;2o 



NATURE 



[September 21, 1905 



functions to perform. All these are permanent organs of 

 the cell, produced in the first instance as a result of the 

 cell activity, but now capable of an independent existence 

 in the cell] in that they reproduce themselves by division, 

 and are in this way carried on from cell to cell. 



In many cells there are formed at certain stages other 

 organs which appear to be transitory, and are only pro- 

 duced when they are required. Such are the spindle 

 figure, the centrosome, the blepharoplast, and the 

 ccenocentrum. 



So far as we know, the cell is the smallest vital unit 

 that can have a separate e.xistence. But it is only among 

 the unicellular organisms and under certain conditions in 

 the earlier stages of development of the more highly 

 organised multicellular organisms that cells have a per- 

 fectly independent life. Schwann's hypothesis that the 

 multicellular body is a colony of independent vital activities 

 governing the nutrition, growth, and reproduction of the 

 whole is not tenable. The cell cannot be regarded as an 

 independent unity working merely in association with 

 other cells. Its life and existence depend upon these. It 

 is an integral part of an individual organisation, and 

 cannot exist apart from it. But this absolute dependence 

 of individual cells upon the organisation as a whole is only 

 realised in the more highly developed forms. In the lower 

 t\'pe5 of plants (and animals) it is possible, during the 

 early stages of development, to separate a single cell from 

 the whole, which will still continue to live and grow. 

 Each cell is no doubt dependent upon the others to some 

 extent, even at this early stage, but it still retains the 

 power to develop independently if placed under suitable 

 conditions. As cell division continues each cell becomes 

 more and more dependent upon its fellows, until the stage 

 is reached when it no longer has the power to exist by 

 itself. The various functions performed by a cell reside 

 within it as an individual unit, but the exercise of these 

 functions is governed by the organism as a whole. Just 

 as the organism seeks for a state of equilibrium in relation 

 to various external stimuli, so a cell in an organism has 

 to adapt itself to and come into a state of equilibrium 

 with the various cells around it. 



The Nucleus. 



The nucleus is the centre of activity, and governs the 

 vital functions of the cell. All investigations show that 

 in its absence the cell soon ceases to perform its vital 

 functions and dies. 



In all cells, from the algas and fungi upwards, the 

 nucleus is more or less clearly delimited from the cyto- 

 plasm by a membrane or limiting layer. The important 

 substance which is thus separated off from the rest of 

 the cell is the chromatin, probably the most complex and 

 most highly differentiated chemical compound or collection 

 of compounds in the cell. It exists in the form of a more 

 or less granular network, and is characterised chemically 

 by the presence of phosphorus, which is in organic con- 

 nection with it. We may look upon the chromatin as 

 the highest point in the development of living substance, 

 upon which the activities of the cell in great measure 

 depend, and as the seat of origin of all those complicated 

 changes which have for their ultimate aim the division 

 of the cell. 



The division of the nucleus begins by a series of trans- 

 formations in the chromatin network which lead to the 

 differentiation in it of chromosomes. We know very little 

 of what actually takes place during these changes, and 

 practically nothing of the forces at work to bring them 

 about. But the visible result is that the chromatin 

 granules gradually fuse together, or become restricted to 

 certain areas by the increased vacuolation of the ground 

 substance of the nucleus to form a thick, more or less 

 regular thread, in which can be observed at certain stages 

 a differentiation into alternate regions of stainable and 

 unstainable substance — chromatin and achromatin — which 

 finally breaks up into equal or unequal lengths to form 

 the chromosomes. In some cases the chromatin granules 

 or network become aggregated into a definite number of 

 irregular masses which form the chromosomes directly 

 without the production of a distinct chromatin thread. 



