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 



op.Ves. 

 Mesenc 



FIG. 74. Head of the same embryo 

 from the ventral side. Abbreviations 

 as before. 



Rec.opt 



FIG. 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. pi., 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, etc.); the side limbs of the head-fojd are 

 then elongated backwards, and are here known as the lateral 

 folds of the amnion; these rise up and arch over the embryo 



