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

similarity of development of vegetable

and animal tissues and the cellular nature of the ultimate

constitution of both was supported by a mass of carefully

gathered evidence which a multitude of microscopists at once

confirmed, so Schwann’s work became a classic almost from the

moment of its publication. Of course various other workers at

once disputed Schwann’s claim to priority of discovery, in

particular the English microscopist Valentin, who asserted, not

without some show of justice, that he was working closely along

the same lines. Put so, for that matter, were numerous others,

as Henle, Turpin, Du-mortier, Purkinje, and Muller, all of whom

Schwann himself had quoted. Moreover, there were various

physiologists who earlier than any of these had foreshadowed the

cell theory—notably Kaspar Friedrich Wolff, towards the close of

the previous century, and Treviranus about 1807, But, as we have

seen in so many other departments of science, it is one thing to

foreshadow a discovery, it is quite another to give it full

expression and make it germinal of other discoveries. And when

Schwann put forward the explicit claim that “there is one

universal principle of development for the elementary parts, of

organisms, however different, and this principle is the formation

of cells,” he enunciated a doctrine which was for all practical

purposes absolutely new and opened up a novel field for the

microscopist to enter. A most important era in physiology dates

from the publication of his book in 1839.

THE CELL THEORY ELABORATED

That Schwann should have gone to embryonic tissues for the

establishment of his ideas was no doubt due very largely to the

influence of the great Russian Karl Ernst von Baer, who about ten

years earlier had published the first part of his celebrated work

on embryology, and whose ideas were rapidly gaining ground,

thanks largely to the advocacy of a few men, notably Johannes

Muller, in Germany, and William B. Carpenter, in England, and to

the fact that the improved microscope had made minute anatomy

popular. Schwann’s researches made it plain that the best field

for the study of the animal cell is here, and a host of explorers

entered the field. The result of their observations was, in the

main, to confirm the claims of Schwann as to the universal

prevalence of the cell. The long-current idea that animal tissues

grow only as a sort of deposit from the blood-vessels was now

discarded, and the fact of so-called plantlike growth of animal

cells, for which Schwann contended, was universally accepted. Yet

the full measure of the affinity between the two classes of cells

was not for some time generally apprehended.

 

Indeed, since the substance that composes the cell walls of

plants is manifestly very different from the limiting membrane of

the animal cell, it was natural, so long as the, wall was

considered the most essential part of the structure, that the

divergence between the two classes of cells should seem very

pronounced. And for a time this was the conception of the matter

that was uniformly accepted. But as time went on many observers

had their attention called to the peculiar characteristics of the

contents of the cell, and were led to ask themselves whether

these might not be more important than had been supposed. In

particular, Dr. Hugo von Mohl, professor of botany in the

University of Tubingen, in the course of his exhaustive studies

of the vegetable cell, was impressed with the peculiar and

characteristic appearance of the cell contents. He observed

universally within the cell “an opaque, viscid fluid, having

granules intermingled in it,” which made up the main substance of

the cell, and which particularly impressed him because under

certain conditions it could be seen to be actively in motion, its

parts separated into filamentous streams.

 

Von Mohl called attention to the fact that this motion of the

cell contents had been observed as long ago as 1774 by

Bonaventura Corti, and rediscovered in 1807 by Treviranus, and

that these observers had described the phenomenon under the “most

unsuitable name of ‘rotation of the cell sap.’ Von Mohl

recognized that the streaming substance was something quite

different from sap. He asserted that the nucleus of the cell lies

within this substance and not attached to the cell wall as

Schleiden had contended. He saw, too, that the chlorophyl

granules, and all other of the cell contents, are incorporated

with the “opaque, viscid fluid,” and in 1846 he had become so

impressed with the importance of this universal cell substance

that be gave it the name of protoplasm. Yet in so doing he had no

intention of subordinating the cell wall. The fact that Payen, in

1844, had demonstrated that the cell walls of all vegetables,

high or low, are composed largely of one substance, cellulose,

tended to strengthen the position of the cell wall as the really

essential structure, of which the protoplasmic contents were only

subsidiary products.

 

Meantime, however, the students of animal histology were more and

more impressed with the seeming preponderance of cell contents

over cell walls in the tissues they studied. They, too, found

the cell to be filled with a viscid, slimy fluid capable of

motion. To this Dujardin gave the name of sarcode. Presently it

came to be known, through the labors of Kolliker, Nageli,

Bischoff, and various others, that there are numerous lower forms

of animal life which seem to be composed of this sarcode, without

any cell wall whatever. The same thing seemed to be true of

certain cells of higher organisms, as the blood corpuscles.

Particularly in the case of cells that change their shape

markedly, moving about in consequence of the streaming of their

sarcode, did it seem certain that no cell wall is present, or

that, if present, its role must be insignificant.

