Thyroid body. In the Ascidians the origin of a groove-like diverticulum of the ventral wall of the branchial sack, bounded by two lateral folds, and known as the endostyle or hypopharyngeal groove, has already been described (p. [18]). This groove remains permanently open to the pharyngeal sack, and would seem to serve as a glandular organ secreting mucus. As was first pointed out by W. Müller there is present in Amphioxus a very similar and probably homologous organ, known as the hypopharyngeal groove.
Fig. 415. Diagrammatic transverse sections through the branchial region of young larvæ of Petromyzon. (From Gegenbaur; after Calberla.)
d. branchial region of throat.
In the higher Vertebrata this organ never retains its primitive condition in the adult state. In the larva of Petromyzon there is, however, present a ventral groove-like diverticulum of the throat, extending from about the second to the fourth visceral cleft. This organ is shewn in longitudinal section in [fig. 414], h, and in transverse section in [fig. 415], and has been identified by W. Müller (Nos. [565] and [566]) with the hypopharyngeal groove of Amphioxus and Ascidians. It does not, however, long retain its primitive condition, but its opening becomes gradually reduced to a pore, placed between the third and fourth of the permanent clefts ([fig. 416], th). This opening is retained throughout the Ammocœte condition, but the organ becomes highly complicated, with paired anterior and posterior horns and a median spiral portion. In the adult the connection with the pharynx is obliterated, and the organ is partly absorbed and partly divided up into a series of glandular follicles, and eventually forms the thyroid body.
From the consideration of the above facts W. Müller was led to the conclusion that the thyroid body of the Craniata was derived from the endostyle or hypopharyngeal groove. In all the higher Vertebrata the thyroid body arises as a diverticulum of the ventral wall of the throat in the region either of the mandibular or hyoid arches ([fig. 417], Th), which after being segmented off becomes divided up into follicles.
In Elasmobranch embryos it appears fairly early as a diverticulum from the ventral surface of the throat in the region of the mandibular arch, extending from the border of the mouth to the point where the ventral aorta divides into the two aortic branches of the mandibular arch ([fig. 417], Th). Somewhat later it becomes in Scyllium and Torpedo solid, though still retaining its attachment to the wall of the œsophagus. It continues to grow in length, and becomes divided up into a number of solid branched lobules separated by connective tissue septa. Eventually its connection with the throat becomes lost, and the lobules develop a lumen. In Acanthias the lumen of the gland is retained (W. Müller) till after its detachment from the throat. It preserves its embryonic position through life. In Amphibia it originates, as in Elasmobranchii, from the region of the mandibular arch; but when first visible it forms a double epithelial wall connecting the throat with the nervous layer of the epidermis. It subsequently becomes detached from the epidermis, and then has the usual form of a diverticulum from the throat. In most Amphibians it becomes divided into two lobes, and so forms a paired body. The peculiar connection between the thyroid diverticulum and the epidermis in Amphibia has been noted by Götte in Bombinator, and by Scott and Osborn in Triton. It is not very easy to see what meaning this connection can have.
Fig. 416. Diagrammatic vertical section through the head of a larva of Petromyzon.
The larva had been hatched three days, and was 4.8 mm. in length. The optic and auditory vesicles are supposed to be seen through the tissues. The letter tv pointing to the base of the velum is where Scott believes the hyomandibular cleft to be situated.
c.h. cerebral hemisphere; th. optic thalamus; in. infundibulum; pn. pineal gland; mb. mid-brain; cb. cerebellum; md. medulla oblongata; au.v. auditory vesicle; op. optic vesicle; ol. olfactory pit; m. mouth; br.c. branchial pouches; th. thyroid involution; v.ao. ventral aorta; ht. ventricle of heart; ch. notochord.