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 lateral limiting sulci (Fig. 69) 



\CJ}or. 



U.S. 



"iS^^psiS^i 





^P^X -% 



yoM^. 



fi , c .-'.'firs 



AfPCrc'-.r' 



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-cmbry-= 

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

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

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

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



come together and fuse both in a caudal direction from the fore- 

 gut, and subsequently in a cephaHc direction from the hind-gut (see, 

 below), so as to convert the splanchnic gutter into a tube (the ali-. 

 mentary canal). There is thus a ventral suture along the ali- 

 mentary canal in which the entoderm of the alimentary canal 

 becomes separated from the extra-embryonic entoderm, 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. 



