ORGANIC DEVELOPEMENT. 609 



Such is obviously the permanent condition of all 

 the organs of sensation, and also of the appa- 

 ratus for locomotion : and it has just been shown 

 that those portions of the nervous system which 

 are situated in the mesial plane, such as the 

 spinal cord and the brain, consisted originally of 

 two separate sets of parts, which are brought 

 together and conjoined into single organs. In 

 like manner we have seen that the constituent 

 laminae of the heart are at first double, and 

 afterwards form by their union a single cavity. 

 The operation of the same law has been traced 

 in the formation of those vascular trunks, situated 

 in the mesial plane, which are usually observed 

 to be single, such as the aorta and the vena 

 cava : for each were originally formed by the 

 coalescence of double vascular trunks running 

 parallel to each other, and at first separated by 

 a considerable interval ; then approaching each 

 other, adhering together, and quickly converted, 

 by the obliteration of the parts which are in 

 contact, into single tubes, throughout a consider- 

 able portion of their length.* 



Nature, ever vigilant in her anticipations of 



* These facts were first observed by Serres (Annales des Sc, 

 Nat. xxi. 8.), and their accuracy has been confirmed by the ob- 

 servations of Dr. Allen Thomson. In Reptiles this union of the 

 two constituent trunks of the aorta is effected only at the pos- 

 terior part, while the anterior portion remains permanently 

 double. (See Fig. 357, vol. ii. p. 274.) • 



VOL. II. R B 



