Henry Van Dyke - "Golden Stars"

Anonymous - "History of Steam on the Erie Canal"

Louis Joseph Vance - "The Bronze Bell"

Marcus Tullius Ciceronis - "De Amicitia, Scipio's Dream"

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




We owe to Owen the first clear distinction between "homologous" and
"analogous" organs; it was he who first proposed the terms "homologue"
and "analogue," which he defined as follows:--"_Analogue_. A part or
organ in one animal which has the same function as another part or organ
in a different animal." "_Homologue_. The same organ in different
animals under every variety of form and function."[165]

He introduced also useful distinctions between Special, General, and
Serial Homology. "The relations of homology," he writes, "are of three
kinds: the first is that above defined, viz., the correspondency of a
part or organ, determined by its relative position and connections, with
a part or organ in a different animal; the determination of which
homology indicates that such animals are constructed on a common type;
when, for example, the correspondence of the basilar process of the
human occipital bone with the distinct bone called 'basi-occipital' in a
fish or crocodile is shown, the _special homology_ of that process is
determined. A higher relation of homology is that in which a part or
series of parts stands to the fundamental or general type, and its
enunciation involves and implies a knowledge of the type on which a
natural group of animals, the Vertebrate, for example, is constructed.
Thus when the basilar process of the human occipital bone is determined
to be the 'centrum' or 'body' of the last cranial vertebra, its _general
homology_ is enunciated.

"If it be admitted that the general type of the vertebrate endoskeleton
is rightly represented by the idea of a series of essentially similar
segments succeeding each other longitudinally from one end of the body
to the other, such segments being for the most part composed of pieces
similar in number and arrangement, and though sometimes extremely
modified for special functions, yet never so as to wholly mask their
typical character--then any given part of one segment may be repeated in
the rest of the series, just as one bone may be reproduced in the
skeletons of different species, and this kind of repetition or
representative relation in the segments of the same skeleton I call
'serial homology'" (p. 7). As an example of serial homology we might
take the centra of the vertebrae--the vomer, the presphenoid, the
basisphenoid, the basioccipital and the series of centra in the spinal
column. Such serially repeated parts are called _homotypes_ (p. 8).

Not all the bones of the vertebrate skeleton are included in the
archetype as constituents of the vertebrae. Thus the branchial and
pharyngeal arches are accounted part of the splanchnoskeleton, as
belonging to the same category as the heart bone of some ruminants, and
the ossicles of the stomach in the lobster (p. 70). The ossicles of the
ear in mammals are "peculiar mammalian productions in relation to the
exalted functions of a special organ of sense" (p. 140, f.n.). This
recognition of a possible development of new organs to meet new
functions shows unmistakably the influence of Cuvier. Owen was indeed
well aware of the importance of the functional aspect of living things,
and he often adopted the teleological point of view. As a true
morphologist, however, he held that the principle of adaptation does not
suffice to explain the existence of special homologies. The ossification
of the bones of the skull from separate centres may be purposive in
Eutheria, in that it prevents injury to the skull at birth; but how
explain on teleological principles the similar ossification from
separate centres in marsupials, birds and reptiles? How explain above
all the fact that the centres are the same in number and relative
position in all these groups? Surely we must accept the idea of an
archetype "on which it has pleased the divine Architect to build up
certain of his diversified living works" (p. 73).

