DEVELOPMENT OF BLOOD-VESSELS. 



373 



original nuclei of those cells, imbedded in it here and there. Subsequently the 

 protoplasm becomes differentiated around the nuclei into the flattened cells which 



<t * 



Fig. 431. PART OF THE NETWORK OF DEVELOPING BLOOD-VESSELS IN THE VASCULAR AREA OF THE 



GUINEA-PIO. (E. A. S.) 



U, blood-corpuscles becoming free in an enlarged and hollowed-out part of the network. The smaller 

 figure on the left represents a of the larger figure, more highly magnified, showing the vacuolation 

 of the cell ; d, a nucleus within it undergoing division. 



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, 



Fig. 432. CELLS FROM MIDDLE LATER OF CHICK'S 



BLASTODERM UNDERGOING DEVELOPMENT INTO 

 BLOOD-VESSKLS. MAGNIFIED. (Klein. ) 



a, cavity of cell ; b, wall of cell ; /, /, cells not 

 yet holJowed out ; d, blood-corpuscles. 



from other cells which apply themselves 

 to the exterior of the previously simple 

 endothelial tubes and produce the plain 

 muscular and other tissues of which those 

 coats consist. 



Within the body of the embryo, vessels 

 are formed in like manner from cells 

 within the connective tissue, especially in 

 rapidly growing vascular organs like 



the liver. One of the most favourable objects for the study of the develop- 

 ment 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 may observe that many of the connective tissue corpuscles are much 

 vacuolated, and that the protoplasm of some of them has a decided reddish tinge 

 (fig. 433, h}. In others the red matter has become condensed in the form of 

 globules within the cells (h', h", &c.), varying in size from minute specks to sphe- 

 roids 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." The 

 cells become elongated and pointed at their ends, sending out processes also to unite 

 with neighbouring cells. At the same time the vacuoles in their interior become 

 enlarged, and coalesce to form a cavity within the cell (fig. 434, a), in which the 



