182 



BLOOD-VESSELS. 



&c.), varying in size from minute specks to spheroids of the diameter 

 of a blood-corpuscle, or more. At some parts the tissue is completely 

 studded with these cells, each containing a number of such spheroids, 

 and forming, as it were, " nests " of blood-corpuscles or minute " blood- 

 islands." After a time the cells, previously rounded, become elongated 

 and pointed at their ends, sending out processes also to unite with 



neio-hbourino; cells. At the same 



Fig. 123. 



Fig. 123. — Capillary Blood-Vessels of 

 THE Tail of a very Young Frog Larva, 

 MAGNIFIED 350 DiAMETERS (after Kol- 

 liker). 



a, capillaries permeable to blood ; h, 

 granules, attached to the walls of the vessels 

 and concealing nuclei ; c, hollow prolongation 

 of a capillaiy, ending in a point ; d, a 

 branched cell, containing a nucleus and 

 granules, and communicating by three 

 branches with prolongations of caiiillaiies 

 already formed ; e, blood-corpuscles. 



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 forma- 



* A formation of blood-corpuscles and vessels from cartilage-cells has been described 

 by Heitzmann as occun-ing in ossifying cartilage. 



time the vacuoles in their interior 

 become enlarged, and coalesce to 

 form a cavity within the cell, in 

 which the reddish globules, which 

 are now becoming disk-shaped, 

 are found. Finally the cavity 

 extends through the cell processes 

 into those of neighbouring cells 

 and into those sent out from pre- 

 existing capillaries (c'). Young 

 capillaries do not exhibit the 

 well-known silver lines when 

 treated with that reagent, for the 

 diflFerentiation of the hollowed 

 cells and cell-processes into flat- 

 tened cellular elements is usually 

 a subsequent process. 



The mode of extension of the 

 vascular system in growing parts, 

 as well as in pathological new 

 formations, is quite similar to 

 that here described, except that 

 blood-corpuscles are not as a 

 rule previously developed within 

 the cells which are to form the 

 blood-vessels.* 



In the tail of batrachian larva3 the 

 process has long been studied and is 

 represented in the adjoining figure 

 (fig. 123) by KoUiker, in which the 

 processes of a stellate cell are seen to 

 meet and join with similar 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 



