176 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



The Thyroid becomes gradually constricted off from the pharynx 

 (Fig. 99, B and C) remaining for a time connected by a narrow stalk 

 or duct with the pharyngeal or rather buccal floor just in front of 

 the primary tongue (see Fig. 82, p. ,149). This stalk of attachment 

 becomes nipped across and the thyroid forms a mass (Fig. 99, D) or 

 vesicle rounded in form or somewhat elongated in an antero-posterior 

 direction lying in the mid-ventral line beneath the pharynx and 

 just in front of the ventral aorta. 



The originally simple vesicle undergoes a process of sprouting 

 and division by which it becomes converted into a mass of rounded 

 vesicles, each possessing a wall composed of a single layer of cubical 

 epithelial cells and separated from its neighbours by highly vascular 

 mesenchyme which penetrates in between the vesicles to form the 

 stroma of the organ. 



During later development the Thyroid undergoes characteristic 

 changes of form in different subdivisions of the Vertebrata. Thus in 

 Teleosts it frequently assumes a more or less diffuse character, the 

 follicles being distributed in the neighbourhood of the ventral aorta 

 and roots of the afferent branchial vessels. In the Amphibia and 

 Amuiota the organ becomes deeply constricted into two laterally 

 placed lobes which may remain connected or may become separated, 

 so that it assumes a paired character as happens in Amphibians 

 and Birds. 



With the processes of differential growth involved in the develop- 

 ment of the neck, the thyroid may undergo considerable displace- 

 ment from its point of origin. Thus in adult Lizards it lies across 

 the trachea well forwards from its hind end while in other reptiles 

 and in birds it lies farther back close to the roots of the great 

 arteries. 



It is now generally accepted that the clue to the phylogenetic 

 history of the Thyroid is afforded by its development in Petromyzon 

 (W. Miiller, 1871). Here there develops a mid- ventral outgrowth of 

 the pharyngeal floor, forming a short gutter in the branchial region, 

 the lining of which is composed partly of glandular cells which 

 secrete a sticky mucus and partly of cells which bear powerful 

 flagella. Morphologically this gutter is the same as the endostyle 

 of Amphioxus and during larval life its function is also similar : it 

 appears to be in fact simply a shortened up endostyle. The slit-like 

 pharyngeal opening becomes gradually reduced in length till it forms 

 merely a small pore. 



At the time of metamorphosis the pore becomes obliterated so 

 that the organ becomes a closed vesicle underlying the pharynx. 

 This vesicle divides up into a number of small vesicles and its 

 mucous secretion accumulates in their interior as a colloid substance 

 like that of the Thyroid vesicles of the Gnathostomata. In a word, 

 the endostyle of the Ammocoetes stage becomes the Thyroid of the 

 adult, and there seems no reason to doubt that the same has 

 happened in phylogeny and that the thyroid of the Vertebrate is 