In his study of centres of ossification, Owen made in point of theory a
distinct advance on his predecessors. We saw that Geoffroy recognised
the importance of studying the ossification of the skeleton, and that
Cuvier accepted such embryological evidence as an aid in determining
homologies. Owen pointed out that it was necessary to distinguish
between centres of ossification which were teleological in import and
such as were purely indicative of homological relationships. Many bones,
single in the adult, arise from separate centres of ossification, but we
must distinguish between "those centres of ossification that have
homological relations, and those that have only teleological ones;
_i.e._, between the separate points of ossification of a human bone
which typify vertebral elements, often permanently distinct bones in the
lower animals; and the separate points which, without such
signification, facilitate the progress of osteogeny, and have for their
obvious final cause the well-being of the growing animal" (p. 105).
There is, for example, a teleological reason why in mammals and leaping
Amphibia (_e.g._, frogs), the long bones should ossify first at their
ends, for the brain is thus protected from concussion; in reptiles that
creep there is less danger of concussion, and the long bones ossify in
the middle (p. 105). But there is no teleological reason why the
coracoid process of the scapula should in all mammals develop from a
separate centre. The coracoid is however a real vertebral element
(haemapophysis), and in monotremes, birds and reptiles it is in the adult
a large and separate bone. Its ossification from a separate centre in
mammals has therefore a homological significance. The scapula in mammals
is an example of what Owen calls a "homologically compound" bone. All
those bones which are formed by a coalescence of parts answering to
distinct elements of the typical vertebra are "homologically compound"
(p. 105). On the other hand, "All those bones which represent single
vertebral elements are 'teleologically compound' when developed from
more than one centre, whether such centres subsequently coalesce, or
remain distinct, or even become the subject of individual adaptive
modifications, with special joints, muscles, etc., for particular
offices" (p. 106). The limb-skeleton, corresponding as it does to a
single bone of the archetype, is the typical example of a teleologically
compound bone. Owen in his definition of teleological compoundness has
combined two kinds of adaptation--(1) temporary adaptation of bones to
the exigencies of development, birth and growth (_e.g._, development of
long bones from separate centres); (2) definitive adaptation of a
skeletal part to the functions which it has to perform (_e.g._,
teleological structure of limbs). Such adaptations are, so to speak,
grafted on the archetype.

Owen's general views on the nature of living things merit some
attention. Organic forms, according to Owen, result from the
antagonistic working of two principles, of which one brings about a
vegetative repetition of structure, while the other, a teleological
principle, shapes the living thing to its functions. The former
principle is illustrated in the archetype of the vertebrate skeleton, in
the segmentation of the Articulates, in the almost mathematical symmetry
of Echinoderms, and the actually crystalline spicules of sponges. It is
the same principle which causes repetition of the forms of crystals in
the inorganic world. "The repetition of similar segments in a vertebral
column, and of similar elements in a vertebral segment, is analogous to
the repetition of similar crystals as the result of polarising force in
the growth of an inorganic body" (p. 171). This "general polarising
force" it is which mainly produces the similarity of forms, the
repetition of parts, and generally the signs of the unity of
organisation. The adaptive or "special organising force" or [Greek:
idea], on the other hand, produces the diversity of organic beings. In
every species these two forces are at work, and the extent to which the
general polarising or "vegetative-repetition-force" is subdued by the
teleological is an index of the grade of the species.

This view is analogous to the Geoffroyan conception that the diversity
of form is limited by the unity of plan. Owen thus ranges himself with
Geoffroy against Cuvier, who considered that diversity of form is
limited only by the principle of the adaptation of parts.

[164] Owen introduced most of the names of bones now
current.

[165] _Lectures on Invertebrate Animals_, pp. 374, 379,
1843.




CHAPTER IX

KARL ERNST VON BAER


Von Baer was recognised as the founder of embryology even by his
contemporaries. His predecessors, Aristotle,[166] Fabricius,[167]
Harvey,[168] Malpighi,[169] Haller,[170] Wolff,[171] had made a
beginning with the study of development; von Baer, by the thoroughness
of his observation and the strength of his analysis, made embryology a
science.

It was to one of the German transcendentalists that von Baer owed the
impulse to study development. Ignatius Doellinger, Professor in Wuerzburg,
induced three of his pupils, Pander, d'Alton and von Baer, to devote
themselves to embryological research. The development of animals was at
this time little known, in spite of recent work by Meckel (1815 and
1817), Tiedemann (_Anatomie u. Bildungsgeschichte des Gehirns_, 1816),
by Oken (_loc. cit., supra_, p. 90), and some others.

