36 PRELIMINARY ACCOUNT. [CHAP- 



the embryo, so that ultimately the embryonic sac alone re- 

 mains. 



6. The embryo, then, is formed by a folding off of a 

 portion of the blastoderm from the yolk-sac. The general 

 outline of the embryo is due to the direction and shape of 

 the several folds which share in its formation ; these, while 

 preserving a nearly perfect bilateral symmetry, present marked 

 differences at the two ends of the embryo. Hence from the 

 very first there is no difficulty in distinguishing the end which 

 will be the head from that which will be the tail. 



In addition to this, the tubular sac of the embryo, while 

 everywhere gradually acquiring thicker and thicker walls, 

 undergoes at various points, through local activities of growth 

 in the form of thickenings, ridges, buds or other processes, 

 many modifications of the outline conferred upon it by the 

 constituent folds. Thus bud- like processes start out from 

 the trunk to form the rudiments of the limbs, and simila,r 

 thickenings and ridges give rise to the jaws and other parts 

 of the face. By the unequal development of these outgrowths 

 the body of the chick is gradually moulded into its proper 

 outward shape. 



7. Were the changes which take place of this class only, 

 the result would be a tubular sac of somewhat complicated 

 outline, but still a simple tubular sac. Such a simple sac 

 might perhaps be roughly taken to represent 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 represented by the bodies 

 of the vertebra and their continuation 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, salivary glands, lungs, &c., which are 

 fundamentally mere diverticula 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 



