148 Vertebrate Embryology 



indefinite network of blood vessels lies all in 

 one plane, and the meshes are more or less 

 filled with mesoblast cells. 



The blood islands, which appear first in the 

 area opaca but soon are found also in the area 

 pellucida, are spots where there are collections 

 of cells attached to the walls of the blood ves- 

 sels. The development of haemoglobin in 

 these cells gives the reddish color that makes 

 the blood islands so conspicuous in surface 

 views of fresh specimens. The development 

 of the blood islands is more marked around 

 the caudal end of the embryo. 



Soon after the blood vessels have become 

 hollow, the blood islands, which in cross sec- 

 tion appear as local thickenings usually of the 

 dorsal walls of the vessels, bud off cells into 

 the cavity of the blood vessels ; these cells 

 form the first blood corpuscles. It is from 

 these primitive corpuscles, according to a 

 commonly accepted hypothesis, that all of the 

 colored corpuscles of the body are descended. 

 They are at first characterized by the posses- 

 sion of a rounded nucleus, with a distinct nucle- 

 olus and a very small amount of protoplasm. 

 After being set free in the lumen of the blood 

 vessel the protoplasm of each cell increases, 



