アートとサイエンス

細胞説 (Cell theory)の形成と発展

 
この記事を書いている人 - WRITER -

以下は、An introduction to cytologyで細胞説に言及されている箇所(太字強調は当ウェブサイトによる)。

The history of cytology falls naturally into three periods, of which the first begins with the discovery of the cell by Robert Hooke in 1665. the second with the foundation of the Cell Theory by Schleiden and Schwann in 1838-9, and the third with the important researches of Strasburger, Hertwig, Biitschli, and others’ between 1870 and 1880.

Marcello Malpight (1628-1694), an Italian physiologist and professor of medicine at Bologna, Pisa and Messina, is best known for his important pioneer work in anatomy and embryology. Most of his observations on plants wore included in his Anatome Plantarum (1675) and had to do largely with the various kinds of elements making up the body of the vascular plant. Malpighi made a distinct step in advance in studying tissues with the cell as a unit; a clear fore-shadowing of the Cell Theory is seen in his remarks concerning the importance of the “utriculi” in the structure of the body.

Hugo von Mohl (1805-1872), in spite of his many valuable observations on the growth of algae, in 1835 agreed essentially with Mirbel. He made a step in advance, however, when he described carefully for the first time the division of a cell. We shall see further on that von Mohl’s later researches contributed largely to the upbuilding of an adequate theory of the cell.

F. J. F. Meyen (1804-1840) held that there are three fundamental forms of elementary organs: cells, spiral tubes, and sap vessels. He noted the wide occurrence of cell-division but did not describe the process in detail. Meyen apparently made the first attempt to distinguish cell- division from the free cell-formation described by previous workers. It has been pointed out by Sachs that if this short step had been clearly taken earlier the peculiar theory of coil-formation later developed by Schleiden would have been impossible. Von Mohl also had made observations ruling out Schleidcn’s idea, but his excessive caution prevented him from making a decisive statement on the subject. H. J. Dutrochet (1776-1847) in 1837 described the body as being composed of solids and fluids, the former being aggregations of cells of a certain degree of firmness, and the latter, such as blood, being made up of cells freely floating. He believed that although the cell contents may be more or less solid, the highest degree of vitality is compatible only with the liquid condition. He further recognized muscle fibers as elongated cells.

To all the above workers the important elementary unit was the “globule.” It was customary to refer to this conception as the Globular Theory, in contradistinction to the curious and fanciful Fiber Theory put forth by Haller (1708-1777) many years before (1757), according to which the organism is made up of slender fibers cemented together by “organized concrete/‘ For some the term “globule” stood for the granules seen in the cell contents, whereas for others it meant the cell itself. As observations multiplied and ideas became more definite the Cell Theory of Schleiden and Schwann was more and more distinctly fore-shadowed. Before turning to the Cell Theory, however, we must notice briefly a few observations which had been made on the cell contents.

The Foundation of the Cell Theory. The year 1838 marks an epoch in the history of biology. In this and the following year Schleiden and 
Schwann founded the Cell Theory, which, in view of its enormous influence upon all branches of biological science, may be regarded as second 
in importance only to the Theory of Evolution. We have seen that cells had been observed by various workers during many years, and had been 
recognized as being constantly present in the bodies of living organisms, but it remained for Schleiden and especially Schwann to formulate a 
comprehensive theory embracing the known facts and affording a starting point for further researches.

The Cell Theory stated primarily that the body is composed entirely of cells and their products, the cell being the unit of structure and function 
and the primary agent of organization. Subsidiary to this was Schleiden's theory of cell-formation, which should not be confused with the main 
thesis just stated.

Theodor Schwann (1810-1882) was associated as a student with Johannes Miiller, the great physiologist, first at Wiirzburg and later at 
Berlin. It was in the latter place that he put forth his statement of the Cell Theory. Immediately afterward he went to Louvain, where he was 
a professor for nine years, and later transferred to Li&ge. In disposition he contrasted strongly with Schleiden, being described as " gentle and 
pacific. " It is said that Schleiden, while dining with Schwann, discussed with him some of his ideas regarding cells in plants, which he had been studying in his laboratory. Schwann had been making similar observations on 
animals, and after the meal the two went to Schwann's laboratory, where they came to the conclusion that cells are fundamentally alike in both 
kingdoms. Schleiden's treatise on the subject, Beitrdge zur Phylogenesis, appeared in 1838 and dealt mainly with the origin of cells. Robert Brown 
had recently discovered the nucleus, and about it Schleiden built up his theory of "free cell-formation, " which was essentially as follows: In the 
general cell contents or mother liquor (" cytoblastcma ") there are formed, by a process of condensation, certain small granules (later called "nu- 
cleoli" by Schwann). Around these many other granules accumulate, thus forming nuclei ("cytoblasts"). Then, "as soon as the cytoblasts 
have attained their full size, a delicate transparent vesicle appears upon their surface. This vesicle in each case enlarges and forms a new cell, 
and, since it arisee upon the surface of the cytoblast (nucleus), "the cytoblast can never lie free in the interior of the cell, but is always en- 
closed [i.e., imbedded] in the cell wall . . . " Schleiden thus regarded new cell-formation as endogenous ("cells within cells") rather than the 
result of cell-division. With respect to the main proposition of the Cell Theory he says in the opening paragraphs: "... every plant developed 
in any higher degree, is an aggregate of fully individualized, independent, separate beings, even the cells themselves. Each cell leads a double 
life: an independent one, pertaining to its own development alone; and another incidental, in so far as it has become an integral part of a 
plant. It is, however, easy to perceive that the vital process of the individual cells must form the first, absolutely indispensable fundamental 
basis, both as regards vegetable physiology and comparative physiology in general; . . . " 

