GROWTH OF BLOOD-VESSELS IN FROG LARVAE 



55 



erty most important for the development of a system of anas- 

 tomosing vessels, namely, that blood-vascular endothelium has 

 an affinity for blood-vascular endothelium, of such a nature 

 that if two processes of blood-vascular endothelium draw near one 

 another in their growth, a union will be formed between them 

 ('cytotropism,' Roux). Equally important is the fact, readily ob- 

 servable in the tail of the frog larva, that blood-vessel endothelium 

 avoids, in its growth, the cells of other tissues among which it 

 grows, such as mesenchyme, and lymphatic endothelium. As a 

 rule, the lumen eventually extends through the entire extent of 

 this new sprout, it widens, and after a varying amount of time 



