II.] THE EMBRYONIC SAC. 35 



model, which he easily can do by spreading a cloth out flat 

 to represent the blastoderm, placing one hand underneath it, 

 to mark the axis of the embryo, and then tucking in the 

 cloth from above under the tips of his fingers. The fingers, 

 covered with the cloth and slightly projecting from the level 

 of the rest of the cloth, will represent the head, in front of 

 which will be the semicircular or horse-shoe-shaped groove 

 of the head-fold. 



At its first appearance the whole 8 may be spoken of as 

 the head-fold, but later on it will be found convenient to 

 restrict the name chiefly to the lower limb of the 8. 



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

 altogether similar but less conspicuous fold makes its ap- 

 pearance, 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. 8, C). 



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

 folds appear, one on either side. These are simpler in cha- 

 racter 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. 



As these several folds become more and more developed, 

 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-narrowing 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. 8), 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 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 consider- 

 able 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. 8, iV) is slipped into the body of 



3—2 



