122 THE DEVELOPMENT OF THE CHICK 



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 aortse. The tissue in which these arches run is 

 destined to form the mandibular arch or lower jaw. The two 

 dorsal aortse 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 aortse disappear in the general extra- 

 embr3'onic network. Slight diverticula of the dorsal aortse 

 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 (cf. 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 10 s, 

 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. 



