THE ORIGIN OF THE BLOOD-VESSELS AND BLOOD. 91 



veins differ but little, if at all, in histological structure during early embryonic 

 stages, and are distinguished chiefly by the direction of blood-currents passing 

 through them. Capillary blood-vessels and sinusoids have, as a rule, throughout 

 life merely an endothelial wall. Arteries and veins become strengthened by the 

 development of special coats around the endothelium which arise by transforma- 

 tions of the mesenchymal cells in the immediate neighborhood of the vessels. 



The Development in the Chick. — The first indication of the blood-vessels is a 

 reticulate appearance, which can be recognized in the mesoderm in surface views 

 of the fresh or hardened embryo at the end of the first day. The reticulate 

 structure increases rapidly in extent and distinctness during the second day of 

 incubation. It is confined to the region of the mesoderm surrounding the em- 

 bryo proper, and which is, therefore, known as the area vasculosa, as above 

 stated. As soon as there are several primitive segments in the embryo, the net- 

 work in the mesoderm shows traces of coloration in irregularly shaped reddish- 

 yellow spots, which are largest and most numerous around the caudal end of the 

 embryo. These spots are called blood-islands because the cells in them are 

 transformed into the first blood-corpuscles. The network appearance is due to 

 the development of the angioblast, which is a set of cells delaminated from the 

 N^J entoderm or the yolk, and intervening between the mesoderm proper and the 

 entoderm. The angioblast at first assumes the form of more or less solid cords. 

 The meshes of the angioblast are partly or wholly filled by mesodermic cells. 

 The ccelom now appears in the extra-embryonic area, and thereafter the anlages 

 of the blood-vessels are connected with the splanchnic mesoderm only. The 

 anlages of the blood-vessel at this stage form a thick network without distinction 

 of stem or branch, except that the edge of the area, bounded by a broad band of 

 angioblast, gives rise to a single large vessel, which is known as the sinus ter- 

 minalis. The anlages are all in one layer, none overlying the others, and up to 

 this stage they are all solid. The terminal sinus becomes connected with the 

 venous system. 



The blood-islands are spots where there is a cluster of cells, which remain 

 attached to one another and to the walls of the vessels. The cells develop hemo- 

 globin in their interior, hence the clusters have a reddish color which renders the 

 islands very conspicuous in surface views of fresh specimens. Blood-islands 

 appear first in the area opaca, but almost immediately after in the pellucida also. 

 They have at first a rounded or branching form. In the inner part of the latter 

 they are small and stand alone. Toward the periphery they are larger, closer 

 set, and more united with one another. Their development is greater around 

 the caudal end of the embryo. 



In the next stage the vascular anlages become hollow, and then may be 



