THE DEVELOPMENT OF THE VASCULAR SYSTEM 



217 



development, the peripheral part of the area opaca caudal and lateral to the 

 primitive streak presents a mpttled appearance (Fig. 1940). This indicates 

 the beginning of the area vasculosa, which subsequently extends forward in 

 the peripheral portion of the opaque area, lateral to the developing body, 

 and becomes reticulated in appearance (Fig. 1946). 



Sections of the blastoderm show that the mottled surface appearance is 

 due to clusters of cells amidst the mesoderm, known as blood islands (Fig. 

 195). These are composed of rounded cells which have developed from the 

 branched mesodermal (mesenchymal) cells, and are situated in close apposi- 

 tion to the entoderm. Subsequently, when the ccelom appears in this region, 

 they lie in the visceral, or splanchnic, layer of mesoderm (Fig. 196). 



Ectoderm' 

 Mesoderm 



Entoderm 

 (yolk cells) 



Blood island 



FIG. 195. Section of blastoderm (area opaca) of chick of 27 hours' incubation. Photograph. 



The early changes that occur in the blood islands are important as re- 

 gards both developing vessels and blood cells. The superficial cells of an 

 island are transformed into flat cells placed edge to edge which surround 

 the remaining rounded cells. The flat cells constitute the endothelium of a 

 primitive blood space, while the cells within the space comprise primitive 

 blood cells (Fig. 196). These early spaces in the area vasculosa join one 

 another and become continuous to form a net-work, or plexus, of channels 

 to which is due the reticulated appearance referred to above (Fig. 1946). 

 This is known as the vitelline plexus. The groups of primitive blood cells 

 within the channels will be considered in detail in a subsequent section 

 (page 268). 



During the second day of incubation in the chick the peripheral 



