Figure 16_d_ is in the region of the heart (_ht_) and lungs (_lu_). The former is an irregular cavity whose walls, especially on the ventral side (_mes′_), are becoming very thick and much folded. Although thin, the body wall completely surrounds the heart, as would be expected, since this was true of the preceding stage. The lung rudiments (_lu_) and the foregut from which they have arisen have the same appearance as in the chick; they consist of three small, thick-walled tubes so arranged as to form a nearly equilateral triangle. They are surrounded by a swollen, rounded mass of mesoblast which almost completely fills the surrounding portion of the body cavity (_bc_). The pleural sides of these crescentic portions of the body (or pleural) cavity--that is, the boundary of the mass of mesoblast just mentioned--are lined with a thickened layer of cells, shown by the solid black lines in the figure. The lung rudiments may be traced through about fifty sections of this series, or about one twelfth of the entire series. At the dorsal angle of the part of the body cavity (_bc_) just described, near the dorsal aorta (_ao_), are two dark, granular masses (_ge_), which, under a higher magnification than is here used, are seen to consist of a small group of blood-vessels filled with corpuscles; although several sections in front of the anterior limits of the kidneys these are evidently glomeruli. They may be traced, though diminishing in size, far toward the tail, in close connection with the Wolffian bodies. At intervals they are connected by narrow channels with the dorsal aorta; no such connection was present in the section drawn. The notochord (_nt_), spinal cord (_sc_), muscle plates (_mp_), and spinal ganglia (_sg_) need no special mention. The mesoblast is beginning to condense in the neighborhood of the notochord, and the ectoderm is slightly thickened laterally and dorsally.