Maria Antonia Field - "Chimes of Mission Bells"

Mary Belle Freeley - "Fair to Look Upon"

Frances (Fanny) Burney (Madame d'Arblay) - "Cecilia vol. 2"

Ben Field - "Exciting Adventures of Mister Robert Robin"

Annual Bibliography of Commonwealth Literature 2007
This paper argues that discourses of love in Ghanaian market literature for youth offer a view into complex negotiations of agency and empowerment. Drawing on Deborah Durham's notion of youth as "social `shifters'" and Francis Nyamnjoh's conception of the "interconnectedness" of agency, I take Ghanaian market literature as one specific case of how African literature for youth foregrounds questions of continuity and change as African societies enter into increasingly complex global relations. In this literature for youth, received notions of love, often constructed out of impressions from American pop and hip hop music, carry new notions of agency that compete with existing "domesticated" forms. Authors like Ike Tandoh and Evelyn Tay employ discourses of love to offer youth alternative avenues for empowerment in a context of socio-economic disenfranchizement. In a creative process of "straddling", this writing both reveals and reproduces the contradictions that obtain in youth configurations of agency.

Form and Function

E >> E. S. (Edward Stuart) Russell >> Form and Function




[224] "Ueber das aeussere und innere Skelet," Meckel's
_Archiv_, pp. 327-76, 1826.

[225] _Vergl. Entwick. d. Kopfes d. nackten Amphibien_ (p.
186).

[226] _Arch. f. mikr. Anat._, xi., Suppl., 1874.

[227] "Om Primordial-Craniet," _Foerhandlingar Skand.
Naturf. Moele_, Stockholm, 1842.

[228] Vol. I., General part, pub. 1844.

[229] _Entosphenoid_, Owen.

[230] _Zweiter Bericht zootom. Anstalt zu Wuerzburg_, 1849.

[231] _Zeits. f. wiss. Zool._, ii., pp. 281-91.

[232] Mueller's _Archiv_ for 1849, pp. 443-515.

[233] _Zeits. f. wiss Zool._, ix., 1858.

[234] _Entw. d. Wirbelthiere_, pp. 139-40, 1861.

[235] _Lectures on the Elements of Comparative Anatomy_.

[236] _On the Archetype of the Vertebrate Skeleton_, p. 5,
1848.

[237] _System der thierischen Morphologie_, Leipzig, 1853.




CHAPTER XI

THE CELL-THEORY.


With the founding of the cell-theory by Schwann in 1839 an important
step was taken in the analysis of the degrees of composition of the
animal body. Aristotle had distinguished three--the unorganised
material, itself compounded of the four primitive elements, earth and
water, air and fire, the homogeneous parts or tissues and the
heterogeneous parts or organs, and this conception was retained with
little change even to the days of Cuvier and von Baer. Those of the old
anatomists who speculated on the relations of organic elements to one
another were dominated by Aristotle's simple and profound
classification, and proposed schemes which differed from his only in
detail. Bichat enlarged and deepened the concept of tissue, but the
degree of composition below this was for him, as for all anatomists of
his time, a fibrous or pulpy "cellulosity," living, indeed, but showing
no uniform and elemental structure. It was Schwann's merit to interpose
between the tissue and the mere unorganised material a new element of
structure, the cell. And, as it happened, a few years before Schwann
published his cell-theory, Dujardin hinted at another degree of
composition which was later to take its place between the cell and the
chemical elements--sarcode or protoplasm.

As is well known, the concept of the cell arose first in botany. Robert
Hooke discovered cells in cork and pith in 1667, and his discovery was
followed up by Grew and Malpighi in 1671, and by Leeuenhoek in 1695. But
they did not conceive the cell as a living, independent, structural
unit. They were interested in the physiology of the plant as a whole,
how it lived and nourished itself, and they studied cells and
sieve-tubes, wood fibres and tracheae with a view rather to finding out
their functions and their significance for the life of the plant than to
discovering the minutiae of their structure. The same attitude was taken
up by the few botanists who in the 18th century paid any heed to the
microscopical anatomy of plants. For C. F. Wolff,[238] the formation of
cells was a result of the secretion of drops of sap in the fundamental
substance of the plant, this substance remaining as cell-walls when
cell-formation was completed--no idea here of cells as units of
structure.