 

And so histologists came to question whether, after all, the cell

contents rather than the enclosing wall must not be the really

essential structure, and the weight of increasing observations

finally left no escape from the conclusion that such is really

the case. But attention being thus focalized on the cell

contents, it was at once apparent that there is a far closer

similarity between the ultimate particles of vegetables and those

of animals than had been supposed. Cellulose and animal membrane

being now regarded as more by-products, the way was clear for the

recognition of the fact that vegetable protoplasm and animal

sarcode are marvellously similar in appearance and general

properties. The closer the observation the more striking seemed

this similarity; and finally, about 1860, it was demonstrated by

Heinrich de Bary and by Max Schultze that the two are to all

intents and purposes identical. Even earlier Remak had reached a

similar conclusion, and applied Von Mohl’s word protoplasm to

animal cell contents, and now this application soon became

universal. Thenceforth this protoplasm was to assume the utmost

importance in the physiological world, being recognized as the

universal “physical basis of life,” vegetable and animal alike.

This amounted to the logical extension and culmination of

Schwann’s doctrine as to the similarity of development of the two

animate kingdoms. Yet at the, same time it was in effect the

banishment of the cell that Schwann had defined. The word cell

was retained, it is true, but it no longer signified a minute

cavity. It now implied, as Schultze defined it, “a small mass of

protoplasm endowed with the attributes of life.” This definition

was destined presently to meet with yet another modification, as

we shall see; but the conception of the protoplasmic mass as the

essential ultimate structure, which might or might not surround

itself with a protective covering, was a permanent addition to

physiological knowledge. The earlier idea had, in effect,

declared the shell the most important part of the egg; this

developed view assigned to the yolk its true position.

 

In one other important regard the theory of Schleiden and Schwann

now became modified. This referred to the origin of the cell.

Schwann had regarded cell growth as a kind of crystallization,

beginning with the deposit of a nucleus about a granule in the

intercellular substance—the cytoblastema, as Schleiden called

it. But Von Mohl, as early as 1835, had called attention to the

formation of new vegetable cells through the division of a

pre-existing cell. Ehrenberg, another high authority of the time,

contended that no such division occurs, and the matter was still

in dispute when Schleiden came forward with his discovery of

so-called free cell-formation within the parent cell, and this

for a long time diverted attention from the process of division

which Von Mohl had described. All manner of schemes of

cell-formation were put forward during the ensuing years by a

multitude of observers, and gained currency notwithstanding Von

Mohl’s reiterated contention that there are really but two ways

in which the formation of new cells takes place—namely, “first,

through division of older cells; secondly, through the formation

of secondary cells lying free in the cavity of a cell.”

 

But gradually the researches of such accurate observers as Unger,

Nageli, Kolliker, Reichart, and Remak tended to confirm the

opinion of Von Mohl that cells spring only from cells, and

finally Rudolf Virchow brought the matter to demonstration about

1860. His Omnis cellula e cellula became from that time one of

the accepted data of physiology. This was supplemented a little

later by Fleming’s Omnis nucleus e nucleo, when still more

refined methods of observation had shown that the part of the

cell which always first undergoes change preparatory to new

cell-formation is the all-essential nucleus. Thus the nucleus was

restored to the important position which Schwann and Schleiden

had given it, but with greatly altered significance. Instead of

being a structure generated de novo from non-cellular substance,

and disappearing as soon as its function of cell-formation was

accomplished, the nucleus was now known as the central and

permanent feature of every cell, indestructible while the cell

lives, itself the division-product of a pre-existing nucleus, and

the parent, by division of its substance, of other generations of

nuclei. The word cell received a final definition as “a small

mass of protoplasm supplied with a nucleus.”

 

In this widened and culminating general view of the cell theory

it became clear that every animate organism, animal or vegetable,

is but a cluster of nucleated cells, all of which, in each

individual case, are the direct descendants of a single

primordial cell of the ovum. In the developed individuals of

higher organisms the successive generations of cells become

marvellously diversified in form and in specific functions; there

is a wonderful division of labor, special functions being chiefly

relegated to definite groups of cells; but from first to last

there is no function developed that is not present, in a

primitive way, in every cell, however isolated; nor does the

developed cell, however specialized, ever forget altogether any

one of its primordial functions or capacities. All physiology,

then, properly interpreted, becomes merely a study of cellular

activities; and the development of the cell theory takes its

place as the great central generalization in physiology of the

nineteenth century. Something of the later developments of this

theory we shall see in another connection.

ANIMAL CHEMISTRY

Just at the time when the microscope was opening up the paths

that were to lead to the wonderful cell theory, another novel

line of interrogation of the living organism was being put

forward by a different set of observers. Two great schools of

physiological chemistry had arisen—one under guidance of Liebig

and Wohler, in Germany, the other dominated by the great French

master Jean Baptiste Dumas. Liebig had at one time contemplated

the study of medicine, and Dumas had achieved distinction in

connection with Prevost, at Geneva, in the field of pure

physiology before he turned his attention especially to

chemistry. Both these masters, therefore, and Wohler as well,

found absorbing interest in those phases of chemistry that