in the Foetus of Vertehrated Animals 99 



same time, the formation of the external gills, or more special 

 respiratory organ of the foetus of these animals, commences. 



The rudiments of the external gills, very similar in their 

 commencement to the branchiae of the foetal fish, are to be 

 found at a very early period. Some days before the foe- 

 tus of the Salamander quits the eggy they are indicated by 

 four transverse opaque bands on the fore part of the body, or 

 pharyngeal portion of the intestine. These transverse bands by 

 their farther development form branchial hoops on each side of 

 the neck. The integuments then begin to bud out at the upper 

 and lateral parts of these hoops, so as to form three small 

 projections or folds of the skin, placed severally opposite the 

 interstices between the hoops. According to Rusconi *, be- 

 fore these parts, which arc the commencing external gills, re- 

 ceive any vessels, the distribution of arteries in the neck is 

 very simple, and analogous to that noticed by Rathke in the 

 foetus of the osseous fish. The arterial vessel prolonged from 

 the bulb of the aorta advancing forwards below the neck, is di- 

 vided into four pairs of smaller vessels, four of which passing 

 round each side of the pharynx, unite with those from the 

 other side below the vertebral column, to form the descend- 

 ing aorta ; each pair of vessels, as it is given off by the aorta, 

 passes along one of the branchial hoops. Rusconi has ascer- 

 tained, and I have repeated his observations with the same re- 

 sult, that these vessels are at first quite simple in their course 

 round the pharynx, and do not give off any branches. 



At the time when the embryo comes out of the egg^ the little 

 processes which constitute the commencing external gills, are 

 considerably elongated, and are each supplied with a loop of 

 vessel from the outermost part of one of the branchial vascular 

 arches (Fig. 8, m). As the stalk of the gill sprouts out far- 

 ther from the side of the neck, it acquires considerable length, 

 and the loop of the branchial vessel, consisting simply of an 

 outgoing and returning branch, is prolonged into it. When the 

 primitive stalks of the gills have acquired greater length, new- 

 buds of the parenchyma begin to appear on their lower sides, 

 forming the commencing leaflets of the gill (Figs. 8 & 10, A), 



• Amours des Salamandres Aquatiques, et D^veloppement du Tetard, &c. ; 

 and in his Descrizione Anatomica degli organi della circolatione delle larve, 

 Ac. 



g2 



