. An American text-book of obstetrics. For practitioners and students. PREGNANCY. 113 of the yolk-sac immediately in contact with the axial portions of the ento-derm. This differentiation is effected by the ventral extension and approxi-mation of the widely expanded splanchnopleure, which, bending together(Fig. 100), gradually closes to form the primitive gut—at first freely openinginto the yolk-sac, finally completely isolated from the latter except throughthe communication maintained by the narrow umbilical duct. By the fifteenth day the gut has become defined to such extent that threeparts
. An American text-book of obstetrics. For practitioners and students. PREGNANCY. 113 of the yolk-sac immediately in contact with the axial portions of the ento-derm. This differentiation is effected by the ventral extension and approxi-mation of the widely expanded splanchnopleure, which, bending together(Fig. 100), gradually closes to form the primitive gut—at first freely openinginto the yolk-sac, finally completely isolated from the latter except throughthe communication maintained by the narrow umbilical duct. By the fifteenth day the gut has become defined to such extent that threeparts are distinguishable—the fore-gut, the mid-gut, and the hind-gut. Thefore-gut, which includes the cephalic third of the tube, gives rise to the phar-ynx, the esophagus, and the stomach, the latter organ early appearing as afusiform enlargement of the primitive canal. The anterior end of the fore-gut reaches as far forward as the marked cephalic flexure opposite the mid-brain, and at first is separated from the primitive oral invagination, or sto- Amnion. Mesoderm. Visceralmesoderm Pleuropericadial cavity. Pericardialplates. Extensionof celom. Fig. 100.—Transverse section of a sixteen and a half day sheep embryo (Bonnet). matodceum (Fig. 101, A, b), by a septum consisting of the opposed ectodermicand entodermic layers. After the rupture of this partition, which happensduring the fifteenth day, the primitive pharynx and oral cavity are directlycontinuous. A series of four diverticula extend between the visceral arches, and constitutethe pharyngeal pouches or inner visceral furrows (Fig. 106; PI. 16). Theseevaginations of the pharyngeal lining are of interest, since the first pouchbecomes converted into the Eustachian tube and the tympanic cavity, the thirdpouch into the early epithelial thymus body, and the fourth pouch into thelateral portions of the early thyroid body. From the ventral surface of thefore-gut, at the end of its pharyngeal division, there grows out the diver
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectobstetrics, bookyear1
