DEVELOPMENT OF VASCULAR SYSTEM. 767 



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, 

 according to the same plan, of fresh vessels within its 

 meshes. The prolongations by which the vessels commu- 

 nicate 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 processes 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. 226). 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 illus- 

 trate the various steps in the development of blood-vessels 

 from elongating and branching 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. 227). Of these three cavities, which are 

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

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



