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HANDBOOK OF PHYSIOLOGY. 



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

 and the cells of which its walls are comprised are transformed into fibrous 

 and muscular tissues, and into epithelium. In the developing chick 



Fig. 480. 



Fig. 491. 



Fig. 489. Capillary blood-vessels of the tail of a young larval frog, a, capillaries perme- 

 able to blood ; 6, fat granules attached to the walls of the vessels, and concealing the nuclei ; c, 

 hollow prolongation of a capillary, ending in a point ; d, a branching cell with nucleus and fat- 

 granules; it communicates by three branches with prolongation of capillaries already formed ; 

 e, e, blood corpuscles still containing granules of fat. x 350 times. (Kolliker. ) 



Fig. 490. Development of capillaries in the regenerating tail of a tadpole, abed, sprouts 

 and cords of protoplasm. (Arnold.) 



Fig. 491. The same region after the lapse of 24 hours. The "sprouts and cords of proto- 

 plasm " have become channelled out into capillaries. (Arnold.) 



it can be observed with the naked eye as a minute red pulsating little 

 mass before the end of the second day of incubation. 



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

 cording to their size. In the formation of large blood-vessels, masses of 

 embryonic cells similar to those from which the heart and other struct- 

 ures of the embryo are developed, arrange themselves in the position, 

 form, and thickness of the developing vessel. Shortly afterward the cells 

 in the interior of a column of this kind seem to be developed into blood- 



