HEAD-FOLD TO TWELVE SOMITES 117 



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 6 s stage (Figs. 43 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-cardiac 

 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 6 s 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 4 s 

 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 4 s 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 lie 

 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 