Pander, with whom apparently Doellinger and d'Alton collaborated, was the
first to publish his results;[172] von Baer, who through absence from
Wuerzburg had for a time dropped his embryological studies, started to
work in 1819, after the publication of Pander's treatise, and produced
in 1828 the first volume of his master-work, _Ueber
Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion_
(Koenigsberg, 1828). The second volume followed in 1837, but dates really
from 1834, and was published in an incomplete form. This second volume
is intended as an introduction to embryology for the use of doctors and
science students. In it von Baer describes in full detail the
development of many vertebrate types--chick, tortoise, snake, lizard,
frog, fish, several mammals and man, basing his remarks largely upon his
personal observations, but taking account also of all contemporary work.
A separate account of the development of a fish (_Cyprinus blicca_)
appeared in 1835.[173]

We shall concentrate attention on the first volume. This volume contains
the first full and adequate account of the development of the chick,
followed by a masterly discussion of the laws of development in general.

When we consider that von Baer worked chiefly with a simple microscope
and dissecting needles, the minuteness and accuracy of his observations
are astonishing. He described the main facts respecting the development
of all the principal organs, and if, through lack of the proper means of
observation, he erred in detail, he made up for it by his masterly
understanding and profound analysis of the essential nature of
development. His account of the development of the chick is a model of
what a scientific memoir ought to be; the series of "Scholia" which
follow contain the deductions he made from the data, and, in so far as
they are direct generalisations from experience, they are valid for all
time.

The first Scholion is directed against the theory of preformation, and
succeeds in refuting it on the ground of simple observation. The theme
of the second Scholion is that the essential nature (_die Wesenheit_) of
the animal determines its differentiation, that no stage of development
is solely determined by the antecedent stage, but that throughout all
stages the _Wesenheit_ or idea of the definitive whole exercises
guidance. This guidance is shown most clearly in the regulatory
processes of the germ, whereby the large individual variations commonly
presented by the early embryo are compensated for or neutralised in the
course of further development. Baer in this shows himself a vitalist.

It is, however, the third and subsequent Scholia which must here
particularly occupy our attention, for it is in these that von Baer
comes to grips with morphological problems. Already in the second
Scholion he had definitely enunciated the law which runs as a theme
throughout the volume, the observational and the theoretical part alike,
the law that development is essentially a process of differentiation by
which the germ becomes ever more and more individualised. "The essential
result of development," he writes, "when we consider it as a whole, is
the increasing independence (_Selbstaendigkeit_) of the developing
animal" (p. 148). In the third Scholion he elaborates this thought and
shows that differentiation takes place in triple wise. The three
processes of differentiation are "primary differentiation" or
layer-formation, "histological differentiation" within the layers, and
the "morphological differentiation" of primitive organs.

The first of these differentiations in time is the formation of the
germ-layers, which takes place by a splitting or separation of the
blastoderm into a series of superimposed lamellae. Baer's account of the
process in the chick is as follows:--

"First of all, the germ separates out into heterogeneous layers, which
with advancing development acquire ever greater individuality, but even
on their first appearance show rudiments of the structures which will
characterise them later. Thus in the germ of the bird, so soon as it
acquires consistency at the beginning of incubation, we can distinguish
an upper smooth continuous surface and a lower more granular surface.
The blastoderm separates thereupon into two distinct layers, of which
the lower develops into the plastic body-parts of the embryo, the upper
into the animal parts; the lower shows clearly a further division into
two closely connected subsidiary layers--the mucous layer and the
vessel-layer; the original upper layer also shows a division into two,
which form respectively the skin and the parts which I have called the
true ventral and dorsal plates--parts which contain in an
undifferentiated state the skeletal and muscular systems, the connective
tissues, and the nerves belonging to these. In order to have a
convenient term for future use, I have named this layer the
muscle-layer" (p. 153).

The process of delamination results then in the formation of four
layers, of which the upper two (composing the "animal" or "serous"
layer) will give origin to the animal (neuromuscular) part of the body,
the lower pair to the plastic or vegetative organs. The uppermost layer
will form the external covering of the embryo, and also the amniotic
folds; from it there differentiates out at a very early stage the
rudiment of the central nervous system, forming a more or less
independent layer. Below the outermost layer lies the layer from which
are formed the muscular and skeletal systems, and beneath this
"muscle-layer" comes the "vessel-layer," which gives origin to the main
blood-vessels. The innermost layer of the four will form the mucous
membrane of the alimentary canal and its dependencies; at the present
stage, however, it is, like the other layers, a flat plate.