Schleiden shared the results of his observations, including his errors,with Schwann, who was the one to formulate the Cell Theory in a com- 
prehensive manner. Schwann announced the theory in concise form in 1838, and in 1839 published a very full account under the title "Mikro- 
skopische Untersuchungen uber die Uebereinstimmung in der Struktur und dem Wachsthum der Thieve und Pflanzen." He says: "The elementary 
parts of all tissues are formed of cells in an analogous, though very diversified manner, so that it may be asserted that there is one universal prin- 
ciple of development for the elementary parts of organisms^ however different, and that this principle is the formation of cells. )} And further: "The 
development of the proposition that there exists one general principle for the formation of all organic productions, and that this principle is 
tho formation of cells, as well as the conclusions which may be drawn from this proposition, may be comprised under the term Celt Theory . . . " 
" . . .all organized bodies are composed of essentially similar parts, namely, of cells . . . " 

Elaboration of the Cell Theory. The Cell Theory at once became established as one of the main foundation stones of biological research, 
but it underwent considerable modification as investigations proceeded. The main thesis, that the body is composed of cells and their products, 
remained, but other ideas associated with this in the minds of Schleiden and Schwann, particularly that concerning free cell-formation, were 
superseded.

Darwin, aside from his Hypothesis of Pangenesis, scarcely mentioned the theories of the cell; and not until many years later was the cell investigated with reference to these matters. 
Researches on the origin of the germ cells, nuclear division, and fertilization, which brought the Cell Theory and the Theory of Evolution into intimate association, began shortly after 1870 with the works of Schneider (1873), Auerbach (1874), Fol (1875, etc.), Biitschli (1875, etc.), O. Hertwig (1875, etc.), van Beneden (1875, etc.), Strasburger (1875, etc.), Flemming 
(1879, etc.), and Boveri (1887, etc.).

Since the time when the cell was pointed out as the unit of structure and function it has been recognized that the mode of origin of new cells 
is a matter of fundamental importance. We have seen in our historical sketch that cells were believed by the founders of the Cell Theory to arise de novo from a mother liquor, or "cytoblastema," a misconception removed by later investigations in which it was shown beyond question that cells arise only by the division of preexisting cells.

(An introduction to cytology)

 

以下は、The cell and protoplasmからの抜粋。

The sixteen papers comprised in this volume were presented in a Symposium on The Cell and Protoplasm at Stanford University, June 30-July 5, 1939. The occasion commemorated the one hundred years of advancement in knowledge of the protoplasmic unit of living things after Schleiden and Schwann's formulation of the Cell Theory. 

Some of the earlier portions of this review repeat in part a paper entitled "Predecessors of Schleiden and Schwann" (1939) which I gave in a symposium on the centenary of the cell theory at the Richmond, Virginia, meeting of the American Association for the Advancement of Science on December 27, 1938.
Hugo von Mohl (1805-1872) is one of the most important figures in the early development of the cell theory.

 

Literature

  1. Professor John H. Gerould (1922). The Dawn of the Cell Theory. : the great French naturalist, Lamarck (1744-1829), deserves to rank as one of the founders of the cell theory. In his Philosophie ZooUogique published in 1809 he says: "No body can possess life if its containing parts are not a cellular tissue, or formed by cellular tissue."
  2. Sedgwick, A. 1894. On the Inadequacy of the Cell Theory of Development, and on the Early Development of Nerves, Particularly of the Third Nerve and of the Sympathetic in Elasmobranchii. Q\iart. J. Mic. Sci., 37: 87-101.
  3.  TURNER, W. 1890. The cell theory, past and present. Nature 43: 10-15.
  4. HUXLEY, T. H. 1853. The Cell Theory. Brit, and For. Med.-Chir. Rev. 12.
この記事を書いている人 - WRITER -

Copyright© Arts and Sciences , 2017 All Rights Reserved.