52 



CELL 



division of all kinds of cells. Whether the subject 

 of investigation be the pollen-cells of a plant, the 

 skin of a tadpole, the developing ovum, or the 

 growth of a tumour, the same process of ordinary 

 indirect division may be observed to take place 

 along essentially similar lines. This, though but 



Fig. 8. Stages of Transverse Division : 



1, 2, 3, 4, 5, in the epidermis of a tadpole ; 1', V, 3', 4', 5', in 



the epidermis of lily. (After Macfarlane. ) 



a natural consequence of common descent and 

 similar conditions, is not without its marvel when 

 the complexity of the process ( see below ) receives 

 due consideration. Even in detail there is in 

 structural as well as in physiological changes a 

 deep-seated unity of process. But while the 

 essential similarity of all cases of simple ' indirect ' 

 division must be allowed, it is only fair to recog- 

 nise that in minor details very manifold variations 

 occur. Even in those Protozoa where the nuclear 

 changes of division have been followed, consider- 

 able diversity of detail obtains ; nor, within a single 

 genus do the ova of two different species of thread- 

 worm (Ascaris) divide in exactly the same fashion. 

 But neglecting at present the detailed divergences, 

 whether these occur normally and constantly, or as 

 they often do atypically and arbitrarily, it is 

 necessary now to notice the general steps usually 

 observed in cell-division. 



We have already described the nucleus as con- 

 sisting of a readily stainable (chromatin) network 

 or ribbon, and of another substance (so-called 

 achromatin) which does not stain so deeply. As 

 a preliminary to division, the nucleus loses its 

 definite boundary, and the chromatin threads no 

 longer exhibit the regular disposition they have 

 when at rest. The threads form an irregular 

 wreath, and as the loops break, their arrangement 

 is comparable to a star in which the open ends of 

 the loops are directed outwards, and the closed ends 

 lie in the centre. By subsequent movement this 

 position is reversed, the loops gather into two 

 groups, which lie with their open ends towards one 

 another in the middle. Meanwhile, the achromatin 

 elements also exhibit regular arrangement, forming 

 fine streaks stretching from the centre towards the 

 poles, and exhibiting an appearance which is often 

 compared to a striated spindle. At the two poles 

 of the cell the granules of the general protoplasm 



Fig. 9. Typical Division of Nucleus. 

 (From Haddon, after Flemming.) 



are also aggregated into a couple of star-like 

 figures. The chromatin loops now diverge farther 

 and farther from the centre, till they reach a posi- 

 tion at the respective poles. A double chromatin 

 star may then be observed, one at each pole, not to 



je in any way confused with unsubstantial polar 

 stars previously mentioned. Soon after this stage 

 s reached, the real cell-division occurs. The pro- 

 toplasm constricts across the middle of the cell, 

 and the division is accomplished. In plant-cells, 

 and apparently in some animal cells also, the 

 division of the protoplasm is accompanied by the 

 formation of a cellular plate, which bounds the 

 open surfaces of the two daughter- cells. With or 

 without cellular plate, the result is the formation 

 of two daughter-cells, each with half of the original 

 nucleus. But this is not all, the half nucleus 

 formed after the above fashion has to be recon- 

 structed into the original resting form. A series of 

 retrogressive stages occur, in the course of which 

 the nucleus passes from star to wreath, and from 

 wreath into the typical network or twisted coil. 

 In some cases the steps of reconstruction seem to 

 correspond very closely to the various steps of the 

 antecedent upbreaking. 



Death. It seems tolerably certain, as Weis- 

 mann and others have suggested, that the unicel- 

 lular Protozoa are in the great majority of cases 

 practically immortal. These simple organisms have 

 no ' body to keep up, in their functions they appear 

 to be continually self-recuperative, and except 

 from entirely abnormal conditions such cells prob- 

 ably never die. The pool in which they live may 

 dry up for ever, or other animals may swallow and 

 digest them, but such casualties are very different 

 from natural death. They may indeed lose their 

 individuality by doubling in division, or the whole 

 cell may break up into spores, but where there is 

 nothing to be buried we can hardly speak of death. 

 It seems in fact justifiable to say that death began 

 with the formation of a many -celled body. Even 

 there, a certain amount of immortality may be 

 claimed for the reproductive cells, which, becoming 

 separate from the parent organism, proceed to 

 divide into a body which will of course eventually 

 die, but also into reproductive cells, which, as some 

 of them at least will form again fresh organisms 

 and reproductive cells, may be said to be links in 

 a continuous and immortal cellular chain. But 

 leaving aside the really immortal Protozoa, and the 

 logically immortal successful reproductive elements, 

 it must be allowed that cells, like organisms, die. 

 And that not only with the body as a whole, but 

 by themselves. Certain superficial cells are con- 

 stantly being brushed off and replaced by others ; 

 the red blood-corpuscles break up in the fluid ; 

 others become hardened in death into the 

 ' mummified ' cells of supporting and epidermic 

 structures ; others surrender themselves into mucus 

 or in the ejection of lassoes as in the Coelenterates ; 

 others practically die away into fat and reserve 

 products, or may in manifold ways degenerate. 

 Many surfaces, especially in secreting regions of 

 the body, exhibit continual death of cells, and 

 regeneration by the division of the survivors. 



IV. Modern Aspect of the Study of the Cell. 

 With the improvement of appliances and the per- 

 fecting of staining methods, the study of the cell 

 has within late years become at once more accurate 

 and more complex. On the one hand, the labours 

 of the early histologists are being amplified and 

 corroborated with ceaseless industry. The forms 

 of cells in different animals and tissues, the 

 minutiae of their structure, the processes observed in 

 their multiplication, are being each year more and 

 more perfectly investigated. On the other hand, 

 the emphasis which has been laid on the pro- 

 toplasm is finding expression in numerous attempts 

 to explain the forms and phases of cell-life in terms 

 of the underlying protoplasmic changes, and such 

 investigations as those which seek to disclose the 

 mechanics of cell-division and ovum-segmentation, 

 the conditions of cellular equilibrium and change, 