From all these layers tubes are developed by the simple bending round of
their edges. The outermost layer becomes the investing skin-tube of the
embryo; the layer for the nervous system forms the tubular rudiment of
the brain and spinal cord; the mucous layer curls round to form the
alimentary tube; the muscle layer grows upwards and downwards to form
the fleshy and osseous tube of the body wall; even the vessel layer
forms a tube investing the alimentary canal, but a part of it goes to
form the medial "Gekroese," or mesenterial complex, which departs
considerably from the tubular form.

When these tubes or "fundamental organs" are formed the process of
primary differentiation is complete. The fundamental organs, however,
have at no time actually the form of tubes; they exist as tubes only
ideally, for morphological and histological differentiation go on
concurrently with the process of primary differentiation.

Through morphological differentiation the various parts of the
fundamental organs become specialised, through unequal growth, first
into the primitive organs and then into the functional organs of the
body. "Single sections of the tubes originally formed from the layers
develop individual forms, which later acquire special functions: these
functions are in the most general way subordinate elements of the
function of the whole tube, but yet differ from the functions of other
sections. Thus the nerve-tube differentiates into sense-organs, brain
and spinal cord, the alimentary tube into mouth cavity, oesophagus,
stomach, intestine, respiratory apparatus, liver, bladder, etc. This
specialisation in development is bound up with increased or diminished
growth" (p. 155). Rapid growth concentrated at one point brings about an
evagination; in this manner are formed the sense-organs from the
nerve-tube, the liver and lungs from the alimentary tube. Or increased
growth over a section of a tube causes it to swell out; in this wise the
brain develops from the nerve-tube, the stomach from the alimentary
tube. The segmentation which soon becomes so marked, particularly in the
muscle layer, is also due to a process of morphological differentiation.

At the same time that the organs of the body are being thus roughly
blocked out and moulded from the germ-layers the third process of
differentiation is actively going on. "In addition to the
differentiation of the layers, there follows later another
differentiation in the substance of the layers, whereby cartilage,
muscle and nerve separate out, a part also of the mass becoming fluid
and entering the bloodstream" (p. 154). Through histological
differentiation the texture of the layers and incipient organs becomes
individualised. In its earliest appearance the germ consists of an
almost homogeneous mass, containing clear or dark globules suspended in
its substance (ii., p. 92). This homogeneity gives place to
heterogeneity; the structureless mass becomes fibrous to form muscles,
hardens to form cartilage or bone, becomes liquid to form the blood,
differentiates in a hundred other ways--into absorbing and secreting
tissues, into nerves and ganglia, and so forth. It will be noticed that
the concept of histological differentiation is independent of the
cell-theory; it signifies that textural differentiation which leads to
the formation of tissues in Bichat's sense. The tissues and the
germ-layers stand in fairly close relation with one another, for while
certain tissues are formed chiefly but not exclusively in one layer,
others are formed only in one layer and never elsewhere. For example,
peripheral nerves are for the most part formed in the muscle layer,
though the bulk of the nervous tissue is formed in the walls of the
nerve tube; similarly blood and blood-vessels may arise from almost any
layer, though their chief seat of origin is the vessel-layer; on the
other hand, bone is formed only in the muscle-layer (i., p. 155, ii.,
pp. 92-3).

This relation of tissue to germ-layer was more fully discussed and
brought into greater prominence by Remak, from the standpoint of the
cell-theory, and it will occupy us in a later chapter (Chap. XII.).

The fourth Scholion elaborates the analysis of developmental processes
still further, and discusses in particular the scheme of development
which is shown by the Vertebrata. The characteristic structure of the
vertebrate body is brought about by a "double symmetrical" rolling
together of the germ-layers, whereby two main tubes are formed, one
above and one below the axis of the body, which is the chorda. The
dorsal tube is formed by the two animal layers, the ventral tube by all
the layers combined (see Fig. 7).

The process is indicated with sufficient clearness in the diagram. It
will be seen that the real foundation and framework of the arrangement
is the muscle-layer, with its two tubes, one surrounding the central
nervous system and forming the "dorsal plates," the other surrounding
the body cavity and forming the "ventral plates." In the dorsal plates,
which early show metameric segmentation, the investing skeleton of the
neural axis develops; in the ventral plates are formed the ribs, the
ventral arches of the vertebrae, the hyoid, the lower jaw and other
skeletal structures.

