46 



CELL 



Cell, a unit-mass of living matter, whether 

 rounded off' by itself, as in the simplest plants or 

 animals and in the youngest stage of all organisms, 

 or associated with other cells to form a higher 

 unity. The great majority of the Protozoa and 

 Protophyta are single cells, and all other organisms 

 begin where the former leave off. From the double 

 unity resulting from the fusion of two sex -cells the 

 higher plants and animals develop by repeated 

 division, and they may be therefore always resolved 

 into more or less close combinations of variously 



Fig. 1. Dividing Egg-cell (after Gegenbaur). 



modified unit-masses. In most cases these in- 

 dividualities of the simplest order are minute, and 

 their separateness is not to be discerned with the 

 unaided eye, but there are many instances among 

 the simplest plants and animals, as well as in the 

 component elements of higher forms, where the 

 unit-masses are relatively giant-cells and quite 

 visible without the use of the microscope. The 

 giant Amoeba Pelomyxa, the common sun-ani- 

 malcule Actinosphcerium, the Alga Botrydium, 

 and some of the cells (e.g. bast) of plants may be 

 noted as illustrations of cells with considerable 

 dimensions. In the great majority of cases the 

 body of the cell includes a well-defined centre or 

 nucleus ; and the definition may therefore be 

 extended in the statement that a cell is a nucleated 

 unit-mass of living matter or protoplasm. 



I. History. In the article BIOLOGY it has been 

 pointed out that a more and more penetrating 

 scrutiny alike of structure and of function led 

 naturalists from organs to tissues, and from tissue 

 to cell. Some of the steps in this gradually 

 deepening analysis deserve fuller record. 



Discovery of Cells. In the latter half of the 17th 

 century the simple microscope afforded to Malpighi 

 and Leeuwenhoek, to Hooke and Grew, what 

 was literally a vision of a new world. In 

 applying their rough and simple instruments to the 

 study of the structure of plants and animals they 

 became pioneers in the investigation of the infinitely 

 little. Leeuwenhoek (Phil. Trans. 1674) seems to 

 have been the first to observe, what are now so 

 familiar, single-celled organisms. In the 18th cen- 

 tury Swammerdam and others continued with much 

 enthusiasm to describe the minute intricacies 

 which their 'new eyes' revealed; Fontana (1784) 

 observed the kernel of the cell the nucleus and 

 some of the elements of the tissues ; but the founda- 

 tion of scientific histology was not laid until the 

 appearance in 1801 of the Anatomie Generate of 

 Bichat. In this epoch-making work organs were 

 resolved into their component tissues, and their 

 functions were interpreted as the sum-total of the 

 properties of their constituent elements. Such a 

 conclusion was the utmost that could be reached 

 with the appliances then at command. 



Early in this century, however, an improvement 

 in the appliances of observation furnished a fulcrum 

 for a new advance. Fraunhofer discovered the 

 principle of achromatic Lenses (q.v. ; also ACHRO- 

 MATISM) ; these were combined into the compound 

 Microscope (q.v.), and a new era began. 'Fibres' 

 and 'globules,' 'larninse,' 'nuclei,' and even 'cells' 

 were described. In 1831 Robert Brown emphasised 

 the normal presence of the nucleus discovered by 

 Fontana, and made the first important advances in 

 the study of the vegetable cell. Isolated discoveries, 



such as that of the nucleolus by Valentin (1836), 

 occurred in rapid succession during those years. 

 Dujardin in 1835 described the sarcode or livi'ng 

 matter of the Protozoan Foraminifera and of some 

 other cells, and thus emphasised, as Rb'sel von 

 Rosenhof had done many years before (1755) in 

 regard to the ' Proteus animalcule ' or Amoeba, the 

 most important element to be considered in form- 

 ing a true conception of the cell. The importance 

 of his description, of which he was apparently him- 

 self unconscious, had for some time the same fate 

 as that of his predecessor of almost a century 

 before. Observations had in fact to accumulate 

 before any generalisation became possible. The 

 first definite steps towards a co-ordination of 

 results was probably that of Johannes Miiller, who 

 in 1835 pointed out the resemblance between the 

 cells of the vertebrate notochord and the elements 

 observed in plants. The cellular nature of the 

 epidermis and the presence of nuclei therein was 

 next ascertained, and similar discoveries were made 

 in regard to several other tissues. Up to 183& 

 there was in fact a period of research in which cells 

 were observed rather than understood. 



Establishment of the Cell-theory. As early as- 

 1826 Turpin had maintained that plants were 

 formed by an agglomeration of cells. Professor 

 M'Kendrick well points out, what one would of 

 course expect, that for some years before 1838 

 botanists were beginning generally to recognise 

 the cellular composition and origin of plants. The 

 conclusion known as the ' cell-theory was doubt- 

 less vaguely present in many minds. Its definite 

 statement was still awanting. In 1838, however, 

 Schleiden proved that a nucleated cell is the only 

 original component of a plant embryo, and that the 

 development of all tissues might be referred to such 

 cells. In the following year Schwann published 

 at Berlin his famous Microscopic Investigations on 

 the Accordance in the Structure and Growth of 

 Plants and Animals (Trans. Sydenham Society, 

 1847). In this classic work it was shown that all 

 organisms, plants and animals alike, are made up- 

 of cells, ana spring from cells. In composition and 

 in origin there is unity. The generalisation 

 familiarly known as the cell-theory was thus 

 clearly established, and though now a common- 

 place and postulate of histology, it may fairly be 

 described in Agassiz's words as ' the greatest dis- 

 covery in the natural sciences in modern times.' 

 Following up the generalisations of Schwann and 

 Schleiden, come a host of researches by which the 

 essential advance contained in the ' cell-theory ' was 

 more and more fully confirmed. Cells were not 

 only observed, their import was recognised. 



New Conception of the Cell. When the cell-theory 

 was established, the general conception of the cell 

 was far from being either accurate or complete. It 

 was usually described as a vesicle closed by a solid 

 membrane, containing a liquid in which float a 

 nucleus and granular bodies. It was also the 

 general opinion that such cells originated within a 

 structureless ground substance. In two ways 

 these notions were speedily corrected. On the one 

 hand as regards the origin of cells, Prevost and 

 Dumas (1824), Martin Barry (1838-9), Reichert 

 ( 1840), Henle ( 1841 ), Kolliker ( 1846), Remak( 1852 ), 

 showed that in the case of the egg-cell, and in the 

 growth of tissues, each new cell arose by division 

 from a predecessor. This important conclusion 

 was most firmly established by Goodsir in 1845, 

 and Virchow in 1858, who proved that in all cases, 

 normal and pathological alike, cells arose from pre- 

 existing cells, a fact expressed in the axiom omnis 

 cellula e cellula. In the second place it gradually 

 became apparent that too much importance had 

 been attached to the cell-wall and too little to the 

 contained substance. Referring details to the 



