II.] THE TAIL-FOLD, 37 



of as the head-fold, but later on it will be found con- 

 venient to restrict the name chiefly to the lower limb 



of the a 



Some time after the appearance of the head-fold, an 

 altogether similar but at first less conspicuous fold 

 makes its appearance, at a point which will become the 

 posterior end of the embryo. This fold, which travels 

 forwards just as the head-fold travels backwards, is the 

 tail-fold (Fig. 9, G). 



In addition, between the head- and the tail-fold two 

 _ lateral folds appear, one on either side. These are 

 simpler in character than either head-fold or tail-fold, 

 inasmuch as they are nearly straight folds directed 

 inwards towards the axis of the body (Fig. 9,^, and not 

 complicated by being crescentic in form. Otherwise they 

 are exactly similar, and in fact are formed by the con- 

 tinuations of the head- and tail-folds respectively. 



As these several folds become more and more de- 

 veloped, the head-fold travelling backwards, the tail- 

 fold forwards, and the lateral folds inwards, they tend to 

 unite in the middle point ; and thus give rise more and 

 more distinctly to the appearance of a small tubular 

 sac seated upon, and connected, by a continually-nar- 

 rowing hollow stalk, with that larger sac which is formed 

 by the extension of the rest of the blastoderm over the 

 whole yolk. 



The smaller sac we may call the " embryonic sac," 

 the larger one " the yolk-sac." As incubation proceeds, 

 the smaller sac (Fig. 9) gets larger and larger at the 

 expense of the yolk-sac (the contents of the latter being 

 gradually assimilated by nutritive processes into the 

 tissues forming the growing walls of the former, not 



