44 PRELIMINARY ACCOUNT. [CHAP. 



of the semilunar head-fold, a second fold (Fig. 22, also 

 Fig. 9, C), running more or less parallel or rather con- 

 centric with the first, and not unlike it in general 

 appearance, though differing widely from it in nature. 

 In the head-fold the whole thickness of the blastoderm 

 is involved ; in it both somatopleure and splanchno- 

 pleure (where they exist, i. e. where the mesoblast is 

 cleft) take part. This second fold, on the contrary, is 

 limited entirely to the somatopleure. Compare Figs. 

 9 and 10. In front of the head-fold, and therefore alto- 

 gether in front of the body of the embryo, the somato- 

 pleure is a very thin membrane, consisting only of 

 epiblast and a very thin layer of mesoblast ; and the 

 fold we are speaking of is, in consequence, itself thin 

 and delicate. Rising up as a semilunar fold with its 

 concavity directed towards the embryo (Fig. 9, G, af.), 

 as it increases in height it is gradually drawn back- 

 wards over the developing head of the embryo. The 

 fold thus covering the head is in due time accompanied 

 by similar folds of the somatopleure starting at some 

 little distance behind the tail, and at some little dis- 

 tance from the sides (Fig. 9, G, D, E, F, and Fig. 11 am). 

 In this way the embryo becomes surrounded by a 

 series of folds of thin somatopleure, which form a con- 

 tinuous wall all round it. All are drawn gradually 

 over the body of the embryo, and at last meet and 

 completely coalesce (Fig. 9, H, 1), all traces of their 

 junction being removed. Beneath these united folds 

 there is therefore a cavity, within which the embryo 

 lies (Fig. 9, H, ae). This cavity is the cavity of the 

 amnion. The folds which we have been describino- are 

 those which form the amnion. 



