BULLFROG AS TYPICAL VERTEBRATE ANIMAL 79 



to the blood, while resistance in the carotid arteries decreases due 

 to their emptying into capillaries; so the last oxygenated blood from 

 the ventricle passes into the carotids and is conveyed to the head 

 region. 



Blood corpuscles, which are of three kinds, float in the plasma. 

 The erythrocytes are flattened and elliptical, with an oval nucleus 

 in the center. They contain a pigment, hemoglobin, which has the 

 property of absorbing oxygen. The colorless thrombocytes or 

 spindle cells are not as large as the erythrocytes but resemble them 

 except for their tapering ends. When these cells contact certain 

 foreign bodies, they break up, releasing a substance that causes, 

 upon contact with air, the coagulation of certain proteins in the 

 blood plasma in which blood corpuscles become entangled, forming 

 a clot. The insoluble protein strands thus formed are called fibrin 

 (see chapter on Physiology). After the frog has been injured, the 

 formation of a clot prevents indefinite bleeding and makes it pos- 

 sible for the tissues to begin repair. 



The white blood corpuscles or leucocytes are of three kinds : lym- 

 phocytes, monocytes, and granuloctjtes. Their outlines are irregular, 

 due to their amoeboid movement, and the shape of their nucleus 

 varies greatly. They are much less numerous in the blood stream 

 than are the red blood corpuscles and spindle cells. Leucocytes 

 may escape from blood capillaries and engulf bacteria and other 

 harmful substances in the tissues. They are finally returned to the 

 venous system by lymphatic vessels. Worn-out corpuscles are re- 

 moved from the blood stream chiefly by the spleen. The spleen 

 seems to be the primary organ concerned in supplying new leuco- 

 cjrtes, while the bone marrow furnishes most of the new erythrocytes. 

 Leucocytes may also increase by fission. 



Lymphatic System.— The lymphatic system of the bullfrog is an 

 open system comprised of a series of large irregular sinuses in vari- 

 ous parts of the body. It collects lymph from the tissues and 

 eventually returns it to the veins. The lymph is a colorless fluid 

 containing leucocytes but no erythrocytes. It is derived from seep- 

 age of plasma from the capillaries. It bathes all of the cells, col- 

 lects wastes, and distributes food products. In the region of the 

 intestinal tract, lymphatics absorb a considerable amount of fat 

 and are called lacteals. Lymph removes cellular debris and trans- 

 ports leucocytes which engulf harmful material and cleanse the tis- 

 sues of the body, 