In the early 19th century, interest in plant anatomy revived somewhat,
and much work was done by Treviranus, Mirbel, Moldenhawer, Meyen and von
Mohl.[239] As a result of their work the fact was established that the
tissues of plants are composed of elements which can, with few
exceptions, be reduced to one simple fundamental form--the spherical
closed cell. Thus the vessels of plants are formed by coalescence of
cells, fibres by the elongation of cells and the thickening and
toughening of their walls. At this time, interest was concentrated on
the cell-wall, to the almost total neglect of the cell-contents; the
"matured framework" of plant cells, to use Sach's convenient phrase, was
the chief, almost the sole, object of study. And it was natural enough
that the mere architecture of the plant should monopolise interest, that
the composition of the tissues out of the cells, and the fitting
together of the tissues to form the plant should awaken and hold the
curiosity of the investigator; even the modifications of the cell-walls
themselves, their rings and spiral thickenings and pits, offered a
fascinating field of enquiry.

The idea that the cell-contents might show a characteristic and
individual structure had hardly dawned upon botanists when Schleiden
published his famous paper, _Beitraege zur Phytogenesis_.[240] Schleiden's
theme in this paper is the origin and development of the plant cell, a
subject then very obscure, in spite of pioneer work by Mirbel. A few
years before, Robert Brown had called attention to the presence in the
epidermal cells of orchids and other plants of a characteristic spot
which he called the areola or nucleus.[241] Schleiden saw the importance
of this discovery, confirmed the constant presence of the nucleus in
young cells, and held it to be an elementary organ of the cell. He named
it the cytoblast because, in his opinion, it formed the cell. It was
embedded in a peculiar gummy substance, the cytoblastem, which formed a
lining to the cellulose cell-wall. Within the nucleus there was often a
small dark spot or sphere--the nucleolus. The nucleus, Schleiden
thought, originated as a minute granule in the cytoblastem which
gradually increased in size, becoming first a nucleolus (_Kernchen_),
and then, by further condensation of matter round it, a nucleus. Several
nuclei might be formed in this way in a single cell. New cells took
their origin directly from a full-grown nucleus, in a peculiar way which
Schleiden describes as follows:--"As soon as the cytoblasts have reached
their full size a delicate transparent vesicle arises on their surface;
this is the young cell, which at first takes the shape of a very flat
segment of a sphere, of which the plane surface is formed by the
cytoblast, the convex side by the young cell itself, which lies upon the
cytoblast like a watch-glass on a watch" (p. 145). The young cells
increase in size and fill up the cavity of the old cell, which is in
time resorbed. Cell-development always takes place within existing
cells, and either one or many new cells may be formed within the
mother-cell. Schleiden's views on cell-formation were drawn from some
rather imperfect observations on the embryo-sac and pollen-tube, but he
extended his theory to cell-formation in general. Though wrong in almost
all respects the theory had at least the merit of fixing attention upon
the really important constituents of the cell, the nucleus and the
cell-plasma. To Schleiden, too, we owe the conception of the cell as a
more or less independent living unity, whose life is not entirely
identified with the life of the plant as a whole. "Each cell," he
writes, "carries on a double life; one a quite independent and
self-contained life, the other a dependent life in so far as the cell
has become an integral part of the plant" (p. 138).

So long as the definition of the plant cell embraced little more than
the hardened cell-wall it was little wonder that "cells" in this sense
were not recognised in animal tissues, except in a few exceptional
cases--as in the notochord by Johannes Mueller.[242] Careful observation of
animal tissues discovered in some cases the existence of discontinuous
units of structure, but these were not, as a rule, recognised before
1838 as analogous to plant cells. Von Baer, for example, observed that
the young chick embryo was composed partly of an albuminous mass and
partly of _Kuegelchen_ or little globules suspended in it
(_Entwickelungsgeschichte_, i., pp. 19, 144). Since such _Kuegelchen_
disposed in a row formed the notochord (i., p. 145) it seems probable
that his _Kuegelchen_ were really cells. Similarly A. de Quatrefages[243]
in 1834 saw and figured segmentation spheres in the developing egg of
_Limnaea_, but he called them globules and did not recognise their
analogy with the cells of plants. According to M'Kendrick,[244] Fontana,
so far back as 1781,[245] described cells with nuclei in various tissues,
and used acids and alkalis to bring out their structure more clearly.
But it was not till 1836-7-8 that a fairly widespread occurrence of
cells in animal tissues was recognised. The pioneer in this seems to
have been Purkinje, who described cells in the choroidal plexus in
1836,[246] and compared gland cells with the cells of plants in 1837.[247]
Henle in 1837[248] and 1838[249] described various kinds of epithelial
tissue, distinguishing them according to the kind of cell composing
them; he also discovered the mode of growth of stratified epithelium.
Valentin[250] appears to have seen cells in cartilage and epithelium even
before Henle, and to have observed cells in the blastoderm of the chick.
In his report on the progress of anatomy during 1838 Johannes Mueller was
able to refer to quite a number of papers dealing with the occurrence of
cells in animal tissues. In addition to those already noted, he mentions
work by Breschet and Gluge on the cells of the umbilical cord, by
Dumortier on the cells in the liver of molluscs, by Remak and by
Purkinje on nerve cells, by Donne on the cells of the conjuctiva, cornea
and lens. He reports, too, that Turpin had compared the epithelial cells
of the vagina with the cell-tissue of plants. Mueller himself had not
only recognised the cellular nature of the notochord, but had observed
the cells of the vitreous humour, fat cells and pigment cells, and even
the nuclei of cartilage cells. From Schwann (1839) we learn that C. H.
Schults had followed back the corpuscles of the blood to their original
state of nucleated cells, and that Werneck had recognised cells in the
embryonic lens. A preliminary notice of Schwann's own work appeared in
1838 (Froriep's _Notizen_, No. 91, 1838), the full memoir in 1839, under
the title _Mikroskopische Untersuchungen ueber die Uebereinstimmung in
der Struktur und dem Wachstume der Tiere und Pflanzen_.[251]

