Page:The New International Encyclopædia 1st ed. v. 04.djvu/468

CELL. 400 CELL. is at the basis of life. Carl.ou, liydro^'cn. oxygen, uitrofien, are always iiresent, and sulpluir. phos- jiliorus, potassium, anil certain other alkalis and metals are usually found in small quantities. The dead protoplasm consists chiefly of allnnni- lious substances, including nudein. .'54 per cent.; lecithin and fats, 14 per cent.: various albu- minoids. 14 per cent.: and, for the rest, numer- ous other substances. Cell AclivUy.—CeU. apart from the process of division, show active processes in their proto- plasm, especially by their capacity for movc- liicnt and response to slinnilus. CcU-niovenient takes on diverse forms according to circum- stances. In naked, free cells, like the Ama-Ua, the most evident movements are the throwing out of blunt, finger-like processes ( pseudopodia ) . If one of these takes the lead it determines the direction of locomotion of the whole cell, for the rest of the body (lows into the jiseudopod. Often the pseudopodia are numerous and ex- tremely fine : nn<ler these circumstances minute particles may be seen streaming in the current of the protoplasm which moves along the thread. In the case of cells with a firm wall, the proto- jdasm often streams in fine threads through the Ihiid-lilk'd fell-space, or in other cases a mar- velous rotatory movement is seen. The causes of protoplasmic inovement are not altogether clear. r,ccently. however, attention has been called to the fact that other foamy masses — especially a drop of oil filled with vesicles of wat^'r— will send out pseudopodium-likc processes. So that the con- stant niovenicnt of the protoplasm may be the physiological concomitant of its unstable struc- ture. Other forms of cell-movement are the lash- ing of whips (flagella) or little hairs (cilia) covering the free swimming cell (Protozoa and Protophyta ) . Response to SItininhis. — Irritability is a funda- iiiental property of protoplasm, so we find it present in unicellular organisms and in the tissue cells. rroto|)hism is active only witliin certain limits of temperature, beyond which quiescence — heat rigor or cold rigor — occurs. Protoplasm (of :Myxomycetes ) will flow with reference to a source of heat — toward that tem- perature to which it is most 'attuned.' The direc- tion of light-rays may also determine the direc- tion of foeomotion. VVhen a ray of white light passes obliquely through an amoeba, the latter moves from the source of light. Also, the ehloro- phyl bodies of a leaf ill move away from the surface which is too intensely illuminated. Like- wise the electric current, gravity, contact, and chemical agents may awaken definite responses in cellprotoplasm. The forms assumed by cells are most diverse. In the animal's body they are united to form tissues, and the cells of each kind of tissue have their peoiliar form. The cells of the skin are flattened or cuhoidal ; those lining the food-canal are '•oinmnar; nuisilos are com])osed of elongat- ed compound cells and nerves give rise to proc- esses that may l>c two feet or more in length. These kinds of cells will be considered under Histology. The (lent! -Cell. — The egg and the spermato- zoon, the union of which is the initial act in sexual reproduction, are each single cells, de- rived, like the other cells of the body, by the division of preexisting cells. The two kinds of germ-cells are, however, very difl'erent in ap- jjcarance and function. The egg-cell (ovum) is of great size, due to the fact that it is stulTed full of food-material, the so-called yolk. The nucleus is large, and is commonly <alled the germinative vesicle, and in it- open-meslicd filing the chromatic material is lodged. The entire egg is often enveloped in one or more envelo])e3 for its protection. The spermatozoon is of ex- tremely small size. It is typically a thread- like cell, enlarged at one end to form the bead. Behind the head is the middle piece, followed by the lash or tail. The head contains the nucleus, which is here dense, without water, but consist- ing almost wholly of chiomatin. Tlie middle piece seems to contain the centrosome. The outer envelope of head and middle piece and the whole of the tail are cytoplasmic, and play only an incidental part in the fertilization of the cell. See Fekiilization. Consult: E. B. Wilson, The Cell in Develop- ment and Inheritance, page 483 (2d ed., Xew York, 1900) ; O. Hertwig, Die Zelte und die Oewebe (Jena. I., 1803; II., 1899: Eng. trans., Xew York, 1900). CELL (in plants). In its most fundamental characters the plant-cell closely resembles the animal cell. Like that, it possesses a nu- cleus and cytoplasm, and may or may not be surrounded by :i cell-wall. Nearly all cells are microscopic in size, a fair average being about 0.004 to 0.002 inch in diameter. Some cells, however, become much elongated, bast cells over 0.2 inch in length having been reported, and the internodal cells of Chara sometimes reaching a length of 2 inches. On the other hand, the cells of the yeast-plant are only 0.00032 inch in diameter, "and the bacteria are nnich smaller. Vhen cells are fn-e, there is a tendency to assume the spherical form, and this is the most usual shape in one-celled plants: but where the cells are aggregated into tissues vari- Celi. : /, a cell showinir wall (in, eytoplasm (J), vju-uoles (I), uucleus (i>, and iiucfeolus In)' .:'. rrW ivitli heavy wijl in-), and iin elouuat^d nuch'u.s (k) with two nuileoli (n); .'(-.r. with same letterins:; 0, a spindle showingceutrosplieresCz), spindle libres (.i ), and chromosomes (a). ous modifications are met, the cells becoming compressed, elongated, flattened, star-shaped, etc. In all except a few of the lowest plants, the cell has a wall of cellulose, a substance de-