THE BLOOD-CORPUSCLES. 93 



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 penetrating vessels follow certain prescribed paths. Part of the vessels run 

 along the posterior edge of the amniocardiac vesicles, and enter into connection 

 with the posterior end of the heart, which has meanwhile been developing, 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 approaching 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 ompnafo-mesaraic. Another set of vessels penetrates along the 

 sphlanchnopleure of the body on each side until they attain the small space 

 between the notochord and myotome and the entoderm, where they fuse so as 

 to form a longitudinal vessel, the anlage of the descending aorta. 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 ven- 

 trally 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 pha- 

 rynx in the median ventral line, in order to join the anterior end of the tubular 

 heart. The heart begins to beat before 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 

 vessels are formed in the area vasculosa. These buds give rise to the endothe- 

 lium 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. 



The " vasof ormative cells" of Ranvier are probably degenerating blood- 

 vessels and not the anlages of vessels, as Ranvier assumed. 



The Blood=corpuscles. 



The red blood-cells are the only elements contained in the blood during the 

 earliest stages of the vertebrate embryo. When the circulation begins, the num- 

 ber of corpuscles is small, but rapidly increases by division of the corpuscles. 

 The cells in amniota are at first round; in the chick they are from 8.3 to 12.5 ,. 



