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 



0/3. yes. 

 Mesenc 



auJ* 



Vojn 





Fig. 74. — Head of the same embryo 

 from the ventral side. Abbreviations 

 as before. 





Ifec.opt 



^' V^i.","*'''^" '''^'"''' *''="°" °f *•>« '"'^'l of ^^ embryo of 18 s 



the isthmus"V?o1"'preo1al'iuro'rnl o'/^7"^. ^^"'- «<■«-" "f 

 opticus. S.'v.Psi„u!'veno:us! ^'therafbre^Siot as b^frre."^"' ^^^^^^"^ 



immediately turns backwards over the head, forming a complete 



cap (F.gs. 67, 71, 75, etc.); the side limbs of the head-fold' ale 



hen elongated backw^ards, and are here known as the lateral 



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