The alimentary or "mucous" tube and the part of the vessel layer which
invests it become so closely bound up with one another as to form a
single primitive organ--the alimentary canal. The muscles of the
alimentary canal are accordingly in all probability developed in the
investing part of the vessel layer. From the "Gekroese," or remaining
part of the vessel layer develop the Wolffian bodies (_Urnieren_,
Pronephros), the kidneys, the sex glands, and the series of
"blood-glands"--suprarenals, thyroid, thymus and spleen. Baer did not
attach any special morphological significance to the peritoneal lining
of the body cavity, as is done in more modern forms of the germ-layer
theory. The gill-slits were largely formed by outgrowths from the
alimentary canal.

_a._ Chorda.
_b._ Dorsal plates.
_c._ Ventral plates.
_d._ Spinal cord.
_e._ Vessel-layer.
_f._ Alimentary tube.
_g._ Pronephros.
_h._ Skin.
_i._ Amnion.
_k._ Serous membrane.
_l._ Yolk-sac.

In his germ-layer theory von Baer was influenced a good deal by
Pander, to whom the actual discovery of the process of layer-formation
is due. Pander, however, had distinguished only three germ-layers, an
upper "serous" layer, a lower "mucous" layer and a middle
"vessel-layer." He it was who introduced the terms "Keimhaut"
(blastoderm) and "Keimblatt" (germ-layer).

[Illustration: FIG. 7.--Ideal Transverse Section of a Vertebrate Embryo.
(After von Baer.)]

The honour of being the founder of the germ-layer theory is sometimes
attributed to C. F. Wolff, notably by Koelliker and O. Hertwig. Wolff, it
is true, in his memoir _De formatione intestinorum_ (1768-9) showed that
the alimentary canal was first formed as a flat plate which folded round
to form a tube, and in a somewhat vaguely worded passage he hinted that
a similar mode of origin might be found to hold good for the other
organ-systems. But it seems clear that Wolff had no definite conception
of the process of layer-formation as the first and necessary step in all
differentiation. This, at any rate, was von Baer's opinion, who assigns
to Pander the glory of the discovery of the germ-layers. "You," he
writes, "through your clearer recognition of the splitting of the
germ--a process which remained dark to Wolff--have shed a light upon all
forms of development" (p. xxi.).

We have now seen, following von Baer's exposition, how development is
essentially a process of differentiation, a progress from the general to
the special, from the homogeneous to the heterogeneous; we have analysed
the process into its three subordinate processes--primary, histological
and morphological differentiation. So far we have considered development
in general and the laws which govern it; we have now to consider the
varieties of development which the animal kingdom offers in such
profusion, in order to discover what relations exist between them. This
is the problem set in the fifth Scholion. Baer at once brings us face to
face with the solution of the problem attempted in the Meckel-Serres
law. It is a generally received opinion, he writes, that the higher
animals repeat in their development the adult stages of the lower, and
this is held to be the essential law governing the relation of the
variety of development to the variety of adult form. This opinion arose
when there was little real knowledge of embryology; it threw light
indeed upon certain cases of monstrous development, but it was pushed
altogether too far. It complicated itself with a belief in a historical
evolution;--"People gradually learnt to think of the different animal
forms as developed one from another--and seemed, in some circles at
least, determined to forget that this metamorphosis could only be
conceptual" (p. 200). At the same time the theory of parallelism led men
to rehabilitate the outworn conception of the scale of beings, to
maintain that animals form one single series of increasing complexity, a
scale which the higher members must mount step by step in their
development--from which it followed that evolution, whether conceived as
an ideal or as an historical process, could take place only along one
line, could be only progressive or regressive. Not all the supporters of
the theory of parallelism held these extreme views, but conclusions of
this kind were natural and logical enough.

Von Baer had soon found in the course of his embryological studies that
the facts did not at all fit in with the doctrine of parallelism; the
developing chick, for example, was at a very early stage demonstrably a
Vertebrate, and did not recapitulate in its early stages the
organisation of a polyp, a worm or a mollusc. He had published his
doubts in 1823, but his final confutation of the theory of parallelism
is found in this Scholion.

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