626 THE THYROID BODY. 



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 Elasmobrancli embryos it appears fairly early as a diverticulum 

 froin the ventral surface of the throat iu 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 ai'ch (fig. 417, Th). 

 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 nvimber of solid branched 

 lobules sei)arated by connective tissue septa. Eventually its connection 

 with the throat becomes lost, and the lobules develop a lumen. In Acan- 

 thias the lumen of the gland is retained (W. Miiller) till after its detach- 

 ment from the throat. It preserves its embiyonic position through life. 

 In Amphibia it originates, as in Elusmobranchii, from the region of the 

 mandibular arch ; but when first visiide it forms a double epithelial wall 

 connecting the throat with the nervous layer of the epidermis. It sub- 

 sequently 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 })eculiar connec- 

 tion between the thyroid diverticulum and the epidermis in Amphibia 

 has been noted by Gtitte 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 

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

 By the ninth day the whole is invested by a capsule of connective tissue, 

 which sends in septa dividing it into a number of lobes or solid masses of 

 cells, and by the sixteenth day it is a paired body composed of a number 

 of hollow branched follicles, each with a ' membraua propria,' and sepa- 

 rated fi'om each other by septa of connective tissue. It finally travels back 

 to the point of origin of the carotids. 



Amongst Mammalia the thyroid arises iu the Rabbit (KoUiker) and 

 Man (His) as a hollow diverticulum of the throat at the bifurcation of the 

 foremost pair of aortic aiches. It soon however becomes solid, and is 

 eventually detached from the throat and comes to lie on the ventral side 

 of the larynx or windpipe. The changes it undergoes are in the main 

 similar to those in the lower Vertebrata. It becomes partially constricted 

 into two lobes, which remain however united by an isthmus'. The fact 



1 Wolfler (No. 571) states that in the Pig and Calf the thyroid body is formed as 

 a pair of epithelial vesicles, which are developed as outgrowths of the walls of tie 

 first pair of visceral clefts. He attempts to exjilain the contradictory observations of 

 other embryologists by supposing that they have mistaken the ventral ends of visceral 

 pouches for an unpaired outgrowth of the throat. Stieda (No. 569) also states that 

 in tlie Pig and Sheep the thyroid arises as a paired body from the epithelium of a pair 

 of visceral clefts, at a much later jieriod than would nppear from the observati(ms of 



