ORGANIC DEVELOPMENT. 429 



ture of communication between the right and left cavities;, 

 but this aperture is soon closed, and the ventricle is now 

 effectually divided into two. Next the auricle, which at first 

 was single, becomes double; not, however, by the growth of 

 a partition, but by the folding in of its sides, along a middle 

 line, as if it were encompassed by a cord, which was gradu- 

 ally 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 vessels; that which issues 

 from the left ventricle being the aorta; and the other, which 

 proceeds from the right ventricle, being the pulmonary ar- 

 tery; and each being now prepared to exercise its appropri- 

 ate function in the double circulation which is soon to be 

 established.* 



A mode of subdivision af 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 re- 

 semble branchiae. These changes may be very distinctly 

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

 the trunk of the aorta undergoing successive subdivisions, 

 by branches sent off from it, and forming loops, which ex- 

 tend in length and are again subdivided, in a manner not 

 unlike the unravelling of the strands of a rope; each subdi- 

 vision, however, being preceded by the formation of a dou- 

 ble partition in the cavity of the tube; so that at length the 

 whole forms an extensive ramified system of branchial arte- 

 ries and veins. Still all these are merely temporary struc- 

 tures; for when the period of change approaches, and the 

 branchiae are to be superseded in their office, every vessel,, 

 one after another, becomes obliterated, and there remain 

 only the two original aorta, which unite into a single trunk 

 lower down, and from which proceed the pulmonary arte- 

 ries, conveying either the whole, or a portion of the blood, 

 to the newly developed respiratory organs, the lungs. 



* The principal authorities for the facts here stated are Baer and Rolando, 

 See the paper of Dr. Thomson already quoted, 

 f See the investigations of Rusconi, ajid of Baer, on this subject. 



