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 35s 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, and 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 
deseribed. <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 aorte. 
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, 538, and 54). In an embryo 
with ten somites this angle may be traced forward 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 epl- 
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 
