38 PRELIMINARY ACCOUNT. [CHAP. 



directly transferred from one cavity into the other). 

 Within a day or two of- the hatching of the chick, at a 

 time when the yolk-sac is still of some considerable size, 

 or at least has not yet dwindled away altogether, and 

 the development of the embryonic sac is nearly com- 

 plete, the yolk-sac (Fig. 9, iV) is slipped into the body 

 of the embryo, so that ultimately the embryonic sac 

 alone remains. 



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 forma- 

 tion ; 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, 

 Avhile 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 similar 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. 



Were the changes which take place of this class 

 only, the result would be a tubular sac of somewhat com- 

 plicated outline, but still a simple tubular sac. Such 



