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 
magnified. 
For abbreviations see Fig. 46. 
become deeper, owing to the forward 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 
‘aused by the median growth and conerescence 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 
Pies. 50, 52, 62, etc. 
Thus by the 12s 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 
