ORGANIC DEVELOPMENT. 427 



by the obliteration 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, sufficient 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 circum- 

 stances, 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 in- 

 nermost of the pellicles which invest the embryo; that is, on 

 the surface opposite to the one \vhich has produced the spi- 

 nal marrow. These laminse, which are originally separate, 

 and apart from one another, are brought together, and by 

 the junction or soldering of their opposite edges, formed into 

 a tube,t which, from being, at first, uniform in diameter, af- 

 terwards expands into several dilated portions, correspond- 

 ing with the cavities of the stomach, crop, gizzard, c., into 

 which they are to be converted, when the time shall come for 

 their active employment. These new organs are, however, 

 even in this, their rudimental state, trained to the perform- 

 ance of their proper offices, receiving into their cavities, 

 through a tube temporarily provided for that purpose, the 

 fluid of the yelk, and preparing nourishment from it. 



In the mean time, early provision is made for the aera- 

 tion of the fluids by an extensive but temporary system of 



* These facts were first observed by Serres (Annales des Sc. Nat. xxi. 8,) 

 and their accuracy has been confirmed by the observations of Dr. Allen Thorn- 

 son. In Reptiles this union of the two constituent trunks of the aorta is ef- 

 fected only at the posterior part, while the anterior portion remains perma> 

 nently double. (See Fig. 357, vol. ii. p. 197.) 

 is the author of this discover)-. 



