CELL 



(>,. mi. in a* in whether given nuclei have a netted or 

 .oil.-d framework, there is yet greater variety in 

 tin- niiiiiiti-r de>eription and figuring. According 

 to Flriimiing tin- ni-twork is quite disorderly, but 

 i. l.rydig, and inline liavi- docrilicd distinct 



Fig. 3. (After Carnoy) : 

 .A, Cell and typical nucleus : a, slight membrane ; 6, radiating 



protoplasmic network; c. wall of nucleus; d, plasma of 



nucleus ; , nuclear coil. 

 B, Nucleus at rest, showing network. 

 C, Nucleus before division, showing coiled filament. 



radial structure ; according to some the nuclear coil 

 is endless, while others describe it as divided into 

 portions; and when we descend to such subtleties 

 of observation as the intimate structure of the 

 threadwork or the relations between chromatin 

 and achromatin, the diversity is so great that it 

 seems desirable here to leave such minutiae un- 

 touched. 



(2) The Intermediate Nuclear Substance. Be- 

 .sides the nuclear elements of definite form, what- 

 ever that form may precisely be, all investigators 

 describe an intermediate substance of variable 

 consistence, usually semi-liquid, amorphous and 

 .structureless, but with fine granules. It is a clear 

 unstainable ' plasma ' filling up the chinks, but 

 nothing definite is known as to its composition. 

 (3) The nucleolus which lies within the nucleus 

 varies greatly in size and position, and more than one 

 are very generally present. Flemming has defined 

 them as ' portions of the nuclear substance, distinct 

 in structure from network and plasma, definitely 

 limited and smoothed, always rounded in outline, 

 usually suspended in the network, but often inde- 

 pendent of it.' But when the minute structure 

 and the relation of nucleoli to nuclear framework 

 are inquired into, or the question of physiological 

 rdle raised, very great diversity of opinion is found 

 to obtain. ( 4 ) Bodies different in appearance from 

 nucleoli may occur inside the nucleus, but of these 

 little is known. (5) The wall which bounds the 

 nucleus seems to be a true integral part of the 

 latter, but disappears at the beginning of division. 



(d) The Cell-wall. In the older conception of 

 the cell, which was practically that of a closed 

 bag, the wall of the cell figured very prominently. 

 But Niigeli showed (1845) that some vegetable 

 cells were destitute of walls, Leydig (1857) de- 

 lined the cell in respect to its substance, Schultze 

 and others described naked Protozoa, and the 

 progress of the ' protoplasmic movement ' led to 

 the abandonment of the position that the wall was 

 a necessary or important part of the cell. In 

 many cells, indeed, a limiting layer is very clearly 

 present, and a sheath or cyst is especially charac- 

 teristic of passive cells. Plant-cells are almost 

 always distinguished by the possession of a limit- 

 ing wall, of definite chemical composition, consist- 

 ing of what is known as cellulose. An analogous 

 wall occasionally occurs round animal cells. In 

 the latter, however, the membrane is usually a 

 comparatively slight thing, and may arise (1) 

 108 



from an aggregation of the threads and knot* of 

 the framework ; (2) as a cuticle or capsule formed 

 from the matrix or ground substance; (3) from a 

 combination of both these element*. Leydig han 

 shown that in a very wide series of animal cells 

 i In- membrane, such as it is, is penetrated by small 

 but definite pores. It is very important further 

 to remember that both in plants and animals the 

 cells are in a great number of cases connected with 

 one another by intercellular bridges of protoplasm, 

 and are in nowise to be thought of as closed bag*. 

 The cell-wall of plants, which, be it again noted, 

 is a definite chemical substance, grows in extent 

 and thickness by an intricate organic process, in 

 the course of which new infinitesimal elements 

 form apparently as intercalations l>etween the old. 

 The growth is in very many cases far from uni- 

 form ; pits, ridges, and manifold kinds of sculptur- 

 ing thus appear, and give rise to numerous detailed 

 variations. The formation of new boundaries when 

 a cell divides is a question of much difficulty ; but 

 in plant, and apparently in some animal cells, the 

 formation of a ' cellular plate ' is one of the last 

 events in the dividing process. 



III. Physiology of the Cell. When the entire 

 organism is simply a cell, as in most of the Proto- 

 zoa and Protopnyta, all the vital processes which 

 in higher forms have their seat in special sets of 

 cells, known as tissues and organs, are of course 

 discharged by the unit-mass. Thus a unicellular 

 organism like the Amoeba takes in energy as food 

 in nutrition, works it up into living matter in 

 digestion and assimilation, and expends it again 

 in contraction and locomotion. As in any higher 

 organism the oxygen required for the chemical 

 breaking up of the protoplasmic molecules, the 

 air for the vital flame, is taken in by the absorp- 

 tion known as respiration, and the waste carbonic 

 acid gas is in an essentially similar way got rid 

 of. Further, more solid ' ashes ' of the vital com- 

 bustion are formed in the Amoeba and in other 

 actively living cells, and may pass out in excre- 

 tion along with the refuse of unusable food- 

 material. The absence of a circulating fluid, of 

 digestive glands, nerves, sense-organs, lungs, 

 kidneys, and the like, does not in any way 

 restrict the vital functions of a unicellular organ- 

 ism. All goes on as usual, only with greater 

 chemical complexity, since all the different pro- 

 cesses have but a unit-mass of protoplasm in 

 which they occur. The physiology of independent 

 cells, instead of being very simple, must ne very 

 complex, just because structure or differentiation 

 is all but absent. It is, however, possible to ex- 

 press the manifold processes in a comparatively 

 simple way by remembering what Claude Bernard 

 was one of the first clearly to emphasise, that 

 vital processes must be really only twofold 

 building up and breaking down of living matter. 

 On the one hand the protoplasm or real living 

 matter is being by a series of chemical processes 

 built up or constructed ; on the other hand, in 

 activity it is breaking down or being destroyed. 

 The income of food or energy is, at the expense 

 of the cellular organism, gradually raised into 

 more and more complex and unstable compounds, 

 until the genuine most complex and more un- 

 stable living matter itself is reached. On the 

 opposite side, with liberation of energy in the 

 form of work, this living matter breaks down 

 into simpler and simpler compounds, until only 

 the work, the waste products, and heat remain as 

 the equivalent of the income of energy or food on 

 the other side of the life-equation. On the one 

 hand there are constructive processes, on the other, 

 destructive ; chemical synthesis and chemical dis- 

 solution is another expression of the contrast ; 

 while the two sets of processes are in more modern 