Theodor Schwann was a pupil of Johannes Mueller, and we know that Mueller
took much interest in the new histology. It is probably to his influence
that we owe Schwann's brilliant work on the cell, which appeared just
after Schwann left Berlin for Loewen. Schwann was himself, as his later
work showed, more a physiologist than a morphologist; he did quite
fundamental work on enzymes, discovering and isolating the pepsin of the
gastric juice; he proved that yeast was not an inorganic precipitate but
a mass of living cells; he carried out experiments directed to show that
spontaneous generation does not occur. We shall see in his treatment of
the cell-theory clear indications of his physiological turn of mind.
Schwann was only twenty-nine when his master-work appeared, and the book
is clearly the work of a young man. It has the clear structure, the
logical finish, which the energy of youth imparts to its chosen work. So
the work of Rathke's prime, the _Anatomische-philosophische
Untersuchungen_ of 1832 shows more vigour and a more reasoned structure
than his later papers. Schwann's book is indeed a model of construction
and cumulative argument, and even for this reason alone justly deserves
to rank as a classic.

The first section of his book is devoted to a detailed study of the
structure and development of cartilage cells and of the cells of the
notochord, and to a comparison of these with plant cells. He accepts
Schleiden's account of the origin and development of nuclei and cells as
a standard of comparison; and he seeks to show that nucleus and
nucleolus, cell-wall and cell-contents, show the same relations and
behave in the same manner in these two types of animal cells as in the
plant-cells studied by Schleiden. The types of cell which he chose for
this comparison are the most plant-like of all animal cells, and he was
even able to point to a thickening of the cell-wall in certain cartilage
cells, analogous to the thickening which plays so important a part in
the outward modification of plant-cells. The analogy indeed in structure
and development between chorda and cartilage cells and the cells of
plants seemed to him complete. The substance of the notochord consisted
of polyhedral cells having attached to their wall an oval disc similar
in all respects to the nucleus of the plant-cell, and like it containing
one or more nucleoli. Inside the mother-cell were to be found young
developing cells of spherical shape, lacking however a nucleus.
Cartilage was even more like plant tissue. It was composed of cells,
each with its cell membrane. The cells lay close to one another,
separated only by their thickened cell-wall and the intercellular
matrix, showing thus even the general appearance of the cellular tissue
of plants. They contained a nucleus with one or two nucleoli, and the
nucleus was often resorbed, as in plants, when the cell reached its full
development. Other nuclei were in many cases present in the cell, round
which young cells could be seen to develop, in exactly the same manner
as in plants. These nuclei had accordingly the same significance as the
nuclei of plants, and deserved the same name of cytoblasts or
cell-generators. The true nucleus of the cartilage cell was probably in
the same way the original generator of the mother-cell.

Having proved the identity in structure and function of the cells of
these selected tissues with the cells of plants, as conceived by
Schleiden, Schwann had still to show that the generality of animal
tissues consisted either in their adult or in their embryonic state of
similar cells. This demonstration occupies the second and longest
section of his book.

His method is throughout genetic; he seeks to show, not so much that all
animal tissues are actually in their finished state composed of cells
and modifications of cells, as that all tissues, even the most complex,
are developed from cells analogous in structure and growth with the
cells of plants.

