564 



TEXT-BOOK OF EMBRYOLOGY. 



on each side of the neural groove between ectoderm and entoderm the mesoderm 

 is a solid mass of cells, while more laterally the mesoderm is split, its peripheral 

 layer with the adjacent ectoderm forming the somatopleure, its central layer 

 with the adjacent entoderm forming the splanchnopleure; that between soma- 

 topleure and splanchnopleure is the body cavity. Ventral to the neural groove 

 is the notochord, while ventral to the latter is the primitive gut, the roof of which 

 is formed of entoderm (Fig. 52), 



The first indication of amnion formation is the appearance of a fold the 

 head amniotic fold just in front of the anterior union of the neural folds (Figs. 



ar. op. 



ar. peL 



FIG. 478 Dorsal view of embryo of bird (Phaeton rubricauda) with fifteen pairs of 



primitive segments. Schauinsland. 



ar. op* y Area opaca, portion in which mesoderm is not yet present; ar. op*, area opaca; ar. pel.-, 

 area pellucida; cce., bladder-like dilatation of ccelom; ed. mes., edge of mesoderm; h. am. /., 

 head amniotic fold; pr. seg., primitive segments; x, portion of amniotic fold containing no 

 mesoderm. 



478 and 484, b) . This occurs during the second day of incubation. After the 

 head fold has become well developed and extends back over the embryo like a 

 hood (Fig. 480), similar lateral and tail folds make their appearance (Figs. 479 

 and 484, a and b). The folds continue to grow over the dorsum of the 

 embryo and finally meet and fuse in the mid-dorsal line, forming the amniotic 

 suture (Fig. 481). 



The amniotic folds from the beginning involve the somatopleure, that is, 

 the ectoderm and parietal mesoderm. But since they arise some distance from 

 the developing embryonic body, the extraembryonic portions only are involved. 

 At the same time a portion of the extraembryonic body cavity is also carried 

 dorsally within the folds (Figs. 479 and 482) . When the folds unite over the 



