88 



GENERAL PRINCIPLES OF ZOOLOGY. 



Formation of Bone. The stratification of bone is caused by its 

 mode of origin. Where the bone borders upon the Haversian 

 canals, the marrow-cavity, and the periosteum, there is transiently 

 or permanently an epithelial-like layer of cells, osteoblasts, which 

 secrete the bone-substance on their surface. Certain cells in the 

 matrix participate in this secretion, and here give rise to the 

 bone-corpuscles, which are distinguished from the cartilage -cells 

 by their numerous processes ramifying through the matrix. The 

 processes of a bone-corpuscle branch, and unite with the neighbor- 

 ing cells through fusion of the processes, an arrangement most 

 beautifully seen in dried bone, because here the cavities and the 

 canals of the matrix are filled with air. Special modification of 

 bony tissue, the substance of fish-scales and of the teeth, called 

 also ivory or dentine, should be mentioned. 



Blood and Lymph, here treated in connexion with the connec- 

 tive substances, are in reality not tissues at all, but nutritive 

 fluids. Two kinds of nutritive fluids occur in the vertebrates, red 

 blood and the colorless, weakly opalescent, or cloudy white lymph. 

 The blood of man and other vertebrates, consists of a fluid and 

 the organized constituents. The fluid or blood-plasma is, apart 

 from inorganic constituents, specially rich in proteids; after the 

 removal of the blood from the blood-vessels a part of these separate 

 by coagulation and form the blood-clot, made up of fibrin, leaving 

 a fluid poor in proteids, the blood-serum. The organized con- 

 stituents, the blood-cells, are distin- 

 guished as red and white blood-cor- 

 puscles. The latter, the leucocytes, 

 are present in smaller numbers and 

 have great similarity to the amoebae 

 found in water; they are particles 

 of protoplasm, contain a nucleus, 

 devour foreign bodies (for example, 

 carmine granules injected into the 

 blood), and move in the ' amoeboid ' 

 manner by putting out pseudopodia 

 (fig. 45). 



Red Blood-corpuscles. In the 

 mature condition > the red blood- 

 corpuscles of vertebrates (fig. 46) 

 are circular or oval discs, which by external influences (e.g., by 

 pressure) may temporarily be bent, incised, or otherwise modified 

 in form, but cannot actively change their shape, because they no 



FIG. 45 White blood-corpuscles, o, 

 of man; b, of the crab (n, the nu- 

 cleus). 