All animals develop from an ovum; it was his first task to discover
whether the ovum was or was not a cell. It happened that, some years
before Schwann wrote, a good deal of work had been done on the minute
structure of the ovum, particularly by Purkinje and von Baer. Purkinje
in 1825[252] discovered and described in the unfertilised egg of the fowl
a small vesicle containing granular matter, which he named the
_Keimblaeschen_ or germinal vesicle. It disappeared in the fertilised
egg. As early as 1791 Poli had seen the germinal vesicle in the eggs of
molluscs, but the first adequate account was given by Purkinje. In
1827[253] von Baer discovered the true ova of mammals and cleared up a
point which had been a stumbling block ever since the days of von Graaf,
who had described as the ova the follicles now bearing his name.[254] Even
von Graaf had noticed that the early uterine eggs were smaller than the
supposed ovarian eggs; Prevost and Dumas[255] had observed the presence in
the Graafian follicle of a minute spherical body, which, however, they
hesitated to call the ovum; it was left to von Baer to elucidate the
structure of the follicle and to prove that this small sphere was indeed
the mammalian ovum. His discovery was confirmed by Sharpey and by Allen
Thomson. Von Baer found the germinal vesicle in the eggs of frogs,
snakes, molluscs, and worms, but not in the mammalian ovum; he
considered the whole mammalian ovum to be the equivalent of the germinal
vesicle of birds--a comparison rightly questioned by Purkinje (1834). In
1834 Coste[256] discovered in the ovum of the rabbit a vesicle which he
considered to be the germinal vesicle of Purkinje; he observed that it
disappeared after fertilisation. Independently of Coste, and very little
time after him, Wharton Jones[257] found the germinal vesicle in the
mammalian ovum. Valentin in 1835,[258] Wagner in 1836,[259] and Krause in
1837,[260] added considerably to the existing knowledge of the structure
of the ovum. Wagner in his _Prodromus_ called attention to the
widespread occurrence, within the germinal vesicle of a darker speck
which he called the _Keimfleck_ or germinal spot, known sometimes as
Wagner's spot. He recognised the _Keimfleck_ in the ova of many classes
of animals from mammals to polyps. Frequently more than one _Keimfleck_
occurred.

Schwann had therefore a good deal of exact knowledge to go upon in
discussing the significance of the ovum for the cell-theory. There were
two possible interpretations. Either the ovum was a cell and the
germinal vesicle its nucleus, or else the germinal vesicle was itself a
cell within the larger cell of the ovum and the germinal spot was its
nucleus. Schwann had some difficulty in deciding which of these views to
adopt, but he finally inclined to the view that the ovum is a cell and
the germinal vesicle its nucleus, basing his opinion largely upon
observations by Wagner which tended to prove that the germinal vesicle
was formed first and the ovum subsequently formed round it. But the ovum
was not, in Schwann's view, a simple cell, for within it were contained
yolk-granules, one set apparently containing a nucleus, the others not.
Even the second set, those composing the yellow yolk, were considered by
Schwann to deserve the name of cells, because, although a nucleus could
not be observed in them, they had a definite membrane, distinct from
their contents--a conception of the cell obviously dating from the
earliest botanical notions of cells as little sacs. The yolk cells were
not mere dead food material but living units which took part in the
subsequent development of the egg. The relation between the unfertilised
egg and the blastoderm which arises from it is not made altogether clear
by Schwann. According to his account the cells of the blastoderm are
formed actually in the ovum. Round the nucleus of the egg appears a
_Niederschlag_ or precipitate which is the rudiment of the blastoderm
(p. 68). When the egg leaves the ovary the nucleus disappears, leaving
behind it this rudiment of the blastoderm, which rapidly grows and
increases in size. The blastoderm of the chick before incubation is
found to be composed of spherical anucleate bodies which Schwann
considers to be cells, because they almost certainly develop into the
cells of the incubated blastoderm, which are clearly recognisable as
such after eight hours' incubation. The serous and mucous layers can be
distinguished after sixteen hours' incubation, and it is found that the
cells of the serous layer contain definite nuclei, though such seem to
be absent in the cells of the mucous layer. Between the two layers other
cells are formed belonging to the vessel layer, which is, however, in
Schwann's opinion not a very definitely individualised layer.

Schwann's next step is a detailed demonstration of the origin of each
tissue from simple cells such as those composing the incubated
blastoderm.

