II GILLS 41 



namely, the mandibular, the hyoidean, and the Univ l)ianeliial 

 arches, correspond, long before they are cartilaginnus, with tmir 

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

 arteria hyo-mandibularis, belongs to the hyoidean and mandibular 

 segments, the second to the tirst branchial, the thii'd to the second 

 branchial, while the fourth soon splits in two lur 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 brancliials, appear in newts, 

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

 future neck, lietween the arches the pharynx gradually l)ulges 

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

 the mandibular and the hyoidean arch, the second l)etween the 

 hyoidean and the first branchial arch, etc. Iliese pouches soon 

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

 first pouch in I^rodela. Before the l)reaking through of the 

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

 each arch a little knob, which 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 l)reaks 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 looj) sprouts out 

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

 producing the gill, the middle portion of the i)rimary arch 

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

 partly obliterates, and lienceforth acts as the internal ejf'erenf 

 vessel of the gill. AVhen, (hiring 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. 



Tiie arterial arches themselves are modilied as follows: The 

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

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

 formed into the pulmonary arteries and "ductus Botalli." tlie list 

 arterial arch having previously sent a branch into the tlevelopiug 

 lungs. In the Anura the third arch obliterates. 



Tlie gills and clefts present various modifications. 'J'he 

 Urodela possess three pairs of gills, one each ui-on the dorsal 



