II.] THE MEDULLARY CANAL. 39 



a simple sac might perhaps be roughly taken to repre- 

 sent the body of many an invertebrate animal ; but the 

 typical structure of a bird or other vertebrate animal is 

 widely different. It may very briefly be described as 

 follows. 



First there is, above, a canal running lengthways 

 along the body, in which are lodged the brain and 

 spinal cord. Below this neural tube is an axis repre- 

 sented by the bodies of the vertebrae and their con- 

 tinuation forwards in the structures which form the base 

 of the skull. Underneath this, again, is another tube 

 closed in above by the axis, and on the sides and below 

 by the body-walls. Enclosed in this second tube, and 

 suspended from the axis, is a third . tube, consisting of 

 the alimentary canal with its appendages (liver, pan- 

 creas, lungs, &c., which are fundamentally mere diver- 

 ticula from one simple canal). The cavity of the outer 

 tube, which also contains the heart and other parts of 

 the vascular system, is the general body cavity ; it con- 

 sists of a thoracic or pleural, and an abdominal or peri- 

 toneal section ; these two parts are, however, from their 

 mode of origin, portions of one and the same tube. 

 Thus a transverse section of a vertebrate animal always 

 shews the same fundamental structure : above a single 

 tube, below a double tube, the latter consisting of one 

 tube enclosed within another, the inner being the ali- 

 mentary canal, the outer the general cavity of the body. 

 Into such a triple tube the simple tubular embryonic 

 sac of the chick is converted by a series of changes of a 

 remarkable character. 



The upper or neural tube is formed in the following 

 way. At a very early period the upper layer of the 



