FROM TWELVE TO THIRTY-SIX SOMITES 137 



(a diagrammatic outline is first given and a detailed description 

 follows). The somatopleure becomes elevated m the form ot 

 a told surrounding the embryo; this fold begins fii-st m front ot 

 the head of the embryo as the head-foU of the amnion, which 



lenc: 



— ojo.l^es. 

 Mesenc 



op.Yes 





Pjq 74, _ Head of the same embryo 

 from the ventral side. Abbreviations 

 as before. 



V /sfh. 



///"Am. 



Rec.opf 



Fig 75. - Median sagittal section of the head of an embryo of 18 s^ 



H. F. Am., Head-fold of the amnion. P^. Pharynx^ Isth Region o 

 the isthmus pr'o. g., Preoral gut. or. pi., Oral plate. Rec. opt., Kecessus 

 opticus! S 'v., Sinus venosus. Other abbreviations as before. 



immediately turns backwards over the head, forming a complete 

 cap (Fio-s 67, 71, 75, etc.); the side Umbs of the head-fold are 

 then elongated backwards, and are here known as the lateral 

 fold, of the amnion; these rise up and arch over the embryo 



