DEVELOPMENT OF BLOOD-VESSELS. 



Fig. XCIX. 



pointed processes which shoot out from the sides of neighbouring capillary vessels, 

 and in this manner the new vessels are adopted into the existing system. The 

 junctions of the cells with each other 

 or with capillary vessels are, at first, of 

 great tenuity, and contrast strongly 

 with the central and wider parts of the 

 cells; they appear then to be solid, 

 but they afterwards become pervious 

 and gradually widen, blood begins to 

 pass through them, and the capillary 

 network acquires a tolerably uniform 

 calibre. The original vascular network 

 may become closer by the formation of 

 new vessels in its interstices, and this 

 is effected by similarly metamorphosed 

 cells, arising in the areolse and joining 

 at various points with the surrounding 

 vessels, and also simply by pointed 

 offshoots from the existing capillaries 

 stretching across the intervals and 

 meeting from opposite sides, so as when 

 enlarged to form new connecting arches. 

 From observations made on the foetal 

 membranes of sheep, Mr. Paget has 

 found that the mode of formation of 

 capillaries described by Kolliker in 

 batrachians, holds good also in mam- 

 miferous animals.* The simple homo- 

 geneous coat of the capillaries is thus 

 formed out of the walls of the coa- 

 lescing cells; the lining epithelium 

 must be a subsequent formation. Whilst 

 the finest capillaries retain this simple 

 structure, those that are larger acquire 

 the additional coats already described ; 

 and arteries and veins, as already stated, 

 especially the smaller ones, appear to 

 be formed in the same manner ; indeed, 

 it would seem not unreasonable to 

 presume, that the several gradations of 

 structure seen as permanent conditions Fi S- XCIX - CAPILLARY BLOOD-VESSELS OP 



THE TAIL OP A VERY YOUNG FROG LARVA. 



MAGNIFIED 350 DIAMETERS (after Kol- 

 liker). 



in vessels of successively larger calibre, 

 may represent the successive steps by 

 which a vessel, having originally the 

 small size and the simple membrane of 

 a fine capillary, increases in width and 

 acquires the complex tunics of a vein or 

 artery. Further observations, however, 

 are required on this point. Kolliker 

 states, that many vessels which eventu- 

 ally attain a medium size, are originally 

 derived from round cells, which unite in 

 single or double rows and form the 

 primitive simple membranous tube of such vessels, by coalescence of their cavities and 

 walls. 



The blood-vessels may be said to increase in size and capacity in proportion to the 

 demands made on their service. Thus, as the uterus enlarges in pregnancy, its 

 vessels become enlarged, and when the main artery of a limb is tied, or otherwise 

 permanently obstructed, collateral branches, originally small and insignificant, 



* Supplement to Miiller's Physiology, by Baly and Kirkes, 1848, p. 104. 



a, capillaries permeable to blood; &, 

 granules, attached to the walls of the vessels 

 and concealing nuclei ; c, hollow prolongation 

 of a capillary, ending in a point; d t a 

 branched cell, containing a nucleus and 

 communicating by three 

 prolongations of capillaries 

 e, blood-corpuscles. 



granules, and 

 branches with 

 already formed 



