550 THE RKPRODUCTIVE FUNCTIONS. 



had divided the ventricle, extends itself into the 

 trunk of the main artery, which it divides into two 

 channels ; and these afterwards become two sepa- 

 rate vessels; that which issues from the left ven- 

 tricle being the aorta; and the other, which pro- 

 ceeds from the right ventricle, being the pulmo- 

 nary artery ; and each of these vessels is now 

 prepared to exercise its appropriate function in the 

 double circulation which is soon to be established.* 

 A mode of subdivision of blood vessels, very 

 similar to that just described, takes place in those 

 which are sent to the first set of organs provided 

 for aeration, and which resemble branchi^. These 

 changes may be very distinctly followed in the 

 Batrachia;-\ for we see, in those animals, the 

 trunk of the aorta undergoing successive subdi- 

 visions, by branches sent off from it and forming 

 loops, which extend in length and are again sub- 

 divided, in a manner not unlike the unravelling 

 of the strands of a rope ; each subdivision, how- 

 ever, being preceded by the formation of a double 

 partition in the cavity of the tube; so that at 

 length the whole forms an extensive ramified sys- 

 tem of branchial arteries and veins. Still all these 

 are merely temporary structures; for when the 

 period of change approaches, and the branchia 

 are to be superseded in their office, every vessel, 

 one after another, becomes obliterated ; and there 

 remain only the two original aortte, which unite 

 into a single trunk lower down, and from which 



* The principal authorities for the facts here stated are Baer 

 and Rolando. See the paper of Dr. Thomson aheady quoted. 



t See the investigations of Rusconi, and of Baer, on this subject. 



