78 TEXT-BOOK OF EMBRYOLOGY. 



thus augmenting the layer generally, there is also evidence that new meso- 

 dermal cells are added by differentiation of the entodermal elements of the 

 germ wall. This probable origin of mesodermal cells from the yolk cells of 

 the germ wall is comparable with the formation of mesoderm around the gut 

 in the frog (p. 60). As the germ wall recedes by encroachment of the 

 entoderm upon the yolk, and the pellucid area becomes correspondingly more 

 extensive, the mesoderm likewise increases in extent. 



^j Almost as soon as the margin of the mesoderm overlaps the germ wall 

 and begins to extend over the area opaca, small dense clusters of cells appear 

 in the mesoderm in close relation to the yolk cells. These are the blood is- 

 lands from which arise the early blood vessels and blood cells, and the area 

 occupied by them is known as the area vasculosa (Fig. 51). They appear, 

 first caudal to the primitive streak and then farther forward on both sides, 

 almost keeping pace with the mesoderm' itself. The islands increase in 

 size and coalesce irregularly to form a network or plexus which is one of the 

 conspicuous features of the blastoderm (Fig. 156). The cells at the pe- 



Neural plate Notochord 



I' Ectoderm 



Mesoderm 

 . . Entoderm 

 Archenteron 



FIG. 50. Transverse section of blastoderm of chick (40 hours' incubation). Hertwig. Section 

 taken short distance anterior to Hensen' node. 



riphery become flat and arranged edge to edge to form endothelial tubes or 

 vessels within which the central cells are contained as primitive blood cells 

 (Fig. 158). The plexus then gradually extends across the pellucid area 

 toward the axis of the blastoderm where the embryonic body is developing. 

 It is obvious that blood vessels and blood cells develop relatively earlier in 

 the bird than in either Amphioxus or the frog. 



The notocord develops out of the fused mass of entoderm and meso- 

 derm of the primitive axis, whether from the one layer or the other being diffi- 

 cult to determine. It thus becomes a rod of cells extending forward from 

 the front end of the primitive streak and separating the mesoderm on one 

 side from that on the other. The formation of the notocord as the axial 

 structure of the future embryo destroys the primitive axis, and the ento- 

 derm now becomes a distinct and separate layer (Fig. 50) . 



Summing up, it may be said that about the beginning of the second day 

 of incubation the mesoderm comprises a sheet of cells between ectoderm 

 and entoderm. It extends caudally and laterally from the primitive streak 

 where it i? merged with the other two layers. In front of the streak it ex- 



