122 THE STRUCTURE OF THE FOWL 



islands — the predecessors of blood-vessels — 

 make their appearance and the vascular area 

 thus comes into existence. Towards the end 

 of the first day a distinction can be made 

 between the mesoderm in the immediate 

 vicinity of the neural groove and notochord 

 and that which is more lateral. The mesoderm 

 in the region of the notochord constitutes the 

 paraxial mesoblast, while the more peripheral 

 mesoderm is known as the lateral plate. Very 

 soon after the formation of the head-fold, the 

 paraxial mesoblast begins to be invaded by 

 transverse clefts, and is thus cut up into blocks 

 or segregations of cells known as the mesohlastic 

 somites. The first somite to be formed lies 

 close to the anterior end of the primitive 

 streak, and the others form in succession 

 behind it. Nevertheless there is always a 

 certain amount of unsegmented mesoblast 

 between the anterior end of the primitive 

 streak and the somite last formed. 



Within the lateral plate of the mesoblast a 

 splitting also takes place, but in a horizontal 

 instead of a transverse direction. A series 

 of small cavities appear within the plate, and 

 by their fusion divide it into an outer somatic 

 layer (Figs. 60, 61, and 62) and an inner or 



