THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



237 



into visceral and parietal layers, come to lie in the visceral (splanchnic) layer 

 (Fig. 211). They are composed of masses of cells known as blood islands which 

 are the anlagen of both the blood cells and the endothelium of the blood vessels. 

 The superficial cells of an island become transformed into fiat cells which sur- 

 round the remaining cells of the island and form the endothelial walls of a 

 primitive blood vessel (Fig. 211). A number of these vessels then anastomose to 

 forma net- work of channels containing, at intervals, groups of cells which repre- 

 sent the central cells of the blood islands, and which are the forerunners of the 



Ccelom 



Parietal mesoderm 



Ectoderm 



Visceral mesoderm 



Blood islands 



Fig. 211. — Section of blastoderm of chick of 42 hours' incubation. Photograph. The cells of the 

 blood islands are differentiated into nucleated red blood cells (erythroblasts) and the endo- 

 thelium, of the vessels. 



red blood cells. Around the border of the area vasculosa the vascular channels 

 then unite to form a vessel — the sinus terminates — which is continuous except at 

 the head end of the embryo (Fig. 212). During the second day of incubation 

 the vascularization of the splanchnic layer of mesoderm gradually extends 

 through the area pellucida toward the embryo (Fig. 212). Some of the channels 

 become larger and form arteries and veins which extend into the embryo and 

 finally unite with the heart in a definite way' (p. 224). 



The question of the growth of blood vessels is not yet settled. Are they formed by a 

 progressive differentiation of the mesenchymal tissue, or do they grow out as sprouts or buds 

 from vessels already present? It is obvious that the first vessels in the area opaca are 

 formed in situ by a differentiation of the tissue already present. In the vascularization 

 of the area pellucida, do the new vessels represent independently formed structures which 

 unite secondarily with those of the area opaca, or are they the result of outgrowths from 

 the primary vessels in the area opaca ? This same question arises in regard to the for- 

 mation of the first vessels inside the embryonic body, and in fact in regard to the formation 



