GENERATION AND DEVELOPMENT. 



277 



forms a communication with the great vessels in contact with it, and the 

 cells of which its walls are composed are transformed into fibrous and 

 muscular tissues, and into epithelium. In the developing chick it can he 

 observed with the naked eye as a minute red pulsating point before the 

 end of the second day of incubation. 



Blood-vessels. Blood-vessels appear to be developed in two ways, 

 according to the size of the vessels. In the formation of large blood- 

 vessels, masses of embryonic cells similar to those from which the heart 



FIG. 433. Capillary blood-vessels of the tail of a young larval frog, a, capillaries permeable to 

 blood; b, fat-granules attached to the walls of the vessels, and concealing the nuclei; c, hollow pro- 

 longation of a capillary, ending in a point; d, a branching cell with nucleus and fat-granules; it com- 

 municates by three branches with prolongation of capillaries already formed; e, e, blood corpuscles 

 still containing granules of fat. x 350 times. (Kolliker.) 



and other structures of the embryo are developed, arrange themselves in 

 the position, form, and thickness of the developing vessel. Shortly after- 

 ward the cells in the interior of a column of this kind seem to be devel- 

 oped into blood-corpuscles, while the external layer of cells is converted 

 into the walls of the vessel. 



Capillaries. In the development of capillaries another plan is pur- 

 sued. This has been well illustrated by Kolliker, as observed in the tails 

 of tadpoles. The first lateral vessels of the tail have the form of simple 



