. Embryology. Embryology; Embryology; genealogy. DIFFERENTIATION IN THE FOREGUT REGION 187 the endoderm gives rise to the cells which secrete the digestive enzymes, as well as those which secrete certain hormones. While the various folds are closing off the embryonic digestive system from the yolk sac, differentiation of the individual structures such as lungs, liver, and pancreas proceeds. We shall discuss the formation of some struc- tures which are not a part of the digestive system but arise in close association with it, as, for example, the thyroid and thymus glands. Differentiation in th
. Embryology. Embryology; Embryology; genealogy. DIFFERENTIATION IN THE FOREGUT REGION 187 the endoderm gives rise to the cells which secrete the digestive enzymes, as well as those which secrete certain hormones. While the various folds are closing off the embryonic digestive system from the yolk sac, differentiation of the individual structures such as lungs, liver, and pancreas proceeds. We shall discuss the formation of some struc- tures which are not a part of the digestive system but arise in close association with it, as, for example, the thyroid and thymus glands. Differentiation in the foregut region Referring to the diagram of the foregut (Fig. 112), we see that the ectoderm fuses with the endoderm of the anterior ventral region of the foregut and forms an oral plate. This plate becomes perforated and the oral cavity and mouth differentiate. A little farther back the tongue arises as two growths from the floor of the pharynx. These two growths fuse and form the lining and glands of the tongue, while the muscle comes from the nearby mesoderm. Between the two primordia of the tongue a pocket develops. This appears first as a single outgrowth but later it splits into two ESOPHAGUS GLOTTIS TRACHEA EPITHELIAL BODIES / 2 OUTGROWTHS OF TONGUE PAIRED THYROIDS Fig. 112. Derivatives of the foregut and some associated structures. The visceral pouches, lettered a through e, are endodermal outgrowths of the foregut. The mesodermal arches are solid black and labeled 1 through 5. Some of the derivatives of the visceral pouches and visceral arches are shown in the figure. Others are described in the text. The thyroid, the lining of tongue and lungs, and the trachea come from the endoderm. The hypophysis is ectodermal in origin and bears no relation to the development of the Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not p
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