88 



GENEliAL Pl'JNCJI'LES OF ZOOLOGY. 



Formation of Bone. — I'he stratification of ?jone is caused by its 

 mode of origin. Whore the bone borders upon the Haversian 

 canals, the marro\y-cavity, and the periosteum, there is transiently 

 or piermanently an epithelial-like layer of cells, odeohlads, 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, whicli 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, Ijecause 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 lymijh. 

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

 the organized constituents. The fluid or hlood-plasma is, aj^art 

 from inorganic constituents, sjiecially rich in 23roteids; after tlie 

 removal of the blood from the blood-vessels a jjart of these sej^arate 

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

 a fluid i^oor in proteids, the blood-serum. The organized con- 

 ^ stituents, the hlood-ccUs, are disiin- 



guished as red and white blood-cor- 

 puscles. The latter, the Icucoci/tes, 

 are present in smaller numbers and 

 have great similarity to the amtebaj 

 found in water: they are particles 

 of protoplasm, contain a nucleus, 

 devour foreign lioilies (for example, 

 I'arniine granules injected into the 

 blood), and move in the ' anueboid " 

 manner l)v puttino- out pseudopodia 

 (fig, 4.-S). " 



Red Blood-corpuscles. — In the 

 mature conditidu the red blood- 

 corjniscles of vertebrates (fig. 4(1) 

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

 pressure) may tem2>orarily be bent, incised, or otherwise modified 

 in form, l.iut cannot actively change their shape, because they no 



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Frr; 45 - Wliite hlood corpuscU'w. <i 

 oC man; /.(, uf the crab (ii, thu iiu 



