THE DEVELOPMENT OF THE FROG 275 



from a rete mirabile or plexus of blood-vessels in connection 

 with its communicating vessel. The connection between 

 the first and second aortic arches persists in the adult as 

 the ductus Botalli. 



The second aortic arch retains its connection with the 

 aorta, and becomes the systemic arch of the adult. 



The third aortic arch loses its connection with the aorta, 

 and disappears altogether. 



The fourth aortic arch becomes the pulmo-cutaneous artery 

 of the adult, after losing its connection with the aorta. 



Thus the arrangement of the arteries in the adult frog 

 is brought about by the modification of an arrangement 

 characteristic of fishes — is due, in fact, to a branchial respira- 

 tion having preceded a pulmonary respiration. The same 

 may be said of the arterial system of all higher vertebrates, 

 though in them the changes are greater, and a branchial 

 respiration is not established at any period of their existence, 

 the gill-slits developed in their embryos being functionless. 



The excretory system of the tadpole makes its appearance 

 at the time when the neural folds are closing in, some 

 considerable time before hatching. It is formed as a 

 thickening of the somatic layer of the lateral mesoblast 

 oppo-site the second mesoblastic somite. The thickening 

 is continued backwards in the substance of the somatopleur 

 as far as the seventh somite, and its anterior end becomes 

 enlarged. Both the enlargement and its posterior continuation 

 become hollow, the former becoming the pronephros or head 

 kidney, the latter the segmental' duct. The pronephros is 

 divided into three tubes, which grow towards and open 

 into the coelom by ciliated funnels or nephrostomes. These 

 tubes, and the portion of the segmental duct immediately 

 behind them, grow in length, become much convoluted, and 

 are invested by a capsule of somatic mesoblast. At the 

 same time the splanchnopleur opposite the pronephric funnels 

 forms an irregular sacculated outgrowth, the glomerulus, 

 which becomes connected with and filled with blood from 

 the dorsal aorta. At the time of hatching, the segmental 

 duct becomes attached to the wall of the cloaca, and opens 

 into it. 



Some time after this, when the tadpole is about 12 mm. in 

 length, the middle kidney, mesonephros, or Wolffian body, is 



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