DEVELOPMENT OF VASCULAR SYSTEM. 599 



sels, masses of embryonic cells similar to those from which the 

 heart and other structures of the embryo are developed, ar- 

 range themselves in the position, form, and thickness of the 

 developing vessel. Shortly afterwards the cells in the interior 

 of a column of this kind seem to be developed into blood-cor- 

 puscles, while the external layer of cells is converted 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, passing between the main artery 

 and vein, and are produced by the junction of prolongations, 

 sent from both the artery and vein, with certain elongated or 

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

 arches are formed and are permeable to blood, new prolonga- 

 tions pass from them, join other radiated cells, and thus form 

 secondary arches. In this manner, the capillary network ex- 

 tends 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 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 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 per- 

 meable to blood, it enlarges, and a cavity then forms in its in- 

 terior (see Fig. 232). With Kolliker's account, our own ob- 

 servations, 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 bloodvessels from 

 elongating and branching cells. 



About the time that the heart at its lowest extremity re- 

 ceives the venous trunks, and at its upper extremity gives off 

 the large arterial trunk, it becomes curved from a straight into 

 a horseshoe form, and shortly divides into three cavities (Fig. 

 233). Of these three cavities, which are developed in all 

 Vertebrata, the most posterior is the simple auricle; themidde 

 one the simple ventricle ; and the most anterior the bulbus 

 arteriosus. These three parts of the heart contract in succes- 

 sion. The auricle and the bulbus arteriosus at this period lie 

 at the extremities of the horseshoe. The bulging out of the 

 middle portion inferiorly gives the first indication of the future 



