EXTRA-EMBRYONIC MEMBRANES 87 



The caudally-directed limbs of the head fold of the amnion 

 are continued posteriorly along either side of the embryo as 

 the lateral amniotic folds. The lateral folds of the amnion 

 grow dorso-mesiad, eventually meeting in the mid-line dorsal 

 to the embryo (Fig. 30, A-C). 



During the third day, the tail-fold of the amnion develops 

 about the caudal region of the embryo. Its manner of de- 

 velopment is similar to that of the head fold of the amnion 

 but its direction of growth is reversed, its concavity being 

 directed anteriorly and its progression being cephalad (Fig. 

 32, B, C). 



Continued growth of the head, lateral, and tail folds of the 

 amnion results in their meeting above the embryo. At the 

 point where the folds meet, they become fused in a scar-like 

 thickening termed the amniotic raphe (sero-amniotic raphe). 

 (Fig. 32, C). The way in which the somatopleure has been 

 folded about the embryo leaves the amniotic cavity completely 

 lined by ectoderm which is continuous with the superficial 

 ectoderm of the embryo at the region where the yolk-stalk 

 enters the body (Fig. 30, D). 



All the amniotic folds involve doubling the somatopleure on 

 itself. Only the inner layer of the somatopleuric fold is in- 

 volved in the formation of the amniotic cavity. The outer 

 layer of somatopleure becomes the serosa (Fig. 30, B}. The 

 cavity between serosa and amnion (sero-amniotic cavity) is part 

 of the extra-embryonic coelom. The continuity of the extra- 

 embryonic ccelom with the intra-embryonic ccelom is most 

 apparent in early stages (Fig. 30, A and B). They remain, 

 however, in open communication in the yolk-stalk region until 

 relatively late in development. 



The rapid peripheral growth of the somatopleure carries the 

 serosa about the yolk-sac, which it eventually envelops. The 

 albumen-sac also is surrounded by folds of serosa, and the 

 allantois after its establishment develops within the serosa, 

 between it and the amnion. Thus the serosa eventually 

 encompasses the embryo itself and all the other extra-embryonic 

 membranes. The relationships of the serosa and allantois 

 and the functional significance of the serosa will be taken up 

 after the allantois has been considered. 



