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NA TURE 



[September 6, 1900 



1839 Theodore Schwann announced the important generalisation 

 that there is one universal principle of development for the 

 elementary part of organisms, however different they may be in 

 appearance, and that this principle is the formation of cells. 

 The enunciation of the fundamental principle that the elementary 

 tissues consisted of cells constituted a step in the progress of 

 biological science which will for ever stamp the century now 

 drawing to a close with a character and renown equalling those 

 which it has derived from the most brilliant discoveries in the 

 physical sciences. It provided biologists with the visible 

 anatomical units through which the external forces operating on, 

 and the energy generated in, living matter come into play. It 

 dispelled for ever the old my^tical idea of the influence exercised 

 by vapours or spirits in living organisms. It supplied the 

 physiologist and pathologist with the specific structures through 

 the agency of which the functions of organisms are discharged 

 in health and disease. It exerted an enormous influence on the 

 progress of practical medicine. A review of the progress of 

 knowledge of the cell may appropriately enter into an address 

 on this occasion. 



Structure of Cells. 



A cell is a living particle, so minute that it needs a micro- 

 scope for its examination ; it grows in size, maintains itself in a 

 state of activity, responds to the action of stimuli, reproduces its 

 kind, and in the course of time it degenerates and dies. 



Let us glance at the structure of a cell to determine its con- 

 stituent parts and the rdle which each plays in the function to be 

 discharged. The original conception of a cell, based upon the 

 study of the vegetaljle tissues, was a minute vesicle enclosed by 

 a definite wall, which exercised chemical or metabolic changes 

 on the surrounding material and secreted into the vesicle its 

 characteristic contents. A similar conception was at first also 

 entertained regarding the cells of animal tissues ; but as 

 observations multiplied, it was seen that numerous elementary 

 particles, which were obviously in their nature cells, did not 

 possess an enclosing envelope. A wall ceased to have a primary 

 value as a constituent part of a cell, the necessary vesicular 

 character of which therefore could no longer be en'ertained. 



The other constituent parts of a cell are the cell plasm, which 

 forms the body of the cell, and the nucleus embedded in its sub- 

 stance. Notwithstanding the very minute size of the nucleus, 

 which even in the largest cells is not more than i/5ooth inch 

 in diameter, and usually is considerably smaller, its almost 

 constant form, its well-defined sharp outline, and its power 

 of resisting the action of strong reagents when applied to the 

 cell have from the period of its discovery by Robert Brown 

 caused histologists to bestow on it much attention. Its structure 

 and chemical composition ; its mode of origin ; the part which 

 it plays in the formation of new cells and its function innutrition 

 and secretion have been investigated. 



When examined under favourable conditions in its passive or 

 resting state, the nucleus is seen to be bounded by a membrane 

 which separates it from the cell plasm and gives it the character- 

 istic sharp contour. It contains an apparently structureless 

 nuclear substance, nucleoplasm or enchylema, in which are 

 embedded one or more extremely minute particles called 

 nucleoli, along with a network of exceedingly fine threads or 

 fibre-s, which in the active living cell play an essential part in 

 the production of new nuclei within the cell. In its chemical 

 composition the nuclear substance consists of albuminous plastin 

 and globulin ; and of a special material named nuclein, rich in 

 phosphorus and with an acid reaction. The delicate network 

 within the nucleus consists apparently of the nuclein, a substance 

 which stains with carmine and other dyes, a property which 

 enables the changes, which take place in the network in the 

 production of young cells, to be more readily seen and followed 

 out by the observer. 



The mode of origin of the nucleus and the part which it plays 

 in the production of new cells have been the subject of much 

 discussion. Schleiden, whose observations, published in 1838, 

 were made on the cells of plants, believed that within the cell a 

 nucleolus first appeared, and that around it molecules aggre- 

 gated to form the nucleus. Schwann again, whose observations 

 were mostly made on the cells of animals, considered that an 

 amorphous material existed in organised bodies, which he called 

 cytoblastema. It formed the contents of cells, or it might be 

 situated free or external to them. He figuratively compared it 

 to a mother liquor in which crystals are formed. Either in the 

 cytoblastema within the cells or in that situated external to 

 them, the aggregation of molecules around a nucleolus to form 



NO. 1610, VOL. 62 1 



a nucleus might occur, and, when once the nucleus had been 

 formed, in its turn it would serve as a centre of aggregation of 

 additional molecules from which a new cell would be produced. 

