A History of Science, vol 4, Henry Smith Williams [best books to read in life txt] 📗
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other observers had come very near the truth regarding them,
unanimity of opinion was possible only after the verdict of the
perfected microscope was given.
These blood corpuscles are so infinitesimal in size that
something like five millions of them are found in each cubic
millimetre of the blood, yet they are isolated particles, each
having, so to speak, its own personality. This, of course, had
been known to microscopists since the days of the earliest
lenses. It had been noticed, too, by here and there an observer,
that certain of the solid tissues seemed to present something of
a granular texture, as if they, too, in their ultimate
constitution, were made up of particles. And now, as better and
better lenses were constructed, this idea gained ground
constantly, though for a time no one saw its full significance.
In the case of vegetable tissues, indeed, the fact that little
particles encased a membranous covering, and called cells, are
the ultimate visible units of structure had long been known. But
it was supposed that animal tissues differed radically from this
construction. The elementary particles of vegetables “were
regarded to a certain extent as individuals which composed the
entire plant, while, on the other hand, no such view was taken of
the elementary parts of animals.”
ROBERT BROWN AND THE CELL NUCLEUS
In the year 1833 a further insight into the nature of the
ultimate particles of plants was gained through the observation
of the English microscopist Robert Brown, who, in the course of
his microscopic studies of the epidermis of orchids, discovered
in the cells “an opaque spot,” which he named the nucleus.
Doubtless the same “spot” had been seen often enough before by
other observers, but Brown was the first to recognize it as a
component part of the vegetable cell and to give it a name.
“I shall conclude my observations on Orchideae,” said Brown,
“with a notice of some points of their general structure, which
chiefly relate to the cellular tissue. In each cell of the
epidermis of a great part of this family, especially of those
with membranous leaves, a single circular areola, generally
somewhat more opaque than, the membrane of the cell, is
observable. This areola, which is more or less distinctly
granular, is slightly convex, and although it seems to be on the
surface is in reality covered by the outer lamina of the cell.
There is no regularity as to its place in the cell; it is not
unfrequently, however, central or nearly so.
“As only one areola belongs to each cell, and as in many cases
where it exists in the common cells of the epidermis, it is also
visible in the cutaneous glands or stomata, and in these is
always double—one being on each side of the limb—it is highly
probable that the cutaneous gland is in all cases composed of two
cells of peculiar form, the line of union being the longitudinal
axis of the disk or pore.
“This areola, or nucleus of the cell as perhaps it might be
termed, is not confined to the epidermis, being also found, not
only in the pubescence of the surface, particularly when jointed,
as in cypripedium, but in many cases in the parenchyma or
internal cells of the tissue, especially when these are free from
the deposition of granular matter.
“In the compressed cells of the epidermis the nucleus is in a
corresponding degree flattened; but in the internal tissue it is
often nearly spherical, more or less firmly adhering to one of
the walls, and projecting into the cavity of the cell. In this
state it may not unfrequently be found. in the substance of the
column and in that of the perianthium.
“The nucleus is manifest also in the tissue of the stigma, where
in accordance with the compression of the utriculi, it has an
intermediate form, being neither so much flattened as in the
epidermis nor so convex as it is in the internal tissue of the
column.
“I may here remark that I am acquainted with one case of apparent
exception to the nucleus being solitary in each utriculus or
cell—namely, in Bletia Tankervilliae. In the utriculi of the
stigma of this plant, I have generally, though not always, found
a second areola apparently on the surface, and composed of much
larger granules than the ordinary nucleus, which is formed of
very minute granular matter, and seems to be deep seated.
“Mr. Bauer has represented the tissue of the stigma, in the
species of Bletia, both before and, as he believes, after
impregnation; and in the latter state the utriculi are marked
with from one to three areolae of similar appearance.
“The nucleus may even be supposed to exist in the pollen of this
family. In the early stages of its formation, at least a minute
areola is of ten visible in the simple grain, and in each of the
constituent parts of cells of the compound grain. But these
areolae may perhaps rather be considered as merely the points of
production of the tubes.
“This nucleus of the cell is not confined to orchideae, but is
equally manifest in many other monocotyledonous families; and I
have even found it, hitherto however in very few cases, in the
epidermis of dicotyledonous plants; though in this primary
division it may perhaps be said to exist in the early stages of
development of the pollen. Among monocotyledons, the orders in
which it is most remarkable are Liliaceae, Hemerocallideae,
Asphodeleae, Irideae, and Commelineae.
“In some plants belonging to this last-mentioned family,
especially in Tradascantia virginica, and several nearly related
species, it is uncommonly distinct, not in the epidermis and in
the jointed hairs of the filaments, but in the tissue of the
stigma, in the cells of the ovulum even before impregnation, and
in all the stages of formation of the grains of pollen, the
evolution of which is so remarkable in tradascantia.
