DEVELOPMENT. 



687 



within its meshes. The prolongations by which the vessels communi- 

 cate 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 Figs. 470, 472). This tissue is well calculated to illus- 

 trate the various steps in the development of blood-vessels from elon- 

 gating 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 joining 

 of their processes, the multiplication of their nuclei, and the vacuolation 

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



FIG. 473. Capillaries from the vitreous humor of a foetal calf. Two vessels are seen connected 

 by a ' cord " of protoplasm, and clothed with an adventitia, containing numerous nuclei; a, inser- 

 tion of this " cord " into the primary walls of the vessels. (Frey.) 



tions 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 

 united by fine protoplasmic processes: by the extension of the vacuoles 

 into them, capillary tubes are gradually formed. 



Morphology Heart. When it first appears, the heart is approximately 

 tubular in form, being at first a double tube, then a single one. It 

 receives at its two posterior angles the two omphalo-mesenteric or vitel- 

 line veins, and gives off anteriorly the primitive aorta (Fig. 474). The 

 junction of the two veins which pass into the auricle becomes removed 

 farther and farther away from the heart, and the vessel thus formed is 

 called sinus venosusnear to the auricle, and ductus venosus farther away, 

 or if it be called by one name, that of meatus venosus may be used. 



