58 CONNECTIVE TISSUES CELLS. 



shape from rounded elevations or swellings but slightly projecting from the 

 general surface, to long and extremely delicate threads, and the rapidity 

 with which they are shot out and retracted differs with the surrounding con- 

 ditions, and especially with variations in the temperature. The activity of 

 the movements is greatest at about 100 F. At 32 they usually become 

 extremely sluggish or cease altogether. The ova of trout, however, which 

 may be regarded as a kind of Protoplasm, undergo segmentation perfectly 

 in iced water, whilst they soon cease to move at ordinary temperatures. The 

 contractile property of Protoplasm can be brought into play by mechanical 

 and electrical irritation, and by the application of chemical stimuli. The 

 locomotive powers of the masses of Protoplasm enable them to search for, 

 and procure, the materials required for their nutrition. When such mate- 

 rials are met with, the Protoplasm extends itself over and embraces them, 

 the opposite surfaces coalescing, so that the foreign body is entirely included ; 

 its nutritive parts are then extracted, and the remainder, if any, is cast 

 away by the retreat of the Protoplasm from it. The reproduction or 

 rather, multiplication of such masses is effected by the detachment of a 

 larger or smaller portion of the parent mass, the two parts separating to 

 obtain the substances requisite for their own growth and nutrition. Chemical 

 examination of Protoplasm shows that it contains a considerable proportion 

 of myosin, and in some instances, protagon, glycogeu, and cholesteriu have 

 been found to be present. Protoplasm, presenting slight but unimportant 

 differences from the general description above given, is found to form the 

 contents of all Vegetable cells, at least in their early condition, to constitute 

 the essential part of a great group of organisms which have been associated 

 together by Haeckel under the name of Protista, and include such forms as the 

 Flagellata, Diatomaceae, Myxomycetre, and Foraminifera, the last of which 

 recent researches have shown to form a thin, but very widely spread, layer 

 on the bed of the ocean at all depths. And, finally, it forms the living ma- 

 terial in the interior of all animal cells, and to its presence the active pro- 

 cesses they exhibit are essentially due. In the very simplest forms of Animal 

 Life, as in the Monera, no differentiation of structure is visible in the Proto- 

 plasm of which they are composed. But in most of the Amoebae and For- 

 aminifera ; in the ovum at an early stage of its development ; in the free 

 floating corpuscles known as the white corpuscles of the blood ; in granula- 

 tion cells and pus-corpuscles ; in the corpuscles of the ductless glands ; in 

 osteoblasts and elsewhere, the small masses of Protoplasm contain a nucleus; 

 and such bodies may be conveniently designated by the term employed by 

 Haeckel of Cytodes. In most cases the outer surface of the Protoplasm has 

 undergone a certain, though often very slight, degree of consolidation. And 

 when this consolidation has proceeded so far as to form a thin membrane 

 around the protoplasmic mass and nucleus, a cell is formed, the various forms 

 and characters of which we shall now proceed to consider. 



43. Every perfectly formed and active cell is composed of three parts, a 

 cell-wall, a nucleus, and protoplasmic, or other cell-contents. Of such bodies, 

 more or less modified, most of the tissues of the body are composed; and 

 each in maintaining its own nutrition ministers to the functional activity of 

 the special tissue to which it belongs, and is subservient to the well-being of 

 the organism at large. The cell- wall in the majority of instances is a thin 

 and transparent, but resistant membrane, through which osmotic processes 

 can readily occur. It presents, however, in different instances, the most 

 various degrees of density and consolidation, sometimes, as in the hepatic 

 cells, being only slightly firmer than the rest, of the cell ; whilst in cartilage 

 cells it acquires great firmness and thickness, becoming ultimately, by the 

 deposition of calcareous salts, converted into bone. Occasionally it presents 



