cord, considerably dorsal to the notochord, while on the left side the dorsal limit of the body cavity is scarcely level with the lower side of the notochord. Between the dorsal end of the body cavity and the side of the spinal cord, on the left, is a dense mass of mesoblast (_s_), one of the mesoblastic somites. A few sections either anterior or posterior to the one under discussion will show the condition of the two sides reversed--that is, the body cavity will extend to the greater distance on the left and will be interrupted by a mesoblastic somite on the right. It is evident, then, that the upper angle of the body cavity is extended dorsally as a series of narrow pouches between the somites. The mesoblast that lines the body cavity, the splanchnopleure (_sm_) and somatopleure (_so_), is somewhat denser than the general mass of mesoblast, so that these layers are quite distinct, the former (_sm_) extending around the enteron (_ent_) and heart (_ht_), and the latter (_so_) being carried dorsalward as the mesoblastic part of the amnion (_a_). The amnion may be traced through about 130 of the 200 sections into which this embryo was cut.