THE ORIGIN OF THE BLOOD-VESSELS AND BLOOD. 



91 



It is confined to the region of the mesoderm surrounding the embryo proper, and 

 which is, therefore, known as the area vasculosa, as above stated (compare Fig. 131). 

 As soon as there are several primitive segments in the embryo, the network 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 (Fig. 131, Bl.is) because the central 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 en- 

 toderm 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 



FIG. 50. HUMAN EMBRYO OF 9.4 MM. PROBABLE AGE THIRTY DAYS. X 8 diams. 



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, ex- 

 cept 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 terminalis. 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 at- 

 tached 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 all 

 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 



