DEVELOPMENT. 797 



corpuscles, 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, 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 forma- 

 tion, 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 permeable to blood, it 

 enlarges, aCnd a cavity then forms in its interior (see figs. 491, 492). 

 This tissue is well calculated to illustrate the various steps in the devel- 

 opment of blood-vessels from elongating and branching cells. 



In many cases a whole network of capillaries is developed from a net- 

 work of branched, embryonic connective-tissue corpuscles by the join- 



Fig. 492. Capillaries from the vitreous humor of a total calf. Two vessels are seen con- 

 ited by a "cord" of protoplasm, and clothed with an adventitia, containing numerous nuclei, 

 a, insertion of this "cord " into the primary walls of the vessels. (Frey.) 



ing of their processes, the multiplication of their nuclei, and the vacuo- 

 lation of the cell-substance. The vacuoles gradually coalesce till all the 

 partitions are broken down, and the originally solid protoplasmic cell- 

 substance is, so to speak, tunnelled out into a number of tubes. 



Capillaries may also be developed from cells which are originally 

 spheroidal, vacuoles form in the interior of the cells gradually becoming 



