100 



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. 72). 



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. 1 

 ed. mes. 

 h. am. f. 



ar. pel. 



Fig. 91.— Dorsal view of embryo of bird (Phaeton rubricauda) with fifteen pairs of 

 primitive segments. Schauinsland. 

 ar. op. 1 , Area opaca, portion in which mesoderm is not yet present; ar. op. 1 , area opaca; ar. pel., 

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

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

 mesoderm. 



91 and 97, 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. 93), similar lateral and tail folds make their appearance (Figs. 92 and 97, 

 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. 94). 

 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. 92 and 95). When the folds unite over the 



