HEAD-FOLD TO TWELVE SOMITES C7, 
embryonic significance either as to origin or fate, but is to be 
regarded as a mixed tissue. 
The mesoblast of the head is derived from several sources: 
(1) from a continuation forward of the paraxial mesoblast; (2) 
by proliferation from the fore-gut; and (3) from proliferations of 
ectoderm. 
(1) The axial mesoblast of the head is an anterior continua- 
tion of that of the trunk; it terminates at the anterior end of the 
fore-gut with which it is continuous from the stage of the head- 
process up to about the 6s stage (Figs. 48 and 49). In the 
anterior part of the head it is mesenchymal in its general struc- 
ture, grading posteriorly: into the mesothelial paraxial mesoblast 
of the hinder part of the head and trunk. It is continuous at 
first with the lateral mesoblast in which the amnio-cardiae 
vesicles are forming; but this connection is lost in the anterior 
part of the head that projects forward above the blastoderm; 
that is, in front of the head-fold. 
(2) The anterior end of the fore-gut proliferates mesenchyme 
from the time of its first formation to about the 6s stage (Fig. 
49). The proliferation is so rapid that it may give rise to the 
appearance of diverticula. The extreme anterior end of the floor 
forms a sac which lies just in front of the oral plate at the 4s 
stage (Fig. 52 A), but soon after breaks up into mesenchyme. 
There is a considerable mass of mesenchyme formed from this 
source in the space bounded by the anterior end of the fore-gut, 
the neural tube and the ectoderm; at the 4s stage this appears 
fused with the floor of the neural tube and the surface ectoderm, 
and probably receives cells from both; the anterior end of the 
notochord also disappears in this mass (cf. Fig. 67). 
(3) Ectodermal proliferations forming mesenchyme in the 
head. (This subject is discussed in the next chapter.) 
Vascular System. The origin of the blood-islands in the 
opaque area was described in the preceding chapter. They lhe 
between the ccelomic mesoblast and the yolk-sac entoderm de- 
rived from the germ-wall. When the somatopleure and splanch- 
nopleure are formed the blood-islands lie between the two layers 
of the latter, and the somatopleure is entirely bloodless. About 
the stage of 1 somite a vascular network continuous with the 
original network of the opaque area begins to appear in the 
pellucid area, at first at the margin of the opaque area, but by 