“The few indications of the presence of this nucleus, or areola,
that I have hitherto met with in the publications of botanists
are chiefly in some figures of epidermis, in the recent works of
Meyen and Purkinje, and in one case, in M. Adolphe Broigniart’s
memoir on the structure of leaves. But so little importance
seems to be attached to it that the appearance is not always
referred to in the explanations of the figures in which it is
represented. Mr. Bauer, however, who has also figured it in the
utriculi of the stigma of Bletia Tankervilliae has more
particularly noticed it, and seems to consider it as only visible
after impregnation.”[2]
SCHLEIDEN AND SCHWANN AND THE CELL THEORY
That this newly recognized structure must be important in the
economy of the cell was recognized by Brown himself, and by the
celebrated German Meyen, who dealt with it in his work on
vegetable physiology, published not long afterwards; but it
remained for another German, the professor of botany in the
University of Jena, Dr. M. J. Schleiden, to bring the nucleus to
popular attention, and to assert its all-importance in the
economy of the cell.
Schleiden freely acknowledged his indebtedness to Brown for first
knowledge of the nucleus, but he soon carried his studies of that
structure far beyond those of its discoverer. He came to believe
that the nucleus is really the most important portion of the
cell, in that it is the original structure from which the
remainder of the cell is developed. Hence he named it the
cytoblast. He outlined his views in an epochal paper published
in Muller’s Archives in 1838, under title of “Beitrage zur
Phytogenesis.” This paper is in itself of value, yet the most
important outgrowth of Schleiden’s observations of the nucleus
did not spring from his own labors, but from those of a friend to
whom he mentioned his discoveries the year previous to their
publication. This friend was Dr. Theodor Schwann, professor of
physiology in the University of Louvain.
At the moment when these observations were communicated to him
Schwann was puzzling over certain details of animal histology
which he could not clearly explain. His great teacher, Johannes
Muller, had called attention to the strange resemblance to
vegetable cells shown by certain cells of the chorda dorsalis
(the embryonic cord from which the spinal column is developed),
and Schwann himself had discovered a corresponding similarity in
the branchial cartilage of a tadpole. Then, too, the researches
of Friedrich Henle had shown that the particles that make up the
epidermis of animals are very cell-like in appearance. Indeed,
the cell-like character of certain animal tissues had come to be
matter of common note among students of minute anatomy. Schwann
felt that this similarity could not be mere coincidence, but he
had gained no clew to further insight until Schleiden called his
attention to the nucleus. Then at once he reasoned that if there
really is the correspondence between vegetable and animal tissues
that he suspected, and if the nucleus is so important in the
vegetable cell as Schleiden believed, the nucleus should also be
found in the ultimate particles of animal tissues.
Schwann’s researches soon showed the entire correctness of this
assumption. A closer study of animal tissues under the microscope
showed, particularly in the case of embryonic tissues, that
“opaque spots” such as Schleiden described are really to be found
there in abundance—forming, indeed, a most characteristic phase
of the structure. The location of these nuclei at comparatively
regular intervals suggested that they are found in definite
compartments of the tissue, as Schleiden had shown to be the case
with vegetables; indeed, the walls that separated such cell-like
compartments one from another were in some cases visible.
Particularly was this found to be the case with embryonic
tissues, and the study of these soon convinced Schwann that his
original surmise had been correct, and that all animal tissues
are in their incipiency composed of particles not unlike the
ultimate particles of vegetables in short, of what the botanists
termed cells. Adopting this name, Schwann propounded what soon
became famous as his cell theory, under title of Mikroskopische
Untersuchungen uber die Ubereinstimmung in der Structur und dent
Wachsthum der Thiere und Pflanzen. So expeditious had been his
work that this book was published early in 1839, only a few
months after the appearance of Schleiden’s paper.
As the title suggests, the main idea that actuated Schwann was to
unify vegetable and animal tissues. Accepting cell-structure as
the basis of all vegetable tissues, he sought to show that the
same is true of animal tissues, all the seeming diversities of
fibre being but the alteration and development of what were
originally simple cells. And by cell Schwann meant, as did
Schleiden also, what the word ordinarily implies—a cavity walled
in on all sides. He conceived that the ultimate constituents of
all tissues were really such minute cavities, the most important
part of which was the cell wall, with its associated nucleus. He
knew, indeed, that the cell might be filled with fluid contents,
but he regarded these as relatively subordinate in importance to
the wall itself. This, however, did not apply to the nucleus,
which was supposed to lie against the cell wall and in the
beginning to generate it. Subsequently the wall might grow so
rapidly as to dissociate itself from its contents, thus becoming
a hollow bubble or true cell; but the nucleus, as long as it
lasted, was supposed to continue in contact with the cell wall.
Schleiden had even supposed the nucleus to be a constituent part
of the wall, sometimes lying enclosed between two layers of its
substance, and Schwann quoted this view with seeming approval.
Schwann believed, however, that in the mature cell the nucleus
ceased to be functional and disappeared.
The main thesis as to the
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