 He regarded therefore the formation of nuclei and cells as 

 possible in two ways : one within pre-existing cells (endogenous 

 cell-formation), the other in a free blastema lying external to 

 cells (free cell-formation). In animals, he says, the endogenous 

 method is rare, and the customary origin is in an external 

 blastema. Both Schleiden and Schwann considered that after 

 the cell was formed the nucleus had no permanent influence on 

 the life of the cell, and usually disappeared. 



Under the teaching principally of Henle, the famous Professor 

 of Anatomy in Gcitiingen. the conception of the free formation 

 of nuclei and cells in a more or less fluid blastema, by an aggre- 

 gation of elementary granules and molecules, obtained so much 

 credence, especially amongst those who were engaged in the 

 study of pathological processes, that the origin of cells within 

 pre-existing cells was to a large extent lost .sight of. That a 

 parent cell was requisite for the production of new cells seemed 

 to many investigators to be no longer needed. Without doubt 

 this conception of free cell-formation contributed in no sm.all 

 degree to the belief, entertained by various observers, that the 

 simplest plants and animals might arise, without pre-existing 

 parents, in organic fluids destitute of life, by a process of spon- 

 taneous generation ; a belief which prevailed in many minds 

 almost to the present day. If, as has been stated, the doctrine 

 of abiogenesis cannot be experimentally refuted, on the other 

 hand it has not been experimentally proved. The burden of 

 proof lies with those who hold the doctrine, and the evidence 

 that we possess is all the other way. 



Multiplication of Cells. 



Although von Mohl, the botanist, seems to have been the 

 first to recognise (183 = ) in plants a multiplication of cells by 

 division, it was not until attention was given to the study of the 

 egg in various animals, and to the changes which take place in 

 it, attendant on fertilisation, that in the course of time a much 

 more correct conception of the origin of the nucleus and of the 

 part which it plays in the formation of new cells was obtained. 

 Before Schwann had published his classical memoir in 1839, 

 von Baer and other observers had recognised within the animal 

 ovum the germinal vesicle, which obviously bore to the ovum 

 the relation of a nucleus to a cell. As the methods of observa- 

 tion improved, it was recognised that, within the developing 

 egg, two vesicles appeared where one only had previously 

 existed, to be followed by four vesicles, then eight, and so on in 

 multiple progression until the ovum contained a multitude of 

 vesicles, each of which possessed a nucleu?. The vesicles were 

 obviously cells which had arisen within the original germ-cell or 

 ovum. These changes were systematically described by Martin 

 Barry so long ago as 1839 and 1840 in two memoirs communi- 

 cated to the Royal Society of London, and the appearance pro- 

 duced, on account of the irregularities of the surface occasioned 

 by the production of new vesicles, was named by him the 

 mulberry-like structure. He further pointed out that the 

 vesicles arranged themselves as a layer within the envelope of 

 the egg or zona pellucida, and that the whole embryo was com- 

 posed of cells filled with the foundations of other cells. He 

 recognised that the new cells were derived from the germinal 

 vesicle or nucleus of the ovum, the contents of which entered 

 into the formation of the first two cells, each of which had its 

 nucleus, which in its turn resolved itself into other cells, and by 

 a repetition of the process into a greater number. The 

 endogenous origin of new cells within a pre-existing cell 

 and the process which we now term the segmentation 

 of the yolk were successfully demonstrated. In a third 

 memoir, published in 1841, Barry definitely stated that 

 young cells originated through division of the nucleus of the 

 parent cell, instead of arising, as a product of crystallisation, in 

 the fluid cytoblastema of the parent cell or in a blastema situated 

 external to the cell. 



In a memoir published in 1842, John Goodsir advocated the 

 view that the nucleus is the reproductive organ of the cell, and 

 that from it, as from a germinal spot, new cells were formed. 

 In a paper, published three years later, on nutritive centres, he 

 described cells the nuclei of which were the permanent source 

 of successive broods of young cells, which from time to time 

 occupied the cavity of the parent cell. He extended also his 

 observations on the endogenous formation of cells to the carti- 

 lage cells in the process of inflammation and to other tissues 



