History of biology

by L. C. Miall

The Cell Theory.

Any one of the higher animals or plants admits of analysis into organs, each adapted to one or more functions. Bichât (1801) showed that the body of one of the higher animals is not only a collection of organs, but also a collection of tissues, and the same is true of the higher plants. Analysis of the organism was carried a step further when in 1838-9 Schleiden and Schwann announced that all the higher animals and plants are made up of cells, which were at first supposed to consist in every case of a cell-wall, fluid contents, and a nucleus.[38] It was soon discovered that the cell-wall is as often absent as present, and that the cell-contents are not simply fluid; the nucleus is still believed to be universal. Schwann proved that nails, feathers, and tooth-enamel, though not obviously cellular, consist of nothing but cells, and it was afterwards shown that bone, cartilage, fatty tissue, and fibrous tissue arise by the activity of cells which disappear from view in the abundance of their formed products. The individual cells of a complex organism are usually themselves alive; sometimes, as in ciliated epithelium, they give indications of life long after they have been separated from the body. The preponderating importance of the transparent jelly or protoplasm became clear when it was recognised that this alone is invariably present, and that this alone responds to stimuli. The nucleus is believed to be only a specialised part of the cell-protoplasm.

The cell-theory, like nearly every theory, was neither altogether new nor in its first form altogether complete. Before 1838 cell-division, as we should now call it, had been indistinctly seen to be the process by which the body of one of the higher animals is built up. Leeuwenhoek and Swammerdam had found a wholly cellular stage in frog-embryos (see p. 103), while Prévost and Dumas in 1824 had in effect discovered that the cells of which such embryos consist result from repeated division of an egg; Mohl in 1835 observed the actual division. Even Schwann, however, was not acquainted with the important fact that every cell arises by the division of a pre-existing cell.

Swarm-spores of algæ showed that protoplasm, when unenclosed in a cell-wall, can move about, direct its course, and change its shape. Knowledge of this fact did more than rectify the definition of the cell; it effaced one distinction between plants and animals, and gave a hint of the resemblance of primitive cells to such simple organisms as Amœba.

Martin Barry in 1843 announced that certain Protozoa (that name was not yet in use) are simple cells. He pointed out that they possess nuclei, like those of tissue-cells, and compared their increase by fission with the cleavage of the egg. Single cells were thus shown to be not only capable of locomotion, which was already known, but able to provide for their own support. The Protozoa and Protophyta (i.e., the simplest animals and plants, which are not always to be clearly separated) are now known to be autonomous cells, increasing by fission, and often forming colonies. Conjugation (fusion of similar individuals) often precedes fission, and when it was proved (1861-5) that ova and spermatozoa are true cells, it was seen that fertilisation, as we know it in the higher animals, is only a special form of the conjugation observed among the Protozoa. To the Protozoa it is now possible to trace, without any startling break of continuity, all the multicellular organisms, their tissues, the growth of those tissues by repeated fission, their eggs, and the process of fertilisation which precedes cleavage. The old Greek riddle, "Which came first, the fowl or the egg?" may now receive the answer: "Neither; their common starting-point is to be found in the Protozoa, which, even when adult, represent the primitive unicellular condition, to which all the higher animals revert once in every generation."

It is not without reason that biologists dwell on the unifying influence of the cell-theory, which has become a chief support of that still wider unifying influence, the Origin of Species by Natural Selection. When it was discovered that all living things, whether plants or animals, consist of nucleated cells which increase by fission, and that in all of them cell-fission is started anew from time to time by a cell-fusion, it was strongly suggested that resemblances so striking and so universal can only proceed from a common descent.

During the last half-century the study of cells has led to a great increase of knowledge respecting all bodily functions, whether in health or disease. We now look to it as perhaps the most hopeful source of new light upon the important question of hereditary transmission.

The Scientific Investigation of the Higher Cryptogams.

We now resume the history of a study which down to the end of the eighteenth century had yielded only meagre and uncertain results (see above, pp. 85-88). At the date in question it had been ascertained that the spores (then called "seeds") of ferns, and probably of other cryptogams, are capable of propagating the species, but no one knew precisely what part the spore played in the life-history, or could explain the true difference between a cryptogam and a flowering plant. The great improvements in the construction of the compound microscope which were effected between 1812 and 1830 rendered it possible to elucidate much more thoroughly the structure and development of the chief groups of cryptogams. The sexual reproduction of algæ was explored; moving filaments (spermatozoids) were seen to enter the chambers in which embryos afterwards formed; the conjugation of similar cells was observed in algæ and fungi, and recognised as a simple mode of sexual reproduction. The resemblance of the spermatozoids of mosses and ferns to animal spermatozoa was noted, and their participation in the process of fertilisation was more and more closely followed until at length Hofmeister in 1851 saw them fuse with the egg-cell of a fern. Suminski, whose full name, Lesczyc-Suminski, is unpronounceable by Englishmen, had discovered (in 1848) that the prothallus of a fern, which is the product of the germinated spore and had been hitherto taken for the cotyledon, bears two kinds of reproductive organs, one of which liberates spermatozoids, while an egg-cell is developed within the other. He did not correctly describe all the details, but he showed where the essential reproductive organs form, and where fertilisation is effected. The masterly researches of Hofmeister (1849-57) fused what had been a number of partial discoveries into a connected and luminous doctrine. He proved that the prothallus is one of two generations in the life-history; that it begins with a spore and ends with a fertilised egg-cell; that in the higher cryptogams there is a regular alternation of generations; that the prothallus of the fern answers to the leafy moss, while the leafy fern is the equivalent of the moss-capsule; that the egg-cell is the same structure in both cryptogams and flowering plants; that the pollen-tube and the seed are found to-day only in flowering plants; that the gymnosperms make a transition from the higher cryptogams to the angiosperms; that unity of plan pervades the whole series of mosses, ferns, fern-like plants, gymnosperms, and angiosperms. Before Darwin's Origin of Species had appeared Hofmeister presented to evolutionists a clear example of a descent in which every principal term is well authenticated, while the extremes are far apart.