DEVELOPMENT OF BLOOD-VESSELS. 



181 



nuclei, derived by division from the original nuclei of those cells, 

 imbedded in it here and there. Subsequently the protoplasm spreads 

 out and becomes differentiated around the nuclei into the flattened cells 

 which compose the wall of the capillaries, and which form the lining 

 membrane of the arteries and veins. The remaining coats of the larger 

 vessels are developed later from cells which come to apply themselves to 

 the exterior of the previously simple tubes and produce the plain 

 muscular and other tissues of which those coats consist. 



The process of formation of the blood-vessels has not been so com- 

 pletely traced within the hodji of the embryo chick, but there is no 

 doubt that it is in all respects similar. In mammalia they are pro- 

 duced in like manner from vacuolated cells belonging to the con- 

 nective tissue. The most favourable object for the study of the 

 development of the blood-vessels and their contained blood-corpuscles 

 is afforded by the subcutaneous tissue of the new-born rat, especially 

 those parts in which fat is being deposited. 



Here we observe that many of the connective tissue corpuscles 

 are excessively vacuolated (fig. 122, v), and the protoplasm of some 



Fig. 122. 



Fig. 122. — Development op Blood-vessels and BLOOD-CoRPtrscLES in the Connective 

 Tissue. From the Subcutaneous Tissue of the new-born Rat, examined in 

 Salt-solution (^ per cent.), and magnified about 350 Diameters. 



n, n', n", n'", cells containing blood -corpuscles iu various stages of development ; in 

 n"", the reddish matter is mainly in two large roundish ill-defined patches ; in n, the 

 lueinoglobin globules are of nearly equal size, and fill the cell ; a little above this cell 

 another is seen with three such globules ; the nucleus is also apparent ; 11', and n", 

 exhibit in addition a number of vacuoles ; v, vacuolated cell without blood corpuscles, 

 applied to another cell the vacuoles of which have united to form a cavity in which two 

 fully-developed corpuscles are observed ; this cell is joined to the end of the developing 

 capillary vessel c' ; c, portion of a fully-developed capillary ; /, /', cells in which fat is 

 being deposited ; another is seen to the right of the capillary, between it and fbr, fibril- 

 lated cell of the connective tissue ; (/, granular connective tissue corpuscle ; I, leucocyte. 



01 tiiera presents a decided reddish tinge. In others the red matter 

 has become condensed in the form of globules within the cells {n, n\ 



