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, from the extra-embryonic blastoderm. 
In the splanchnopleure the limiting sulci (Fig. 69) come to- 
Fic. 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’pl., Somatopleure. Spl’pl., Splanch- 
nopleure. s., Somite. s. 5, Fifth somite. 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. 
