THE DEVELOPMENT OF THE FROG 273 



for the fourth branchial arch. (Prior to this, there was a stage 

 when only two external gills were present, and only two pairs 

 of afferent arteries running in the first and second branchial 

 arches.) 



The blood passes through capillary loops in the external 

 gills, and is collected and brought back to the right and left 

 branches of the dorsal aorta by three pairs of efferent branchial 

 vessels. Though there are no afferent arteries in the mandibu- 

 lar and hyoid, each of these arches is furnished with a small 

 efferent artery, and there is also an efferent artery in the fourth 

 branchial arch, which, even at this early stage, sends out a 

 branch towards the lungs. 



When the gill-slits open to the exterior, and the internal gills 

 are formed on their walls, the afferent arteries send out vessels 

 to them, and the blood is returned from them to the efferent 

 arteries. The fourth pair of branchial arteries supplies the 

 fourth gill-slit, and Its corresponding efferent vessel becomes 

 functional. Less and less blood passes into the external gills, 

 and eventually they shrivel up and disappear. The circulation 

 is now very like that of the dogfish, except that there are four 

 afferent branchial vessels instead of five. There is a very 

 important difference, however, in the fact that the lungs of 

 the tadpole are by this time fairly well-developed, and are 

 supphed with blood from branches of the fourth pair of 

 efferent arteries, this, of course, being oxygenated blood which 

 has passed through the gills. From the lungs the blood is 

 returned to the heart by two pulmonary veins, which unite 

 and open into the left division of the auricle, cut off from 

 the originally single auricular cavity by the downgrowth of a 

 vertical septum from its dorsal wall. 



Soon after the internal gills are established, and the 

 circulation has assumed the arrangement just described, a 

 direct communication, shown at i in fig. 68, is established 

 between each afferent artery and the ventral end of its 

 corresponding efferent vessel. Thus a certain amount of 

 blood that has not passed through the gills, and therefore 

 is not oxygenated, passes into the general circulation and 

 to the lungs. At first the amount is very small, but at 

 the time of the metamorphosis these communicating vessels 

 are enlarged, and an increasing amount of blood flows through 

 them without having first passed through the gills. The 



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