II GILLS 41 



namely, the mandibular, the hyoidean, and the four branchial 

 arches, correspond, long before they are cartilaginous, with four 

 main arterial arches of the truncus arteriosus. The first, the 

 arteria hyo-mandibularis, belongs to the hyoidean and mandibular 

 segments, the second to the first branchial, the third to the second 

 l»ranchial, while the fourth soon splits in two for the third and 

 fourth or last branchial arch. On the dorsal side these branchial 

 arterial arches combine to form the radix of the dorsal aorta. 

 These arches, especially the three branchials, appear in newts, 

 less clearly in frogs, as transverse ridges on the sides of the 

 future neck. Between the arches the pharynx gradually bulges 

 out in the shape of five lateral gill-pouches; the first between 

 the mandibular and the hyoidean arch, the second between the 

 liyoidean and the first branchial arch, etc. These pouches soon 

 break through to the outside and become gill-clefts, except the 

 first pouch in Urodela. Before the breaking through of the 

 clefts there appears upon the outside of the middle of the rim of 

 each arch a little knob, wliich soon ramifies and forms an external 

 gill. The knob owes its origin to the development of a blood- 

 vessel which buds from the arterial arch, ramifies and breaks up 

 into capillaries, and returns a little further dorsalwards into the 

 arch. A secondary loop to the outside of the primary arterial 

 arch is thus formed ; and whilst this outer loop sprouts out 

 further, driving before it the likewise proliferating skin, and thus 

 producing the gill, the middle portion of the primary arch 

 remains in the Urodela as a short cut, but in the Anura it 

 partly obliterates, and henceforth acts as the internal efferent 

 vessel of the gill. When, during metamorphosis, the gills dis- 

 appear, their intrinsic afferent and efferent vessels vanish likewise, 

 and the short cut completes the circuit. In order to do this 

 they have, in the Anura, to form new connections with the trunks 

 of the afferent vessels. 



The arterial arches themselves are modified as follows : — The 

 first pair become the carotids, the second form the right and left 

 aortic arches, while the tliird and fourth unite and are trans- 

 formed into the pulmonary arteries and " ductus Botalli," the last 

 arterial arch having previously sent a branch into the developing 

 lungs. In the Anura the third arch obliterates. 



Tlie gills and clefts present various modifications. The 

 Urodela possess three pairs of gills, one each upon the dorsal 



