134 DEVELOPMENT OF THE FKOG 



of the first, second, third, and fourth branchial arches respectively 

 are formed. 



Each afferent vessel runs outwards and upwards in its own 

 arch. The efferent branchial vessels lie immediately in front 

 of the corre.sponding afferent vessels, with which they are 

 connected by very numerous capillary loops in the substance of 

 the internal gills, and not shown in the figures. At their upper 

 ends the efferent vessels open, as before, into the dorsal aorta. 

 Fig. 34. 



The venous blood in the heart is driven by the contraction of the 

 ventricle into the truncus arteriosus, and then along the afferent 

 branchial vessels, through the capillary loops of the gills, in 

 which it gets aerated, to the efferent branchial vessels ; and 

 thence to the dorsal aorta, and so all over the body. 



The lungs are by this time of considerable size : they receive 

 blood by the pulmonary arteries, AP, which, as already noticed, 

 are branches from the efferent vessels of the fourth branchial 

 arches, and therefore contain blood which has already passed 

 through the gill capillaries. The blood from the lungs is 

 returned direct to the heart by two pulmonary veins which 

 unite and open into the left auricle, the single auricular cavity 

 of the earlier stage being by this time divided by a vertical 

 septum into right and left auricles. 



One other point of great importance remains to be noticed 

 in the arrangement of the branchial vessels of the tadpole. 

 The afferent and efferent vessels of each arch at first com- 

 municate only through the gill capillaries : but in tadpoles of 

 about 12 mm. length each efferent vessel becomes directly 

 connected at its ventral end with the corresponding afferent 

 vessel. Fig. 34. These direct connections are situated ven- 

 trally to the gills, so that the blood in any one of the 

 afferent branchial vessels has two paths open to it : it may 

 either (1) continue along the afferent vessel, and then reach 

 the efferent vessel by passing through the connecting loops 

 afforded by the gill capillaries ; or (2) it may pass at once 

 into the efferent vessel through the direct communication, 

 and so reach the dorsal aorta without having passed through 

 the gill at all. 



So long as the tadpole is breathing by giUs, these direct com- 

 munications between afferent and efferent vessels, though 

 present in aU four branchial arches, are so small that practi- 



