. Text-book of embryology. Embryology. 362 IIsrVERTEBEATA CHAP. splits in the mesoderm. The coelomic cavities contain a clear fluid, and have a definite epithelial arrangement of the cells forming their walls; whilst the blood spaces contain an albuminous serum which stains, and their walls are often irregular and in some places formed only by the yolk-membrane. The coelom arises as two vesicles lying beneath the mid-gut. Each of these vesicles becomes divided by a con- striction into a dorsal part, which is the rudiment of the pericardium, and a ventral part, the rudiment of the kidney; the c
. Text-book of embryology. Embryology. 362 IIsrVERTEBEATA CHAP. splits in the mesoderm. The coelomic cavities contain a clear fluid, and have a definite epithelial arrangement of the cells forming their walls; whilst the blood spaces contain an albuminous serum which stains, and their walls are often irregular and in some places formed only by the yolk-membrane. The coelom arises as two vesicles lying beneath the mid-gut. Each of these vesicles becomes divided by a con- striction into a dorsal part, which is the rudiment of the pericardium, and a ventral part, the rudiment of the kidney; the connec- tion between the two, though narrow, persists, and forms the reno-peri- cardial canal. Beneath the shell-sac, on the posterior slope of the embryo, a wide blood space arises whose cavity is traversed by cords of mesen- chyme. This is the posterior sinus (Fig. 294). It ex- tends anteriorly round the sides of the gut and there constitutes the two forks of the vena cava. From this a branch extends into the rudiment of the gill, which is the beginning of the branchial heart. In front, these forks unite beneath the gut to form the unpaired vena cava. The heart arises independ- ently of this sinus as two hollow tubes lying inter- nally between the coelomic rudiments. Behind, these tubes unite to form the ventricle, but in front, where kidney and coelom join, they diverge so as to form the auricles (Fig. 295). In the next period of development, the end of which is represented in Fig. 296, the embryo becomes as large as the external yolk- sac, and the funnel is definitely constituted by the union of the free edges of the folds in the mid-ventral line. The arms have now acquired suckers and have extended round the head to the mid- dorsal line, so that the encircling of the head by the fore foot is. Fig. 294.—Two diagrammatic transverse sections through a young emliryo of Loligo vulgaris to illustrate the origin of the coelom and the blood cavities. (After Faussek.) A, The more
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Keywords: ., bookcentury1900, bookdecade1910, bookpubli, booksubjectembryology
