ALIMENTARY CANAL. 



761 



Somewhat later it becomes in Scyllium and Torpedo solid, though still 

 retaining its attachment to the wall of the oesophagus. 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. Miiller) till after its detachment from the 



-- "- 



Pti 



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. 



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 diver- 

 ticulum and the epidermis in Amphibia has been noted by Gotte in 

 Bombinator, and by Scott and Osborn in Triton. It is not very easy to see 

 what meaning this connection can have. 



In the Fowl (W. Miiller) the thyroid body arises at the end of the second 

 or beginning of the third day as an outgrowth from the hypoblast of the 

 throat, opposite the point of origin of the anterior arterial arch. This 

 outgrowth becomes by the fourth day a solid mass of cells, and by the fifth 

 ceases to be connected with the epithelium of the throat, becoming at the 

 same time bilobed. By the seventh day it has travelled somewhat back- 

 wards, and the two lobes have completely separated from each other. By 



