FROM TWELVE TO THIRTY-SIX SOMITES 131 



chapter. The lateral limiting sulci are a continuation of the 

 lateral limbs of the head-fold; they owe their origin to the folding 

 of the splanchnopleure and somatopleure adjacent to the embryo 

 towards the yolk, at the line of junction of embryonic and extra- 

 embryonic parts. The tail-fold arises about the stage of 26 to 

 27 somites (Fig. 93), and is similar to the head-fold, except that 

 it is turned in the opposite direction. The sulci combine to form 

 a continuous ring around the embryo and gradually pinch it off, 

 so to speak, jrom the extra-embryonic blastoderm. 



In the splanchnopleure the limiting sulci (Fig. 69) come to- 



FIG. 69. Transverse section through the fifth somite of the 23 s stage. 



Amn., Amnion. Ao., Aorta, a. i. p., Anterior intestinal portal. Coel., 

 Coelome. Chor., Chorion. Ectam., Ectamnion. E. E. B. C., Extra-embry- 

 onic body-cavity. Int., Intestine. 1. 1. s., Lateral limiting sulcus. My., 

 Myotome. s. a., Segmental artery. So 'pi., Somatopleure. Spl'pl., Splanch- 

 nopleure. s., Somite, s. 5, Fifth somite, v V. O. M. R. and L., Right and left 

 omphalo-mesenteric veins. V. V., Vitelline vein. 



gether and fuse both in a caudal direction from the fore-gut, and 

 subsequently in a cephalic direction from the hind-gut (see below), 

 so as to convert the splanchnic gutter into a tube (the alimentary 

 canal). There is thus a ventral suture along the alimentary 

 canal in which the entoderm of the alimentary canal becomes 

 separated from the extra-embryonic entoderm, thus leaving a 

 double layer of the splanchnic mesoblast (ventral mesentery) 

 connecting the alimentary canal with the extra-embryonic splanch- 

 nopleure; but this disappears everywhere as soon as formed, 

 except in the region of the posterior part of the heart and the 

 liver where it forms the dorsal mesocardium and gastro-hepatic 

 ligament (Fig. 118), and in the region of the neck of the allantois. 



