DEVELOPMENT OF CAPILLARIES 105 



Vnsa vasorum are small blood-vessels which serve to nourish the 

 outer layers of the large arteries and veins. 



^PRACTICAL DEMONSTRATION 



Sections of heart showing pericardium and endocardium, of aorta, of another 

 large artery, and a large vein, should be studied and drawn. The small blood- 

 vessels will be encountered in the various organs. 



The DEVELOPMENT of capillaries is important because the larger 

 vessels first appear in the embryo as capillaries, around whose 

 endothelium the other coats later become differentiated from the 

 neighboring mesoderm. Areas of mesoderrnic cells branch, unite 

 with one another, and become hollowed out to form a system of 

 channels. Part of the protoplasm and nuclei form nucleated red 

 blood -corpuscles; part of them lie outside and constitute the wall 

 of endothelial cells and their nuclei. Later in embryonic life and 

 after birth the formation of capillaries is carried on in much the 

 same way, and becomes of great importance in many pathological 

 processes where new capillaries are required; for instance, in the 

 healing of a wound. In these cases solid protoplasmic outgrowths 

 are protruded from the endothelial cells of existing capillaries. 

 These outgrowths lengthen by multiplication of the endothelial cells 

 or by fusion with connective tissue cells. They branch and also 

 unite with other similar outgrowths to form a network. Vacuoles 

 appear in the middle of the processes, which enlarge, become con- 

 fluent, and make channels through them, which open into the 

 original capillaries. The protoplasm and nuclei of the solid sprouts 

 form the endothelium of the new capillaries. 



The newly forming capillaries may be studied in the thin tail of the young 

 frog-tadpole. Select a tail with as little pigment as possible; harden in 

 Flemming's chromic -acetic solution; wash thoroughly; stain with haematoxylin 

 and eosin; alcohol; oil of cloves; balsam. Focus on a plane below the 

 epithelium of the skin. The capillaries are narrow, dark bands, with nuclei; 

 the large ones containing blood-corpuscles, the small ones solid and showing 

 branches. 



