The Heart.
Fig. 356. Section through the developing heart of an embryo of an Elasmobranch (Pristiurus).
al. alimentary tract; sp. splanchnic mesoblast; so. somatic mesoblast; ht. heart.
The heart is essentially formed as a tubular cavity in the splanchnic mesoblast, on the ventral side of the throat, immediately behind the region of the visceral clefts. The walls of this cavity are formed of two layers, an outer thicker layer, which has at first only the form of a half tube, being incomplete on its dorsal side; and an inner lamina formed of delicate flattened cells. The latter is the epithelioid lining of the heart, and the cavity it contains the true cavity of the heart. The outer layer gives rise to the muscular wall and peritoneal covering of the heart. Though at first it has only the form of a half tube (fig. 356), it soon becomes folded in on the dorsal side so as to form for the heart a complete muscular wall. Its two sides, after thus meeting to complete the tube of the heart, remain at first continuous with the splanchnic mesoblast surrounding the throat, and form a provisional mesentery—the mesocardium—which attaches the heart to the ventral wall of the throat. The superficial stratum of the wall of the heart differentiates itself as the peritoneal covering. The inner epithelioid tube takes its origin at the time when the general cavity of the heart is being formed by the separation of the splanchnic mesoblast from the hypoblast. During this process ([fig. 357]) a layer of mesoblast remains close to the hypoblast, but connected with the main mass of the mesoblast by protoplasmic processes. A second layer next becomes split from the splanchnic mesoblast, connected with the first layer by the above-mentioned protoplasmic processes. These two layers form together the epithelioid lining of the heart; between them is the cavity of the heart, which soon loses the protoplasmic trabeculæ which at first traverse it. The cavity of the heart may thus be described as being formed by a hollowing out of the splanchnic mesoblast, and resembles in its mode of origin that of other large vascular trunks.
Fig. 357. Transverse section through the posterior part of the head of an embryo Chick of thirty hours.
hb. hind-brain; vg. vagus nerve; ep. epiblast; ch. notochord; x. thickening of hypoblast (possibly a rudiment of the subnotochordal rod); al. throat; ht. heart; pp. body cavity; so. somatic mesoblast; sf. splanchnic mesoblast; hy. hypoblast.
