66 EARLY EMBRYOLOGY OF THE CHICK 



The Formation of Extra-embryonic Blood Vessels. In 



dealing with the circulation of the chick we must recognize 

 at the outset two distinct circulatory arcs of which the heart is 

 the common center. One complete circulatory arc is estab- 

 lished entirely within the body of the embryo. A second arc is 

 established which has a rich plexus of terminal vessels located 

 in the extra-embryonic membranes enveloping the yolk. These 

 are the vitelline vessels. The vitelline vessels communicate 

 with the heart over main vessels which traverse the embryonic 

 body. The chief distribution of the vitelline circulation is, 

 however, extra-embryonic. Later in development there arises 

 a third circulatory arc involving another set of extra-embryonic 

 vessels in the allantois, but with that we have no concern until 

 we take up later stages. Neither the intra-embryomc, nor the 

 vitelline circulatory channels have as yet been completed but 

 the heart and many of the main vessels have made their 

 appearance. 



The formation of extra-embryonic blood vessels is presaged 

 by the appearance of blood islands in the vascular area of 

 chicks toward the end of the first day of incubation (see Chapter 

 Vll). Figure 25 shows the differentiation of blood islands to 

 form primitive blood corpuscles and blood vessels. At their 

 first appearance the blood islands are irregular clusters of meso- 

 derm cells lying in intimate contact with the yolk-sac entoderm 

 (Fig. 25, A). When the lateral mesoderm becomes split 

 forming the somatic and splanchnic layers with the ccelom 

 between, the blood islands lie in the splanchnic mesoderm ad- 

 jacent to the entoderm. In embryos of 3 to 5 somites fluid 

 filled spaces begin to appear in the blood islands with the result 

 that in each blood island the peripheral cells are separated from 

 the central ones (Fig. 25, B). As the fluid accumulates and the 

 spaces expand the peripheral cells become flattened andjpushed 

 outward, but they remain adherent to each other and com- 

 pletely enclose the central cells. At this stage the single layer 

 of peripheral cells may be regarded as constituting the endo- 

 thelial wall of a primitive blood channel (Fig. 25, C). Exten- 

 sion and anastomosis of neighboring blood islands which have 

 undergone similar differentiation results in the establishment of 

 a network of communicating vessels. Meanwhile the cells 

 enclosed in the primitive blood channels have become separated 