"The foregoing investigation has taught us that the whole ovum shows
nothing but a continual formation and differentiation of cells, from the
moment of its appearance up to the time when, through the development of
the serous and mucous layers of the blastoderm, the foundation is given
for all the tissues subsequently appearing: we have found this common
parent of all tissues itself to consist of cells; our next task must be
to demonstrate not only in this general way that tissues originate from
cells, but also that the special formative mass of each tissue is
composed of cells, and that all tissues are either constituted by simple
cells or by one or other of the manifold kinds of modified cells" (p.
71). Five classes of tissue can be distinguished, according to the
extent and manner of the modifications which the cells composing them
have undergone. There are first of all independent and isolated cells,
such as the corpuscles of the blood and lymph, not forming a coherent
tissue in the ordinary sense. Next there are the assemblages of cells
lying in contiguity with one another, but not in any way fused; examples
of this class are the epidermal tissues and the lens of the eye. In the
third class come tissues the cells of which have fused by their walls,
but whose cell-cavities are not in continuity, such as osseous tissue
and cartilage. In the tissues of the fourth class, comprising the most
highly specialised of all, not only are the cell-walls continuous but
also the cell-cavities; to this class belong muscle, nerve and capillary
vessels. A fifth class, of rather a special nature, includes the fibrous
tissues of all kinds. This is the first classification of tissues upon a
cellular basis, and it marks the foundation of a new histology which
took the place of the "general anatomy" of Bichat. The exhaustive
account which Schwann gives of the structure and development of the
tissues in this section of his book constitutes the first systematic
treatise on histology in the modern sense, and it is still worth
reading, in spite of many errors in detail.

Schwann found it easy to demonstrate the cellular nature of the tissues
of his first three classes. With the other two classes he had more
difficulty. Fibres of all kinds, he considered, arose by an elongation
of cells, which afterwards split longitudinally into long strips,
forming as the case might be white or elastic fibrous tissue.
Muscle-fibres and nerve-fibres were formed in a totally different way,
by coalescence of cells; each separate muscle-fibre and nerve-fibre was
thus a compound cell. Capillaries, Schwann held, were formed by cells
hollowed out like drain-pipes, and set end to end--a mistaken view soon
corrected by Vogt (_Embryologie des Salmones_, p. 206, 1842).

In this detail part of his book Schwann accumulates material for a
general theory of the cell which he develops in the third and last
section. Taking up the physiological or dynamical standpoint, he points
out that one process is common to all growth and development of tissues
both in animals and plants, namely, the formation of cells, a process
which he conceives to take place in the following manner. There is,
first of all, a structureless substance, the cytoblastem, the matrix in
which all cells originate. The cytoblastem may be either inside the
cells, or, more usually, in the spaces between them. It is not a
substance of definite chemical and physical properties, for the matrix
of cartilage and the plasma of the blood alike come within the
definition. It has largely the significance of food material for the
developing cells. In plants, according to Schleiden, cells are never
formed in the intercellular substance--the cytoblastem is within the
cells; but extracellular cell formation seems to be the general rule in
animals. An intracellular formation of cells occurs only in the ovum, in
cartilage cells and chorda cells and in a few others, and even there it
is not the exclusive method of formation; a formation of cells within
cells never occurs in muscles and nerves, nor in fibrous tissue (p.
204). In the cytoblastem granules appear, which gradually increase in
size and take on the characteristic shape of nuclei; round each of these
a young cell is formed. Sometimes the young cells appear to have no
nuclei, as in the intracellular brood of chorda cells, but, as a rule, a
nucleus is clearly visible. The nucleus is indeed the most
characteristic constituent of the cell. "The most important and most
constant criterion of the existence of a cell is the presence or absence
of the nucleus," writes Schwann near the beginning of his book (p. 43).

As a general rule the nucleolus is formed first, and round it by a sort
of condensation or concretion the nucleus, which is frequently hollow,
and round this again, by a somewhat similar process, the cell. "The
whole process of the formation of a cell consists in the precipitation
round a small previously formed corpuscle (the nucleolus) of first one
layer (the nucleus) and then later round this a second layer (the cell
substance)" (p. 213). The outermost layer of the cell usually thickens
to form the membrane, but this membrane formation does not always occur,
and the membrane is not present in all cells. The nucleus is formed in
exactly the same manner as the cell, and it might with much truth itself
be called a cell--a cell of the first order, while ordinary nucleated
cells might be designated cells of the second order (p. 212). In
anucleate cells there is probably only a single process of layer
formation round an infinitely small nucleolus. In almost all nucleate
cells the nucleus is resorbed when the cell reaches its full
development, and it is larger and more important the younger the cell
is.

The cell was for Schwann not a morphological concept at all, but a
physiological; the cell was a dynamical, not a statical unit.
Cell-formation was the process at the back of all production of life,
and cells were the centres of all vital activity. Each cell was itself
an organism, and its life and activities were to some extent independent
of the lives and activities of all the other cells. The multicellular
organism was a colony of unicellular organisms, and its life was a sum
of the lives of its constituent elements. This "theory of the organism,"
which holds so important a place in biology even at the present day, is
developed by Schwann in the concluding pages of his book.

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