158 



F(ETAL MEMBRANES. 



looks away from it. The space between the two limbs of the fold, as 

 can easily be seen in fig. 121, 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 pleuroperi- 

 toneal cavity of the body, shaded with dots in tbe figure and marked 

 {'p p)', so that it is possible to pass from the cavity between the 

 two limbs of the amniotic folds into the cavity which surrounds 

 the alimentary canal. When the several folds meet and coalesce 

 together above the embryo, they unite in such a way that all their 

 inner limbs unite to form a continuous inner membrane or sack, and 

 all their outer limbs a similarly continuous outer membrane or sack. 

 The inner membrane thus built up forms a completely closed sack 

 round the body of the embryo, and is called the amniotic sack, or 

 amnion proper (fig. 121, H, I, &c. a), and the fluid which it after- 

 wards contains is called the amniotic fluid, or liquor amnii. The space 

 between the inner and outer sack is, from the mode of its formation, 

 simply a part of the general cavity found everywhere between 

 somatopleure and splanchnopleure. The outer sack over the embryo 

 lies close under the vitelline membrane, and the cavity between it 

 and the true amnion is gradually extended over the whole yolk 

 sack. 



The actual manner in which the amniotic folds meet is somewhat 

 peculiar (His and Kolliker). The head fold of the amnion is the earliest 



Fig. 122. DrAGRAMMATic longitudinal section through the axis of an embbto. 



The section is supposed to be made at a time when the head-fold has commenced 

 but the tail-fold has not yet appeared. 



F.So. fold of the somatopleure. F.Sp. fold of the splanchnopleure ; D. fore-gut. 

 pp. pleuroperitoneal cavity between somatopleure and splanchnopleure; Am. com- 

 mencing (head) fold of the amnion. For remaining reference letters vi(U p. 139. 



formed, and completely covers over the head before the end of the second 

 day. The side and tail folds are later in developing. The side folds finally 

 meet in the dorsal line, and their coalescence proceeds backwards from the 

 head fold in a linear direction, till there is only a small opening left over 

 the tail. This also becomes closed early on the third day. 



