Jessie Graham Flower - "Grace Harlowe's Problem"

Sewell Peaslee Wright - "The Infra Medians"

Samuel Adams - "The Writings of Samuel Adams, volume II (1770 1773)"

Sewell Ford - "Torchy"

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




[175] Compare a parallel passage in Prevost et Dumas:--"At
the very first sight one will be struck with the
resemblance between the forms of the very early embryos
of these two classes, a resemblance so extraordinary
that one cannot refuse to admit the conclusions
resulting from it. The resemblance is so striking that
one can defy the most experienced observer to
distinguish in any way the embryos of dog or rabbit ...
from those of fowls or ducks of a corresponding
age."--_Ann. Sci. nat._, iii., p. 132, 1824.

[176] _De l'organisation des Animaux_, i., p. 140, 1822.

[177] "Ueber das aeussere und innere Skelet," Meckel's
_Archiv fuer Anat. u. Physiol._, pp. 327-76, 1826. See,
too, his _Entwickelungsgeschichte_, i., pp. 181, ff.

[178] Von Baer wrote an appreciative biography of Cuvier,
published posthumously in 1897, _Lebensgeschichte
Cuviers_, ed. L. Stieda. French trans. in _Ann. Sci.
Nat._ (_Zool._), ix., 1907.

[179] Cuvier et Valenciennes, _Histoire naturelle des
Poissons_, i., p. 550.

[180] _Mem. Mus. d'Hist. Nat._, iii., pp. 98-119, 1817.

[181] _Lecons d'Anatomie comparee_, 3rd ed., vol. i., p.
414, Bruxelles, 1836.

[182] In the aforementioned paper in Mueller's _Archiv_ he
criticises Carus vigorously and is sarcastic on
Geoffroy.




CHAPTER X

THE EMBRYOLOGICAL CRITERION


Pander's work of 1817 was the forerunner of an embryological period in
which men's hopes and interest centred round the study of development.
"With bewilderment we saw ourselves transported to the strange soil of a
new world," wrote Pander, and many shared his hopeful enthusiasm. K. E.
von Baer's _Entwickelungsgeschichte_ was by far the greatest product of
this time, but it stands in a measure apart; we have in this chapter to
consider the lesser men who were Baer's contemporaries, friends,
followers or critics.

It was largely a German science, this new embryology, and its leaders
were all personally acquainted. Pander, von Baer and Rathke were on
friendly terms with one another; von Baer dedicated his master-work to
Pander; Rathke dedicated the second volume of his _Abhandlungen_ to von
Baer. Interest in the new science was, however, not confined to Germany.
In Italy, Rusconi commenced in 1817 his pioneer researches on the
development of the Amphibia with a _Descrizione anatomica degli organi
della circolazione delle larve delle Salamandre aquatiche_ (Pavia), in
which he traced the metamorphoses of the aortic arches. This was
followed in 1822 by his _Amours des Salamandres aquatiques_ (Milan), and
in 1826 by his memoir _Du developpement de la grenouille_ (Milan). In
this last paper he described how the dark upper hemisphere of the frog's
egg grows down over the lower white hemisphere and leaves free only the
yolk plug; he observed the segmentation cavity and the archenteron, but
thought that the former became the alimentary canal; he observed and
interpreted rightly the formation of the medullary folds. The circular
blastopore in the frog in later years often went by the name of the anus
of Rusconi.

In France Dutrochet[183] investigated the foetal membranes in various
vertebrate classes; Prevost and Dumas studied the very earliest stages
of development in birds, mammals and amphibia (_Ann. Sci. nat._, ii.,
iii., 1824, xii., 1827).

A little later came Duges' studies of the osteology and myology of
developing amphibia (1834),[184] and Coste's careful researches into the
early developmental history of mammals.[185]

[Illustration: FIG. 8.--Gill-slits of the Pig Embryo. (After Rathke.)]

It was in 1825 that Heinrich Rathke (1793-1860), published his famous
discovery of gill-slits in the embryo of a mammal,[186] a discovery which
aroused considerable interest, and greatly stimulated embryological
research. He describes how in a young embryo of a pig he saw four slits
in the region of the neck, going right through into the oesophagus. They
were separated by partitions which he called _Kiemenbogen_
(gill-arches), and immediately in front of the first gill-slit lay the
developing lower jaw. He compared these gill-slits with those of a
dogfish. We reproduce his drawing of the pig-embryo (_Isis_, Pl. IV.,
fig. 1).

Later in the same year Rathke discovered gill-slits in the chick,[187] in
this case finding only three. He described growing out from in front of
the first slit a structure which he compared to the operculum or
gill-cover of a fish.

These discoveries were confirmed and extended for the chick[188] by the
embryologist Huschke, a pupil of Oken. Like Rathke, he found only three
indubitable gill-slits, but he noticed that the body-wall in front of
the first gill-slit was really composed of two arches, which were on the
whole similar to the gill-arches. The hinder of these two seemed to him
to be a horn of the hyoid, the front one, which was bent at an angle, to
be the rudiment of the upper and lower jaws (p. 401). Between these two
arches he found an opening, just as between two gill-arches a gill-slit.
This opening led into the mouth-cavity, and according to Huschke it
became the external ear-passage. He discovered also three pairs of
aortic arches in close relation with the gill-arches, so close indeed,
that he did not hesitate to call them gill-arteries, and to recognise
their resemblance with the aortic arches of fish. He traced, in part at
least, the metamorphosis which these aortic arches undergo. This part of
his discovery he developed in fuller detail in a paper of 1828,[189] in
which he gave some excellent figures.

