“FROM TWELVE TO THIRTY-SIX SOMITES 137 
(a diagrammatic outline is first given and a detailed description 
follows). The somatopleure becomes elevated in the form of 
a fold surrounding the embryo; this fold begins first in front of 
the head of the embryo as the head-fold of the amnion, which 
Fic. 74. — Head of the same embryo 
from the ventral side. Abbreviations 
as before. 
Fic. 75. — Median sagittal section of the head of an embryo of 18 s. 
H. F. Am., Head-fold of the amnion. Ph., Pharynx. Isth., Region of 
the isthmus, pr’o. g., Preoral gut. or. pl., Oral plate. Rec. opt., Recessus 
opticus. S. v., Sinus venosus. Other abbreviations as before. 
immediately turns backwards over the head, forming a complete 
cap (Figs. 67, 71, 75, ete.); the side limbs of the head-fold are 
then elongated backwards, and are here known as the lateral 
folds of the amnion; these rise up and arch over the embryo 
