. A laboratory manual and text-book of embryology. Embryology. 70 THE STUDY OF CHICK EMBRYOS about the cephalad end of the pharynx. Between the ventral wall of the fore-brain and the pharynx is an invagination of the ectoderm, Rathke's pocket. Section through the Otocysts and Second Aortic Arch (Fig. 58).—The otic vesicles are sectioned caudal to their apertures and appear as closed sacs lateral to the wall of the hind-brain. The cavity of the pharynx is somewhat triangular and its dorsad wall is thin. The anterior cardinal veins pass between the otocysts and the wall of the hind-brain. Ventra
. A laboratory manual and text-book of embryology. Embryology. 70 THE STUDY OF CHICK EMBRYOS about the cephalad end of the pharynx. Between the ventral wall of the fore-brain and the pharynx is an invagination of the ectoderm, Rathke's pocket. Section through the Otocysts and Second Aortic Arch (Fig. 58).—The otic vesicles are sectioned caudal to their apertures and appear as closed sacs lateral to the wall of the hind-brain. The cavity of the pharynx is somewhat triangular and its dorsad wall is thin. The anterior cardinal veins pass between the otocysts and the wall of the hind-brain. Ventral to the pharynx the bulbus cordis is sectioned obliquely where it leaves the heart, and at this level gives off laterad the second pair of aortic arches which connect dorsad with the descending aorta?. Surrounding the bulbus cordis is the large pericardial cavity. Between the first and second aortic arches (Fig. 58) is the first pair of pharyngeal pouches, lateral diverticula of the entoderm. The student should note that in the sections of this stage so far studied, the mesen-. Aorticarch z Ventral aorta. Endothelium of bulbus Fig. 58.—Transverse section through the otic vesicles and second aortic arches of a fifty-hour chick embryo. X 50. chyma of the head is undifferentiated, the tissues peculiar to the adult not yet having been formed. Section through the Sinus Venosus and Common Cardinal Veins (Fig. 59)-—At this level, the common trunk formed by the anterior and posterior cardinal veins opens into the thin-walled sinus venosus. The sinus receives all of the blood passing to the heart and is separated only by a slight constriction from the larger atrium. The muscle plates of the first mesodermal segments are seen, and the descending aorta have united to form a single dorsal vessel. On either side of the pharynx are seen subdivisions of the ccelom which will form the pleural cavities. These cavities are separated from the pericardial cavity by the septum trans- versum
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectembryology, bookyear1
