22 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



into the typical flattened endothelial cells of the vascular intima. 

 Excepted from this endothelial transformation are only those 

 cells of the vascular walls which differentiate as young blood cells. 

 Thus eventually the confluence of the originally individual and 

 separate spaces produces a continuous and connected channel 

 system, lined by endothelium, which nearly encircles the blood 

 islands. The latter are therefore now in large part included within 

 the lumen of the capillaries, with whose walls they are from place 

 to place continuous. It only requires a further and complete 

 solution of this continuity, and the accompanying freeing of the 

 blood cells, to add the latter to the plasma circulating in the 

 preformed channels. In the chick, according to Thoma's obser- 

 vations, the resolution of the blood islands into separate blood 

 cells occurs between the 45th and 55th hour of incubation, while 

 the acquisition of haemoglobin by the cells occurs between the 

 40th and 45th hour. With this occurrence the development of 

 the primary blood-vascular channels has reached its definite 

 accomplishment. 



The general picture presented by the earliest development of 

 the blood-vascular system may therefore be summarized as follows: 



(1) Differentiation of certain mesodermal areas and lines by 

 the multiplication of mesodermal cells to form cell-strands of 

 varying density and size (vascular strands). 



This appears to be a common antecedent condition not only 

 of all vascular mesodermal structures, but also of other meso- 

 dermal derivatives eventually destined to obtain a lumen and 

 enter into the formation of canals, as the Wolffian tubules and 

 the cell-strands of the gonad. 



It possibly explains the conditions described, for example, by 

 Sala in the development of the avian thoracic duct, is seen 

 in the developing aortae of early chick embryos, and is especially 

 significant in the pictures furnished by Sabotta of the develop- 

 ment of the aortse in fishes. 



(2) Development in the interior of these cellular strands of 

 intercellular spaces in large numbers. 



(3) Bio-mechanical modification of cells lining these spaces 

 to produce typical flattened vascular endothelium. 



