372 



BLOOD-VESSELS. 



somewhat glandular appearance, but composed almost entirely of a plexus of minute arteries, 

 which are derived in the one case directly from the carotid, in the other from the middle 

 sacral. The plexiform vessels are invested by one or more layers of granular polygonal cells, 

 apparently like those found in the interstitial tissue of some other organs (testis, ovary, 



Fig. 430. A SMALL ARTERY A, WITH A CORRESPONDING VEIN B, TREATED WITH ACETIC ACID, AND 



MAGNIFIED 350 DIAMETERS (after Kollikev). 



a, external coat with elongated nuclei ; )8, nuclei of the transverse muscular tissue of the middle 

 coat (when seen endwise, as at the sides of the vessel, their outline is circular) ; 7, nuclei of the 

 epithelium-cells ; 5, elastic layers of the inner coat. 



thyroid, suprarenal bodies). The whole is invested by connective tissue, which also penetrates 

 between the vessels of the so-called gland, and itself contains numerous granular cells. The 

 true nature and function of these peculiar structures is entirely unknown, but they are 

 probably of embryological significance. 



DEVELOPMENT OP BLOOD-VESSELS. 



The first vessels which appear in the ovuin are formed in the mesoblast, and the 

 process subsequently goes on in the same layer and in its derivatives in all parts of 

 the animal body. New vessels, also, are formed in the healing of wounds, in the 

 restoration of lost parts, and in the production of adventitious growths. The 

 process is in every case essentially the same. 



The first vessels of the embryo, both of the chick and mammal, are formed in 

 the vascular area, and originate from some of the cells of the mesoblast in that 

 situation (fig. 431). Vacuoles are formed within the cells, and as they increase in 

 size run together, and a cavity filled with fluid is in this way produced in the 

 interior of the cell. The nucleus of the cell has meanwhile become multiplied, 

 while blood-corpuscles are formed within the cell-cavity in the manner already 

 described in connection with the blood (p. 217). The cells, whilst these changes are 

 going on, increase largely in size, especially in the chick, where they form vesicles 

 (fig. 432), visible to the naked eye as minute reddish specks, which have been 

 known since the time of Pander as " blood-islands." The cells are united to one 

 another by their processes, and after a time the cavities become extended into the 

 cell-processes, so that a network of vessels is by this means produced. 



The wall of these primary vessels is therefore composed at first merely of the 

 protoplasm of the original embryonic cells with nuclei, derived by division from the 



