CKLL 



51 



-ted is a natural character of the 

 l>l.im-, anil tin- insheathed cells of many animal 

 11 MICH may be similarly expressed an an exhibition 

 Hi the same passive phase. But it is enough here 

 t<> |>oint out the possibility of classifying and in 

 t-i|>n-tin^ tin- various cells composing the tissues 

 of higher organisms in terms or an original life- 

 rvrlf, or deeper still in terms of those twofold pro- 

 to|ilasmii - possibilities which lie behind all forms 

 iiinl phases whether of cells, tissues, or organisms 

 themselves. This conception of a cell-cycle is due 



MM- lii/i/tiii/ni/i/ii/ at rnd of article). 

 Cell-division. When the vital processes are so 

 related that income and upbuilding exceed ex- 

 penditure and dissolution, the cell must obviously 

 accumulate capital and increase in size. In some 

 cases the cell may expand into relatively gigantic 

 proportions, as in the alga Botrydium and in 

 many eggs. Growth, however, brings a nemesis 

 with it, this namely, that the mass to De kept alive 

 increases more rapidly than the surface through 

 which the vital processes are accomplished. In 

 spherical cells the former increases as the cube, 

 the latter as the square of the radius. The bigger 

 the cell gets, the more difficult do its conditions 

 of life become. The supplies of food and oxygen, 



Fig. 7. 



A, Life-history of unicellular plant (Protococcus) : 1, encysted ; 2, 

 quitting its cell ; 3, ciliated ; 4, quiescent ; 6 and 6, dividing. 



B, Life-history of Amoeba : 1, encysted; 2, escaping; 3, free; 

 4, dividing ; 5, free half with vacuole v, nucleus n, and food- 

 particles/; 6, encysting anew. 4 and 6 may also represent 

 the union of two Amoebse (conjugation). 



the means of accomplishing purification and the 

 like, cannot keep pace with the growth of the 

 living mass if the surface increase only at a 

 much less rapid rate. A limit of growth is thus 

 reached. The cell must stop growing, or go on 

 growing at an increasing risk, or in some way 

 restore the balance between mass and surface. 

 This last course is the one most frequently 

 exhibited the cell divides. However this may 

 be effected, the result is in all cases the same 

 namely, the reduction of mass, and corre- 

 sponding increase of surface. Like other organ- 

 i-uis, the cell-organism reproduces at its limit 

 of growth. This rationale of cell-division, due 

 especially to Herbert Spencer, is obviously clearest 

 in reference to free-living cells like Protozoa, 

 Protophyta, blood-corpuscles, reproductive cells, 

 and the like, but the general principle holds good 

 throughout. 



It is evident, however, that such considerations 

 as the above go to justify rather than to explain 

 cell-division. They show why the cell ought to 

 divide, not how it does. The real mechanism of 

 the process is still a riddle. In its very simplest 

 expressions, indeed, the riddle may be partly read. 

 In a simple and primitive Protozoon like Schizo- 



C'-s, the protoplasm seems literally to break, 

 gular fissures appear, as well they might if a 

 condition of unstable vital equilibrium has been 



reached, and portions of the substance are cleft 

 apart from the main IIIOKX. From such a cane 

 to the separation of multiple buds, which are 

 little more than overflowings of too large a 

 cell, or to the commoner occurrence of simple 

 budding, is no great step. The difficulty begins, 

 however, when we consider the ordinary cell- 

 division, which appears in most cases as a deliber- 

 ate and orderly process, including a well-defined 

 series of nuclear changes. As to the mechanics 

 of this process only a few suggestions of moment 

 have been made. Thus Plainer points out that 

 the explanation must be in terms either ( 1 ) of 

 chemical processes influencing the cellular sub- 

 stance, or (2) of protoplasmic movement due to 

 the above or to external influences, or (3) of 

 unknown molecular and attractive forces. He 

 himself finds the condition of nuclear division to be 

 in part a streaming movement of the protoplasm, 

 such as is familiar in many Protozoa, and would 

 regard the division of the protoplasm as a purely 

 mechanical process. In his studies on protoplasmic 

 mechanics, Berthold has also attacked this in- 

 tricate problem, but more in relation to the nature 

 of the dividing partitions than with reference to 

 the forces at work. Professor Van Beneden, who 

 did so much in working out the details of cell- 

 division in the ovum, expressed himself as follows 

 in regard to the deeper problem in a paper pub- 

 lished in 1887: 'All the internal movements 

 which are associated with the cellular division 

 have their immediate cause in the contractility of 

 the fibres of the reticular protoplasm which form 

 two antagonistic groups.' All that one can at 

 present conclude is that the process represents, as 

 above noticed, a physiological necessity, and that 

 it takes place in connection with very intricate 

 physical and chemical changes within the cell. 



Modes of Cell-division. After abstracting the 

 rare occurrence of almost mechanical ruptures 

 and of overflow buds, various modes of orderly 

 division remain to be noticed, (a) The cell may 

 give off a bud, usually smaller than itself. With 

 this a portion of the nucleus is usually associated, 

 as in many Protozoa ; or the processes may occur 

 apart from demonstrable nucleus, as in the common 

 yeast-plant, (b) Division into two is by far the 

 most frequent mode of multiplication, and occurs 

 all but universally. In a small minority of cases 

 the division is accomplished without any intricate 

 nuclear change, the cell being in an apparently 

 simple way divided into two, with half of the 

 nucleus in each daughter-cell. Such divisions are 

 said to be 'direct.' In most cases the nucleus, 

 apparently taking the initiative, undergoes a strik- 

 ing series of orderly changes before the division is 

 perfect. This is the commonly observed condition, 

 and such divisions are termed ' indirect.' (c) But in 

 many cases the division occurs in a very different 

 way, being not single but multiple. "From one 

 cell more than two daughter-cells arise simultane- 

 ously, and that not by external cleavage, but by 

 internal multiplication. Such a mode of multipli- 

 cation is termed endogenous division or 'free' cell- 

 formation, and is well seen in many Fungi and 

 Algae. It may l>e compared with the ordinary 

 process by defining it as division taking place in 

 limited space and time, since the daughter-cells 

 arise wit bin the mother-cell, and simultaneously, 

 not successively. It is, in many cases at least, pre- 

 ceded by the rapid division of the nuclei, to form 

 centres round each of which protoplasmic material 

 then becomes aggregated. In a few cases, Arnold 

 has described a peculiar breaking up of the nucleus 

 which he called fragmentation. 



Karyokinesis. One of the most beautiful results 

 of recent histology is the demonstration of the 

 general unity of process which obtains in the 



