ORGANIC DEVELOPKMENT. 547 



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 in- 

 terval ; then approaching each other, adhering 

 together, and quickly converted, by the oblitera- 

 tion of the parts which are in contact, into single 

 tubes, throughout a considerable portion of their 

 length.* 



Nature, ever vigilant in her anticipations of the 

 wants of the system, has accumulated round the 

 embryo ample stores of nutritive matter, sutiicient 

 for maintaining the life of the chick, and for the 

 building of its frame, while it continues in the egg, 

 and is consequently unable to obtain supplies from 

 without ; yet, with the same foresight of future cir- 

 cumstances, she delays not, longer than is necessary 

 for the complete establishment of the circulation, to 

 construct the apparatus for digestion, on which the 

 animal is to rely for the means of support in after 

 life. The alimentary canal, of which no trace 

 exists at an earlier period, is constructed by the 

 formation of two laminae, arising from folds of the 

 innermost of the pellicles which invest the embryo ; 

 that is, on the surface opposite to the one which 

 has produced the spinal marrow. These laminae, 

 which are originally separate, and apart from one 

 another, are brought together, and by the junction 



* These facts were first observed by Serres (Annales dcs Sc. Nat. 

 xxi. 8.), and their accuracy has been confirmed by the observations 

 of Dr. Allen Thomson. In Reptiles this union of the two constituent 

 trunks of the aorta is effected only at the posterior part, while the 

 anterior portion remains permanently double. (See Fig. 357. vol. ii. 

 p. 248.) 