This nuclear differentiation is usually accompanied by 

 changes in the cytoplasm which lead to the appearance 



NO. 1873, VOL, 72] 



of a fibrillar structure in the form of a more or less 

 regular spindle, the threads of which come into contact 

 with the chromosomes through the breaking down of the 

 nuclear wall. The chromosomes then, by the action of a 

 force or forces of which we as yet know^ very little, arrange 

 themselves in regular order in the equatorial plane of the 

 spindle figure, and some of the spindle fibres become 

 attached to them. The chromosomes become divided longi- 

 tudinally into two apparently exactly equal halves; and 

 then, probably by the exertion of .some sort of contractile 

 force or pull on the part of the spindle fibres, the separate 

 halves are caused to move to opposite poles of the spindle. 

 Here a series of transformations take place, which lead 

 to the constitution of two new nuclei. Such are the 

 essential features in this complex process of nuclear 

 division, and it is a striking fact that they occur with 

 more or less regularity in all nuclei from the algoe and 

 fungi up to the highest plants. 



The Structure of Cytoplasm. 

 In the elucidation of cell structure we owe much to the 

 beautiful methods of staining and fixing which are due 

 especially to Flemming and Heidenhain, to the improved 

 micro-chemical methods which we owe especially to 

 Zacharias and Macallum, and to the investigations of 

 such observers as Fischer and Mann, who have shown us 

 the effects of various reagents upon the living substance, 

 and have thus taught us to be very cautious in our 

 interpretations of the structures seen in dead fi.xed cells. 



The investigations of oil-foams and colloids by Butschli, 

 Hardy, and others have given us a clue to possible ex- 

 planations of the various appearances seen both in the 

 living and dead fi.xed and stained cells, and the intro- 

 duction of the ribbon section cutting microtome into the 

 domain of vegetable histology has enabled us to make the 

 best use of the beautiful apochromatic object-glasses which 

 we owe to the researches of the late Prof. .\bbe. 



It is unfortunate that, so far, very little progress has 

 been made in the examination of the structure of the 

 living cell. We may hope that, with the improved methods 

 of illumination now available, combined with experimental 

 investigation, it will be possible to make some progress in 

 this direction. It is of the greatest importance that we 

 should be able to satisfy ourselves to what extent the 

 various appearances seen in the fixed and stained cell are 

 due to the action of the reagents employed. In this 

 respect a recent discovery by Kohler, which indicates the 

 possibility of making use of the ultra-violet rays in such 

 investigations, is of interest. Kohler {Phys. Zeit., 1904) 

 finds that if the ultra-violet rays from the electric spark 

 between cadmium or magnesium terminals are separated 

 out bv means of quartz prisms, objects illuminated by 

 them, when examined by means of lenses made of quartz, 

 show differentiations of structure which otherwise require 

 staining to make them visible. The chromatin of the 

 nucleus and such substances as cuticle and cork are almost 

 opaque to the ultra-violet rays, and can be made visible 

 on a fluorescent screen or can be photographed. The re- 

 solving power of the microscope is doubled, and Lummer 

 considers that the principle employed is the only one by 

 which further progress in resolving power can be made. 

 If the method is found by cytologists to be a vi-orkable 

 one, it may open up an entirely new field of microscopic 

 investigation by which the protoplasmic differentiation 

 in living cells may be more clearly revealed. 



Many attempts have been made to show that the cyto- 

 plasm possesses a definite morphological structure of its 

 own, which is related to the various functions it performs, 

 and that it is not a formless semi-viscid fluid in which 

 various physical and chemical forces are at work, and 

 upon which the various structures observed depend ; in 

 other words, that it possesses a morphological constitution 

 as opposed to a merely chemical one. 



Fromman and Heitzmann in 1875 described the struc- 

 ture of cytoplasm as consisting of fine threads or fibres 

 in the form of a net with fluid between and forming a 

 sponge-like structure. Flemming in 1882 described it as 

 composed of two substances, one in the form of fibrils 

 (filar substance) embedded in the other, a more or less 

 homogeneous interfilar substance. In 1890 Altmans pro- 

 pounded his interesting hypothesis that all living sub- 



