THE BLOOD-CORPUSCLES. 93 



The Growth of the Vessels into the Embryo. The entrance of the vessels into 

 the embryo chick begins toward the end of the second day. The buds which form 

 the extra-embryonic angioblast grow first toward, then into, the embryo. The pene- 

 trating vessels follow certain prescribed paths. Part of the vessels run along the 

 posterior edge of the amnio-cardiac vesicles, and enter into connection with the pos- 

 terior end of the heart, which has meanwhile been progressing, and which owing 

 to the early separation of the head end of the embryo from the yolk is the only 

 part of the heart which the vessels can reach directly. While the vessels are ap- 

 proaching the heart their differentiation into various sizes is going on, the smallest 

 ones to remain as capillaries, the larger ones to become arteries or veins. The 

 only two veins in the first stage are those above mentioned, which are called the 

 omphalo-mesaraic. Another set of vessels penetrates along the splanchnopleure of 

 the body on each side until they attain the small space between the notochord and 

 somite and the entoderm, where they fuse so as to form a longitudinal vessel, the 

 anlage of the descending aorta (Fig. 143, Ao.D). It should be noted that this 

 anlage is primitively double. The aorta appears first in the region toward the 

 head. It grows forward above the pharynx, bends ventrally just behind the mouth, 

 dividing as it bends, one branch going around each side of the future pharynx, and 

 uniting again on the ventral side of the pharynx in the mediari ventral line, in 

 order to join the anterior end of the tubular heart. The heart begins to beat be- 

 fore the vessels unite with it. The first blood-cells have already been formed; 

 hence as soon as union is accomplished the blood circulation starts up, the blood 

 passing through the aorta to the body, thence by numerous lateral branches to the 

 area vasculosa, and returning by the two omphalo-mesaraic veins to the heart. It 

 will thus be seen that almost the entire circulation is extra-embryonic. 



The other embryonic blood-vessels are developed by buds from the walls of 

 the vessels already present in the embryo, in the same general manner as new ves- 

 sels are formed in the area vasculosa. These buds give rise to the endothelium 

 only of the embryonic vessels. When a vessel becomes an artery or a vein, the 

 media and adventitia are added, as above stated, by differentiation of the 

 surrounding mesenchyma. 



During further development many small blood-vessels abort, and often appear 

 as disconnected bits, closed at both ends and containing corpuscles. Such struc- 

 tures were at one time supposed to be developing blood-vessels and were accord- 

 ingly termed " vaso-f ormative cells." The blood-corpuscles in them are of course 

 not developing, but degenerating . 



The Blood-corpuscles. 



The first blood-corpuscles are free cells of an indifferent character and capable 

 of wandering through the walls of the blood-vessels, which in early stages are easily 

 permeable, as if of a merely gelatinous consistency. The primitive blood-cells, as 

 they may be called, give rise not only to the permanent blood-corpuscles, both red 



