686 HANDBOOK OF PHYSIOLOGY. 



been formed in the cardiac walls. At first they seldom exceed from fif- 

 teen to eighteen in the minute. The fluid within the cavity of the heart 

 shortly assumes the characters of blood. At the same time the cavity 

 itself 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 be 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, ac- 

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

 masses of embryonic cells similar to those from which the heart and 

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

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

 wards the cells in the interior of a column of this kind seem to be de- 



FIG. 461. FIG. 462. 



FIG. 471. Development of capillaries in the regenerating tail of a tadpole, a, 6, c, d, sprouts 

 and cords of protoplasm. (Arnold.) 



FIG. 472. The same region after the lapse of 24 hours. The " sprouts and cords of protoplasm " 

 have become channelled out into capillaries. (Arnold.) 



veloped into blood-corpuscles, while the external layer of cells is con- 

 verted into the walls of the vessel. 



In the development of capillaries another plan is pursued. 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 arches, pass- 

 ing between the main artery and vein, and are produced by the junction 

 of prolongations, sent from both the artery and vein, with certain elon- 

 gated or star-shaped cells, in the substance of the tail. When these 

 arches are formed and are permeable to blood, new prolongations pass 

 from them, join other radiated cells, and thus form secondary arches. 

 In this manner, the capillary network extends in proportion as the tail 

 increases in length and breadth, and it, at the same time, becomes more 

 dense by the formation, according to the same plan, of fresh vessels 



