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 3. 



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, (7). 



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. 8, F), 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 



