A History of Science, vol 4 by Henry Smith Williams (best novels to read for beginners txt) 📖

and though Dr. Thomas Young and various

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