40 PRELIMINARY ACCOUNT. [CHAP. 



At a time when the cleavage of the mesoblast has some- 

 what advanced, there appears, a little way in front of the semi- 

 lunar head-fold, a second fold (Fig. 11, also Fig 8, (7.), running 

 more or less parallel or rather concentric 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 splanch- 

 nopleure (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. 8 and 9. In front of 

 the head-fold, and therefore altogether in front of the body 

 of the embryo, the somatopleure 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. 8, (7, a/.), 

 as it increases in height it is gradually drawn backwards 

 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 be- 

 hind the tail, and at some little distance from the sides (Fig. 8, 

 C, D, E, F). In this way the embryo becomes surrounded 

 by a series of folds of thin somatopleure, which form a 

 continuous wall all round it. All are drawn gradually over the 

 body of the embryo, and at last meet and completely coalesce 

 (Fig. 8, H, /), all traces of their junction being removed. 

 Beneath these united folds there is therefore a cavity, within 

 which the embryo lies (Fig. 8, H y ae). This cavity is the 

 cavity of the amnion. The folds which we have been 

 describing are those which form the amnion. 



Each fold, of course, necessarily consists of two limbs, both 

 limbs consisting of epiblast and a very thin layer of mesoblast; 

 but in one limb the epiblast looks towards the embryo, while 

 in the other it looks away from it. The space between the two 

 limbs of the fold, as can easily be seen in Fig. 8, is really part 

 of the space between the somatopleure and splanchnopleure ; 

 it is therefore continuous with the general space, part of which 

 afterwards becomes the pleuroperitoneal cavity of the body, 

 shaded with dots in the figure and marked (p p). So that it 

 is possible to pass from the cavity between the two limbs of 

 each fold of the amnion into the cavity which surrounds 



