'/'.'uUlffilllll'lll, 



$ 



IP 



Embryonic Development; Differentiation of Tissues - 289 



ilipias 





■tr 1 





Fig. 15-17. Compact bone: cross section, with detail of an osteoblast. 



tributed throughout the body; the yellow 

 type of connective tissue is encountered 

 mainly in the ligaments of the skeletal 

 joints of the body. 



2. Cartilage (Fig. 15-16). The matrix of 

 cartilage is composed mainly of a protein 

 substance, chondrin. The chondrin appears 

 as a homogeneous mass between the cells, or 

 chondroblasts, that secrete it. Cartilage has 

 a flexible, resilient consistency. In lower ver- 

 tebrates cartilage forms the entire skeleton, 

 but in higher vertebrates, cartilage supple- 

 ments bone; that is, cartilage is used as a 

 padding between the bones, and as the skele- 

 ton of parts of the body, such as the nose 

 and ears, where flexibility is a desirable char- 

 acteristic. All kinds of cartilage display fibers, 

 like those of fibrillar connective tissue, em- 

 bedded in the chondrin matrix. 



3. Bone (Fig. 15-17). Bone is the character- 

 istic skeletal material of most vertebrate ani- 

 mals. The matrix of bone is hard and rigid, 

 being composed largely of phosphate and 

 carbonate compounds of calcium. The bone 

 cells, or osteoblasts, which deposit the bony 

 matrix, are small cells with numerous fine 

 cytoplasmic branches (Fig. 15-17). Usually 

 the osteoblasts are arranged in concentric 



Fig. 15-18. Red and white blood corpuscles of man, 

 all drawn to the same scale. A, red blood corpuscle, flat 

 and edgewise; B, lymphocyte; C, mononuclear leucocyte; 

 D, polymorphonuclear leucocyte (granulocyte). 



circles around tubular channels, called Ha- 

 versian canals. These canals penetrate 

 through the bone, allowing for the passage 

 of nerves and blood vessels (Fig. 15-17). 



4. 8/ood and Lymph. Blood (Fig. 15-18) has 

 a fluid matrix, called the plasma. This com- 

 plex aqueous solution (p. 321) serves as the 

 chief medium of transportation throughout 

 the body. In the plasma, the several differ- 

 ent kinds of blood cells are suspended (see 

 p. 321). 



The blood cells of vertebrates are of three 

 types. (1) The erythrocytes, or red corpuscles, 

 contain large quantities of hemoglobin, the 

 red pigment that facilitates the transporta- 

 tion of oxygen. (2) The leucocytes, or white 

 corpuscles, are amoeboid cells, which in some 

 cases can ingest bacteria and other foreign 

 particles, thus counteracting infections, and 

 preventing the blood vessels from clogging. 

 (3) The blood platelets, or thrombocytes, are 

 fragile colorless corpuscles, which initiate a 

 clotting of the plasma — by disintegrating as 

 soon as blood is shed (see p. 323). 



Invertebrate bloods do not contain ery- 

 throcytes, although hemoglobin (or allied 

 respiratory pigments) may be present — dis- 

 solved directly in the plasma. The erythro- 



• m 



B 



