350 THE ANIMAL KINGDOM. 



ment of the mesenteron. The former is a ventral outgrowth of hypoblast 

 into the mesoblast, either double or single at first, but always becoming 

 double at last. The gland tubes develope into a network surrounded by a 

 vascular network, and their tubular character, retained in Amphibia and 

 Reptilia, is lost in Aves and Mammalia by the growth of the lining epi- 

 thelium. The number of bile ducts vary, and there is frequently a gall- 

 bladder or caecum attached to one of them. The fully formed gland is 

 typically bilobed, but its external shape is subject to great changes. The 

 pancreas arises as a single, or in Aves double, dorsal outgrowth of the 

 mesenteron into the mesoblast. It is eventually closely connected (with 

 few exceptions) to the bile duct near its intestinal opening. It is absent 

 in some Pisces. 



A diverticulum from the oesophagus, wanting in Elasmobranchii and 

 Holocephali, becomes the air-bladder of Pisces, the lungs of the higher Ver- 

 tebrata. It is primitively ventral in position, dorsal in nearly all Pisces in 

 which the original communication with the oesophagus is often lost. It is 

 either single or double, but in the case of the lungs always becomes double, 

 remaining attached by a stalk to the oesophagus. The stalk is the trachea, 

 the two appended caeca the bronchi and lungs. In Aves and Mammalia 

 the caeca branch repeatedly forming the bronchia 1 , and the ultimate 

 branches the air-tubes or cells. Certain of the bronchia are prolonged 

 into air-sacs in Aves. The hypoblast cells form the epithelium lining the 

 whole structure, the surrounding mesoblast, the supporting and vascular 

 tissues. The trachea, bronchi, and the bronchia when present, are strength- 

 ened by cartilaginous rings or pieces most complete in the two structures 

 first named. The first rings of the trachea differentiate into a larynx, 

 with which are connected the fibrous bands or cords which produce the 

 voice. The organ of voice, however, in Aves is developed at the junction 

 of the trachea and bronchi, and is known as the syrinx. A valve, the 

 epiglottis, covers the entrance to the larynx in Mammalia. 



Two other structures, the thyroid and thymus, are closely connected 

 to the fore-part of the alimentary canal. The former develops as a ventral 

 diverticulum of the mouth, lying at the anterior end of the ventral aorta, 

 and is probably homologous with the endostyle of Urochorda. It closes 

 off from the mouth, and when fully formed is a solid body either single, 

 bilobed, or broken up into two parts (Amphibia} or many masses (Teleostei)* . 

 The thymus is formed by epithelial growths from the dorsal ends of more 

 or fewer of the branchial (visceral) clefts. These growths fuse together on 

 the right and left sides into a single body. The thymus atrophies in the 

 higher Vertebrata as a rule. 



1 The bronchia, which spring from a spot above, i. e. anterior to the point, where the pulmonary 

 artery crosses the bronchus, are known as ep-arterial ; those below, or posterior to it, as hyp-arterial. 



2 See a paper by Dohrn on the thyroid in Petromyzon, Amphioxus, and Tunicata ( = Urochorda), 

 Mitth. Zool. Stat. Naples, vi. 1885. 



