DEVELOPMENT OF VASCULAR SYSTEM. 765 



sued. This has been well illustrated by Kolliker, as ob- 

 served in the tails of tadpoles. The first lateral vessels of 

 the tail have the form of simple arches, passing between 

 the main artery and vein, and are produced by the junc- 

 tion of prolongations, sent from both the artery and vein, 

 with certain elongated or star-shaped cells, in the sub- 

 stance 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 net-work extends in propor- 

 tion as the tail increases in length and breadth, and it, at 

 the same time, becomes more dense by the formation, ac- 

 cording to the same plan, of fresh vessels within its meshes. 

 The prolongations by which the vessels communicate with 

 the star-shaped cells consist at first of narrow-pointed 

 projections from the side of the vessels, which gradually 

 elongate until they come in contact with the radiated pro- 

 cesses of the cells. The thickness of such a prolongation 

 often does not exceed that of a fibril of fibrous tissue, and 

 at first it is perfectly solid; but, by degrees, especially 

 after its junction with a cell, or with another prolongation, 

 or with a vessel already permeable to blood, it enlarges, 

 and a cavity then forms in its interior (see fig. 232). With 

 Kolliker' s account, our own observations, made on the fine 

 gelatinous tissue conveying the umbilical vessels of a sheep's 

 embryo to the uterine cotyledons, completely accord. This 

 tissue is well calculated to illustrate the various steps in the 

 development of blood-vessels from elongating and branch- 

 ing cells. 



About the time that the heart at its lowest extremity 

 receives the venous trunks, and at its upper extremity 

 gives off the large arterial trunk, it becomes curved from a 

 straight into a horse-shoe form, and shortly divides into 

 three cavities (fig. 233). Of these three cavities, which are 

 developed in all Vertebrata, the most posterior is the sim- 

 ple auricle j the middle one the simple ventricle ; and the 



