FOUR TO Six SOMITE STAGE 



469 



ventral midline and fuses to form the one cavity which it later becomes 

 in all vertebrates. The crescent-shaped line where fore-gut and mid-gut 

 meet is called the anterior intestinal portal (Fig. 266, B). 



It is in the region of the anterior intestinal portal that the coelomic 

 chambers on both sides show a marked enlargement. The enlargement 

 of each side extends mesiad toward the other, and finally both break 

 through into each other ventral to the fore-gut, to form the pericardial 

 cavity. These enlarged regions are called the amnio-cardiac vesicles 

 (Fig. 274) in their early stages; however, it is better to remember what 



X -^ 



opVes 



Fig. 274. 



Ventral views of the head ends of chick embryos. A. Embryo with five pairs 

 of somites (about twenty-three hours). B. Embryo with seven pairs of somites 

 (about twenty-five hours), a.c.v., Amnio-cardiac vesicle; a. i. p., anterior intestinal 

 portal ; End'c.s., endocardinal septum ; FG., fore-gut ; Ht., heart ; My'C., myo- 

 cardium ; N'ch., notochord ; N'ch.T., anterior tip of notochord ; n.F., neural fold, 

 op. Ves., optic vesicle; p.C., parietal cavity (coelomic) ; Pr'a., proamnion ; a.2, s.4, 

 second and fourth mesodermal somites ; V.o.m., omphalo-mesenteric vein. (From 

 Lillie's "Development of the Chick" by permission of Henry Holt & Co., 

 Publishers. ) 



they are to become, and think of them as the pericardial region of the 

 coelom. 



The splanchnic mesoderm is thickened at the point where it lies 

 closely applied to the entoderm at the lateral margins of the portal. It is 

 from these thickened areas that the paired primordia of the heart will 

 arise later. 



It will thus be noted that the heart develops from the ventral and 

 not the dorsal aspect. 



The amnio-cardiac vesicles also become vascularized quite like the 

 rest of the pellucid area, and a pair of ventral aortae is formed beneath 

 the fore-gut. Immediately posterior to the anterior intestinal portal 

 these vessels diverge, passing into the vascular network as the rudiments 

 of the vitelline veins. 



