142 DEVELOPMENT OF THE FROG. 



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 con- 

 nected at its ventral end with the corresponding afferent vessel. 

 Figs. 33 and 34. These direct connections are situated ventrally 

 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 capil- 

 laries ; 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 gills, these direct com- 

 munications between afferent and efferent vessels, though 

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

 cally no blood passes through them, and all the blood is com- 

 pelled to pass through the gills to reach the aorta. 



3. The Changes in the Circulation at the time of the Meta- 

 morphosis. 



At the time of the metamorphosis, however, when the 

 anterior limbs are protruded, and the tail begins to shorten, 



