. Chordate morphology. Morphology (Animals); Chordata. notochord dorsal aorta. notochord nasal pouch Rathke's pouch nasal pouch â ^secondary pharyngeal membrane opens to outside later Rathke's pouch opened into pharynx Figure 12-1. Stages in the development of Rathke's pouch in the lamprey and Eptofrefus. (After Dohrn, and Kupper, 1900) of the protochordate as in Amphioxus. In Amphioxus the bilobed nature of the endostyle when it first appears sug- gests a pair of pharyngeal pouches (Van Wijhe). The endostyle evaginates early in the lamprey and soon differentiates into a complex and apparently
. Chordate morphology. Morphology (Animals); Chordata. notochord dorsal aorta. notochord nasal pouch Rathke's pouch nasal pouch â ^secondary pharyngeal membrane opens to outside later Rathke's pouch opened into pharynx Figure 12-1. Stages in the development of Rathke's pouch in the lamprey and Eptofrefus. (After Dohrn, and Kupper, 1900) of the protochordate as in Amphioxus. In Amphioxus the bilobed nature of the endostyle when it first appears sug- gests a pair of pharyngeal pouches (Van Wijhe). The endostyle evaginates early in the lamprey and soon differentiates into a complex and apparently mucoid sec- retory organ. Some of the cells of this organ take up iodine very much in the way of the thyroid tissue, which develops later. With the onset of metamorphosis, the endostylar gland loses connection with the pharynx and a part of its tissue forms thyroid-like follicles. In the hagfish the thyroid arises as a groove extending the length of the floor of the pharynx. In the actinopterygian fishes the thyroid arises, along a line, from the floor of the pharynx. From this groove, clumps of cells come to lie along the course of the ventral aorta. Each clump forms a follicle or groups of follicles of a typical thy- roid structure. Experimental evidence indicates that their secretion is of a thyroid nature. A few teleosts have a mas- sive thyroid like that of a shark. In the sharks the thyroid tissue is rather gelatinous, but formed of typical follicles, and encapsulated by connective tissue. This mass lies far forward, behind or below the basi- hyal cartilage, anterior to the forking of the ventral aorta. It may be crescentic in shape or irregular. In the shark the gland arises as an evagination from the floor of the pharyngeal cavity with or without a central lumen. As it sinks deeper it loses connection with the phar- ynx. In Chlamydoselachus the duct retains its connection with the pharynx even in the adult. This duct enters the pharynx through a perforation in the bas
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