226 COMPARATIVE ANATOMY. 



constricted off from the pharynx in the course of development, 

 and the communication between them is once for all abolished. 

 (Comp. also small print on p. 225.) As a rule it shows a paired 

 arrangement, and lies right and left of the median line. Internally 

 it consists of closed glandular vesicles, surrounded by a capillary 

 network, or cylindrical branched tubes may be present (Mammals). 

 The whole is lobulated in structure, and is characterised, 

 particularly in Mammals, by a great wealth of blood-vessels. It 

 seems very probable that this organ, which is in many respects 

 rudimentary, has undergone a gradual change of function in the 

 course of phylogenetic development, but as yet it is impossible to 

 explain in what its function consists. As regards position, it either 

 remains throughout life in its locus nascendi on the floor of the 

 mouth, as in Fishes and Amphibia, or it extends a varying 

 distance backwards. In Birds, for instance, it lies on the origin 

 of the carotid artery (Fig. 185, Tr). 1 



The development of the thyroid has been studied most completely in the 

 Fowl and in Mammals. In the former, the organ arises as an unpaired hollow 

 vesicle in the median ventral line of the neck. Later, this becomes solid, 

 and then divided into two halves, which gradually separate from one another 

 until eventually they reach the position they occupy in the adult. 



Amongst Mammals the development of this organ is best known in the Pig. 

 Here the so-called middle lobe or isthmus arises as an outgrowth of the 

 mucous membrane of the floor of the mouth on the level of the second 

 branchial arch. The lateral lobes arise in the region of the third branchial 

 cleft. The different lobes become united later on (comp. also small print at 

 end of section on the tongue). 



Thymus Gland. 



The thymus gland arises on either side from the mucous 

 membrane of the pharynx, as a proliferation of the epithelium 

 of the gill-clefts. It is impossible to state with certainty whe- 

 ther it was originally a glandular, that is, a secretory organ, or 

 whether it consists of material that was at one time designed 

 for the formation of gill-filaments. Certain discoveries in 

 Elasmobranch embryos seem to point to the latter view. In 

 them the organ has a segmental arrangement corresponding to the 

 number of gill-clefts, and in Gymnophiona indications of a 

 similar arrangement are to be seen. 



According to His, the thymus of Man does not arise from the inner or 

 hypoblastic epithelium of the pharynx, that is, of the visceral clefts, but 

 from the epithelial (epiblastic) covering of the fourth, third, and partly also 



1 According to A. Dohrn the hyoid arch corresponds genetically to two arches, 

 the first giving rise to the hyomandibular, and the second to the hyoid arch proper, 

 and the thyroid represents the remnant of a lost gill-cleft between the hyomandibular 

 and hyoid. Dohrn supports this by the relations of the thyroid to the vascular 

 system ; the thyroid artery arises like a true branchial vessel from the hyoid artery. 



