CELL 



article PROTOPLASM, we may note some of the 

 iiii|Mirtiiiit steps. Dujardin (1835) descril>ed the 

 -aicode' of Protozoa and other cells; Purkinje 

 . iv: c i| emphasised the analogy between the 

 'protoplasm' of the animal embryo and the 

 cambium ' of plant-cells; Von Mohl (1846) em- 

 phasised in the clearest way the importance of the 

 protoplasm in the vegetable cell; Ecker (1849) 

 roinpaivd the contractile substance of muscles wil.li 

 i hat oi the amwba ; Donders also referred the con- 

 tract ilit\ from the cell-wall to the contained 

 material; Colin suspected that the 'sarcode' of j 

 animal and the 'protoplasm' of plant-cells must l>e i 

 in the highest decree analogous substance;' and 

 so throughout another decade did botanists and 

 /oologiste unite in laying stress rather on the 

 living matter than on the wall of the cell, and in 

 hinting at the existence of one living substance as 

 the physical basis alike of plants and animals. 

 This view found at length definite expression in 

 I sill, when Max Schultze defined the modern con- 

 ception of the cell as a unit-mass of nucleated 

 protoplasm. Since then the ' protoplasmic move- 

 ment has dominated research, and we think not 

 so much of the cell-containing protoplasm as of the 

 protoplasm which constitutes and gives form to the 

 cell. 



II. Structure of the Cell. While it is impossible 

 to isolate the static from the dynamic aspects of 

 the cell, it will be convenient to discuss the two 

 separately, and to consider the cell at rest and 

 dead, apart from the cell active and alive. In 

 other words, the form, structure, or morphology 

 may be studied for literary clearness apart from tne 

 functions, life, and physiology. 



(a) General Form. The typical and primitive 

 form of the cell is spherical. This is illustrated by 

 many of the simplest plants and animals which live 

 freely, and by young cells such as ova. But the 

 typical form is in many, indeed in most cases, lost ; 

 and the forms assumed are as diverse as the 

 internal and external conditions of life. The cell 

 may be irregular and protean, as in Amoeba 1 , white 

 blood-corpuscles, ana many young eggs ; or 

 squeezed into rectangular shape, as in much of the 

 substance of a leaf ; or flattened into thinness, as in 

 the outer lining of the lips ; or oval and pointed, as 

 in swiftly moving Infusorians and Bacteria ; or 

 much branched, as in multipolar ganglion cells of 

 animals or the latex-containing cells of some plants. 

 The typical spherical and self-contained form is 

 tnat which would naturally be assumed by a com- 

 plex coherent substance situated in a medium 

 different from itself. The other forms are responses 

 to internal and external conditions. Under the 

 heading Cell-cycle below it will be shown how the 

 relative activity and passivity of the cell naturally 

 expresses itself in such extremes as a long-drawn- 

 out Infusorian and a rounded-off Gregarine, or in a 

 highly nourished ovum and a mobile spermatozoon. 

 Further, cells, like entire animals, often show a 

 tendency to become two-ended, to have poles very 

 ditt'erent from one another. Just as an animal may 

 have a highly nourished head and a scantily 

 nourished tail, so a cell may become distinctly 

 bipolar in form. In other cases the cell is altogether 

 plastic, expressing every impulse of internal change 

 and every impact of external influence in some 

 modification of form. Or the state of nutrition of 

 the living matter may cause alteration in the 

 adhesion of the substance all over, or in particular 

 places, and thus condition an outflowing, regular 

 or irregular, in given directions. Furthermore, ex- 

 ternal pressure and limitation of growth may 

 square off the cell into a parallelogram, or restrict 

 it to grow like a bast fibre in length alone and not 

 in breadth. In fact the conditions are most mani- 

 fold, and the resultant forms likewise. 



(b\ General Substance of the Cell. The cell U 

 much more than a max* of highly complex chemical 

 substance : it has an organised structure. ( 1 ) Tin? 

 protoplasm or living matter in the strict/eat MOM 

 is generally supposed to be an intimate mixture of 

 eomplex iuid highly unstable chemical compound-. 

 Inspection under a microscope of such cells a* 

 amo-lia-, white blood -corpuscles, ova, simple alga-, 

 or such as are readily seen in thin slice* of growing 

 plant-shoots, in root-hairs, and transparent parts, 

 will at once furnish an impression or the general 

 aspect of the substance of the cell. Not all that 

 one sees can of course deserve the name of proto- 

 plasm, for apart from definite inclosurcs like starch- 

 grains and fat-globules, much of the remaining 

 slightly clouded substance is hardly to be strictly 

 called protoplasm, but rather represents steps in 

 the ceaseless making and unmaking which form 

 the fundamental rhythm of life. Keeping the 

 definite inclosures and products for the moment 

 aside, we may briefly notice in general outline 

 what has been with most collusiveness observed 

 as to the structure of the general cell-substance or 

 * cytoplasm ' as it is now frequently termed. All 

 ot)servers~agree that the structure is far removed 

 from the homogeneous, though there is much dif- 

 ference of opinion as to the nature of the hetero- 

 geneity. In a large number of cases at least the 

 substance of the cell has been resolved into two 

 distinct portions the one an intricate network, 

 knotted and interlaced in a manner baffling descrip- 

 tion ; the other a clear substance, filling up the 

 interstices or meshes of the living net. Leydig, 

 Frommann, and Heitzmann have ueen peculiarly 

 successful in unravelling this knotted structure in 

 animal cells, and much the same has been recorded 

 by Strasburger and Schmitz as observable in some 

 plants. The reticulate structure is certainly more 

 doubtful in regard to vegetable cells, and even in 

 some animal cells what some have descrilied as a 

 network others have deemed only a minutely 

 bubbled emulsion. 



But besides the real substance of the cell there 

 are to be seen products of various kinds formed 

 from the living matter. The cell may be packed 

 with starch, or laden with fat, or expanded with 

 mucus ; it may contain colouring matter in various 

 forms, as in the familiar chlorophyll bodies of many 

 plant-cells ; its structure mav include, as in some 

 Protozoa, definitely formed fit>rils or yet firmer for- 

 mations of chitin and the like ; and again there are 

 concretions of retained waste and reserve products, 

 sometimes in the form of crystals. Not to be over- 

 looked either is the fine ' dust-cloud ' of minute 

 granules which are seen suspended in the clearer 

 matrix, and which apparently represent aggrega- 

 tions of diverse chemical substances formed in the 

 building up and breaking down of the protoplasm. 

 As the outside of any mass is bound to be in differ- 

 ent conditions from the inside, it is natural to find 

 the appearance of distinct physical and chemical 

 zones in the cell-substance. Thus in many Protozoa 

 the outer portion, needlessly termed 'ectoplasm,' is 

 often denser and more refractive than the more 

 fluid and internal stratum of the 'endoplasm.' Or 

 this may go further, and we may have a sweated- 

 off limiting cuticle, or a definitely organised wall 

 of cellulose in vegetable cells. The cuticle may 

 be further substantiated with secretions of horny, 

 flinty, limy, and other material. Even within 

 the cell a stratified structure may be frequently 

 observed, and Berthold and others have recently 

 emphasised the existence of such concentric layers, 

 each characterised by ite own special set of de- 

 posits. 



Worthy of notice, too, are the various kinds of 

 bubbles or vacuoles which occur in the cell-sub- 

 stance. These may be simply indefinite spaces, 



