ZOOLOGY. 



still firmer intercellular substance ; and when the intercel- 

 lular substance becomes combined with salts of lime form- 

 ing bone, we have hony tissue. 



The blood-corpuscles origmate from the mesoderm as 

 independent cells floating in the circulating fluid, the blood- 

 cells being formed contemporaneously with the walls of the 

 vessels enclosing the blood. In the invertebrates the blood- 

 cells are either strikingly like the Ammba in appearance, or 

 ■mil im ^^® oya\, but still capable of 

 feiil^^l^^i^^l ^^ changing their form. Thus blood- 

 '™^'*™"' -^™'^^^^^^*™' corpuscles arise like other tissues. 

 Fig. 5 -strmied miiKciiiar flhriiia Qxceijt that the blood-cells are 



of a water beetle. — After Minot. i 



free. 



ifuscular tissue is also composed of cells, which are at 

 first nucleated and afterward lose their nuclei. From being 

 at first oval, the cells finally become elongated and unite 

 together to form the fibrillae ; these unite into bundles 

 forming muscular fibres, which in the vertebrates unite to 

 form muscles. Muscular fibrillse may be simple or striated 

 (Fig. 5). The contractility of muscles is due to the con- 

 tractility of the protoplasm 

 originating in the cells forming 

 the filjrillse. 



Nervous tissue is made up 

 of nerve-cells and fibres pro- 

 ceeding from tliem ; the for- 

 mer constituting the centres 

 of nervous force, and usually 

 massed together, forming a 

 gcmglion or nerve-centre from 

 which nerve-fibres pass to the 

 periphery and extremities of °"™* ("^ 

 the body, and serve as conductors of nerve-force (Fig. 6). 



Organs and their Functions. — Having considered the 

 difiierent kinds of cells and the tissues they form, we may 

 now consider the origin of organs and their functions. Tho 

 Protamoeba may be considered as an organless being. In 

 Avmha (Fig. 11) we first meet with a specialized portion of 

 the body, set apart for the performance of a special function. 



nnglion in the clam, with 

 ■) proceeding from it. 



