ORGANIC DEVELOPEMENT. 613 



by a cord, which was gradually tightened. In 

 the mean while the partition, which had divided 

 the ventricle, extends itself into the trunk of the 

 main artery, which it divides into two channels ; 

 and these afterwards become two separate ves- 

 sels; that which issues from the left ventricle 

 being the aorta ; and the other, which proceeds 

 from the right ventricle, being the pulmonary 

 artery ; and each of these vessels is now pre- 

 pared to exercise its appropriate function in the 

 double circulation which is soon to be estab- 

 lished.* 



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 

 branchiae. These changes may be very dis- 

 tinctly followed in the Batrachia ;'\ for we see, 

 in those animals, the trunk of the aorta under- 

 going successive subdivisions, by branches sent 

 off from it and forming loops, which extend in 

 length and are again subdivided, in a manner not 

 unlike the unravelling of the strands of a rope ; 

 each subdivision, however, being preceded by 

 the formation of a double partition in the cavity 

 of the tube ; so that at length the whole forms 



* The principal authorities for the facts here stated are Baer 

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



+ See the investigations of Rusconi, and of Baer, on this 

 subject. 



