116 THE DEVELOPMENT OF THE CHICK 



the overgrowth of the yolk until it extends completely around 

 the latter 



Returning now to the first stages in the formation of the coe- 

 lome. In the 3 s stage it undergoes a precocious expansion in 

 the region lateral to the head of the embryo (Figs. 51, 52, etc.), 

 forming a pair of large cavities known as the amnio-cardiac 

 vesicles, because they participate in the formation of the amnion 

 and pericardium. These cavities extend in rapidly towards the 

 middle line, arid enter the head-fold in the 4-5 s stage (Figs. 52, 

 58). At the stage of 6-7 s they meet in the floor of the fore-gut 

 immediately behind the oral plate and fuse together, thus divid- 

 ing the head-fold into somatic and splanchnic limbs, as previously 

 described. A median undivided portion of the body-cavity 

 known as the parietal cavity (forerunner of the pericardium) 

 is thus established beneath the fore-gut; and it extends back- 

 ward with the elongation of the fore-gut in the manner already 

 described. A pair of blind prolongations of this cavity extends 

 a short distance forward at the sides of the oral plate at the 10-12 s 

 stage (cf. Fig. 62), lying lateral and ventral to the ventral aortaB. 



The median angle of the body-cavity, where the somatic 

 and splanchnic layers meet, is a point of fundamental morpho- 

 logical importance. In the region of the somites the nephrotome 

 is attached here, and in the head the wing of cells leading to the 

 axial mesoblast (cf. Figs. 68 B, 53, and 54). In an embryo 

 with ten somites this angle may be traced forw r ard to near the 

 hinder end of the oral plate, lying beneath the lateral angles of 

 the pharynx. 



Mesoblast of the Head. Mesoblast exists in two forms in 

 the embryo: (1) in the form of epithelial layers or membranes 

 (mesothelium), and (2) in the form of migrating cells which 

 usually unite secondarily to form a syncytium in the form of a 

 network, the meshes of which are filled with fluid; the nuclei 

 lie in the thickened nodes. This form of the mesoblast is known 

 as mesenchyme. It is always derived from a pre-existing epi- 

 thelial layer, usually, but not necessarily, mesothelium, for, as 

 we shall see, parts of it are derived from ectoderm and entoderm; 

 on the other hand, mesenchyme may secondarily take on an 

 epithelial arrangement (endothelium). The terms mesothelium 

 and mesenchyme have therefore merely descriptive significance 

 in the early embryonic stages. The mesenchyme has no single 



