HEAD-FOLD TO TWELVE SOMITES 



93 



II. Formation of the Fore-gut 



The extension of the amnio-cardiac vesicles between the 

 ectodermal and entodermal layers of the head-fold introduces a 

 section of the body-cavity (pericardium) between these layers 

 and at the same time converts the ectodermal limb into a portion 

 of the somatopleure, and the entodermal limb into a portion of 

 the splanchnopleure. (See p. 115.) The splanchnopleuric 

 head-fold extends posteriorly very rapidly after the invasion 

 of the body-cavity, while the somatopleuric fold apparently 

 remains fixed for some time, though the head-fold appears to 



Fig. 47. — Head-fold region of Fig. 46 highly 

 magnified. 

 For abbreviations see Fig. 46. 



become deeper, owing to the forw^ard extension of the head 

 above the blastoderm. The posterior extension of the splanch- 

 nopleuric head-fold lengthens the floor of the fore-gut; it is 

 caused by the median growth and concrescence of folds of the 

 splanchnopleure (Fig. 53). Along with this process is involved 

 the development of the heart described farther on. The growth 

 in length of the fore-gut may be realized by a comparison of 

 Figs. 50, 52, 62, etc. 



Thus by the 12 s stage a considerable section of the fore-gut 

 is already established (Fig. 63); this is the pharyngeal division: 

 from the first it is extremely broad, and lunate in cross-section 

 (Fig. 54), the floor being composed of columnar cells, and the roof 



