468 



EMBRYOLOGY OF THE CHICK- 



derm into somatic and splanchnic layers, the blood vessels remain asso- 

 ciated with the splanchnic layer. 



Just as the somites are beginning to form, blood vessels will be 

 found at the margin of the pellucid area beginning to grow toward the 

 embryo. Only the tubular vessels develop in the area pellucida ; the 

 blood islands, as already noted, develop in the area opaca. This means 

 that the cellular or corpuscular elements of the blood arise in the pos- 

 terior region of the area opaca. 



Fig. 273. 



Beginning of the vascular system in the chick embryo. A, complete blood- 

 island ; B and C, beginning of vacuole formation ; D, vacuoles becoming confluent 

 to form the lumen of the blood vessel. b.hem., primitive red blood cells ; I, lumen ; 

 p, vessel- wall ; v, vacuole or lacunae. (After Uskow). E. A portion of the 

 vascular network destined to become the aorta in the chick embryo, gv, primitive 

 blood forming cells ; Ip, cytoplasmic lamina persisting in the lumen of the blood 

 vessel that has formed ; mar, blood vessels ; p, vessel wall ; p.ac., point of enlarge- 

 ment of network; vac, vacuoles. (From Vialleton.) 



Professor Riickert has worked out the arrangement of the blood 

 vessels (Fig. 284). He says the vessels in the area pellucida are formed 

 by a rearrangement of small groups of cells in the splanchnic mesoderm 

 of the area. First, short sections of tubular vessels are formed, which 

 then connect with the more peripheral vessels of the opaque area, thus 

 forming a continuous vascular network extending toward the embryo 

 and finally reaching it at about the time, six pairs of somites are formed. 



Soon after this, vessels appear in the embryo itself, the first being 

 the paired dorsal aortae in the body region. These are regarded as 

 merely straightened axial margins of the vascular network of the area 

 pellucida. They diverge widely posteriorly, passing as the vitelline 

 arteries into the general vascular network. Anteriorly they are pro- 

 longed forward to the heart region where they connect with a pair of 

 vessels differentiated in the mesenchyme of the head. 



It will be remembered that the coelom is really paired, and extends 

 downward ventrally on both sides until at a later period it meets in the 



