122 THE DEVELOPMENT OF THE CHICK 
mandibular arteries connecting 1 and 3; (3) dorsal aorte; (4) seg- 
mental branches of the dorsal aorte. The ventral aorta is, as 
we have seen, the anterior prolongation of the endocardium 
extending between the extreme anterior end of the heart proper 
and the oral plate. At the oral plate it divides into two branches, 
right and left mandibular arteries or arches, that surround the 
anterior end of the fore-gut, and arch over to be continued into 
the two dorsal aortee. The tissue in which these arches run is 
destined to form the mandibular arch or lower jaw. The two 
dorsal aorte are very large vessels running above the roof of the 
pharynx near its lateral angles. They give off no branches in 
the head. In the trunk they pass backwards in the splanchno- 
pleure beneath the somites (Fig. 68 B), and are connected at 
intervals with the extra-embryonic blood-vessels. These con- 
nections are more important in the region of the primitive streak 
(Fig. 63) where the dorsal aortz disappear in the general extra- 
embryonic network. Shght diverticula of the dorsal aorte 
ascend in the interspaces between successive somites (segmental 
arteries). 
Concerning the veins in the period under consideration there 
is nothing additional to be said. 
V. DESCRIPTION OF AN EMBRYO WITH 10 SOMITES 
It will now be in place to describe rather fully the anatomy 
of the stage at which we have arrived; this will serve as a point 
of departure for the next chapter. 
The blastoderm is a circular membrane covering a consider- 
able portion of the yolk (ef. Fig. 32 A). The embryo appears 
to the naked eye as a whitish streak in the central pear-shaped 
pellucid area. The surface views and the two views of the em- 
bryo viewed as a transparent object show the topography of the 
various parts of the embryo (Figs. 63-66). 
A section across the entire blastoderm at the stage of 10s, 
through the sixth somite (Fig. 68), shows the following parts: 
The ectoderm bounds the section above; it is thickened in the 
angle between the neural tube and the somites, and becomes 
thinner as it is traced peripherally; at the extreme periphery of 
the blastoderm it merges into a mass of cells that interpenetrate 
the yolk. Ventrally the boundary of the section is formed by 
the entoderm which is slightly arched upwards in the middle line. 