Shortly after Huschke's first paper, von Baer published his views and
observations on this subject in a short memoir in Meckel's _Archiv_.[190]
In this paper he confirmed Rathke's discovery, and described the slits
and arches in the dog and the chick. Both Rathke and he found gill-slits
in the human embryo about this time (p. 557). There were generally
present, he found, four gill-slits, and, as Rathke had suggested, the
first gill-arch became the lower jaw. Von Baer also confirmed Rathke's
discovery of the operculum, assigning it, however, to the second
gill-arch. He refused to accept Huschke's derivation of the auditory
meatus from the first gill-slit. Von Baer saw what had escaped Rathke
and Huschke, that there were, not three nor four, but as many as five
aortic arches.

In his view of the metamorphosis of the aortic arches in the chick the
first two pairs disappeared completely, the third pair gave rise to the
arteries of the head and the fore-limbs, the right side of the fourth
arch became the aorta, the left half of the fourth and the right half of
the fifth arch became the pulmonary arteries, while the left half of the
fifth arch disappeared. This schema, which for the last three arches was
the same as Huschke's, von Baer upheld for the chick even in the second
volume of his _Entwickelungsgeschichte_ (p. 116); he rectified it,
however, for mammals in the same volume (p. 212), deriving both
pulmonary arteries from the fifth arch, and the aorta from the fourth
left. He fully recognised the great analogy of the embryonic arrangement
of gill-arches and gill-arteries in Tetrapoda with their arrangement in
fish (i., pp. 53, 73).

Huschke, in a paper of 1832,[191] chiefly devoted to the development of
the eye, figured and described the developing upper and lower jaws, and
maintained against von Baer that the first slit turns into the auditory
meatus and the Eustachian tube.

These were the first papers of the embryological period. Before going on
to discuss the principles which guided embryological research during the
next ten or twenty years it is convenient to note what were the main
lines of work characterising the period.

The typical figure of the period is Rathke, who produced a great deal of
first-class embryological work. He was, even more than von Baer, a
comparative embryologist, and there were few groups of animals that he
did not study. His first large publication, the _Beitraege zur Geschichte
der Thierwelt_ (i.-iv., Halle, 1820-27), contained much anatomical work
in addition to the purely embryological; he commenced here his series of
papers on the development of the genital and urinary organs, continued
in the _Abhandlungen zur Bildungsund Entwickelungs-Geschichte des
Menschen und der Thiere_ (i., ii., Leipzig, 1832-3). A fellow-worker in
this line was Johannes Mueller, whose _Bildungsgeschichte der Genitalien_
(Duesseldorf) appeared in 1830.

In a memoir on the development of the crayfish which appeared in
1829,[192] Rathke found in an Invertebrate confirmation of the germ-layer
theory propounded by Pander and von Baer. He was greatly struck by the
inverted position of the embryo with respect to the yolk. In following
out the development of the appendages he noticed how much alike were
jaws and legs in their earliest stage, and how this supported Savigny's
contention that the limbs of Arthropods belonged to one single type of
structure. In his paper (1832) on the development of the fresh-water
Isopod, _Asellus_,[193] Rathke returns to this point. Commenting on the
original similarity in development of antennae, jaws and legs, he writes,
"Whatever the doubts one may have reserved as to the intimate relation
existing between the jaws and feet of articulate animals after the
researches of Savigny on this subject and mine on developing crayfish,
they must all fall to the ground when one examines with care the
development of the fresh-water Asellus" (p. 147 of French translation).

Further comparative work by Rathke is found in the two volumes of
_Abhandlungen_ and in a book, _Zur Morphologie, Reisebemerkungen aus
Taurien_ (1837), which contains embryological studies of many different
types, including a study of the uniform plan of arthropod limbs. Later
on Rathke devoted himself more to vertebrate embryology, producing among
other works his classical papers on the development of the adder (1839),
of the tortoise (1848), and of the crocodile (1866). He laid the
foundations of all subsequent knowledge of the development of the
blood-vascular system in a series of papers of various dates from 1838
to 1856. The diagrams in his paper on the aortic arches of reptiles
(1856) were for long copied in every text-book.

Rathke was a foremost worker in another important line of embryological
work, the study of the development of the skeleton and particularly of
the skull. We shall discuss the history of the embryological study of
the skull in some detail below; meantime, we note the two other
important lines of research which characterise this period. One is the
intensive study of the development of the human embryo, a study pursued
by, among others, Pockels, Seiler, Breschet, Velpeau, Bischoff, Weber,
Mueller, and Wharton Jones.[194] The other important line--the early
development of the Mammalia--was worked chiefly by Valentin,[195]
Coste,[196] and, above all, by Bischoff, whose series of papers[197] was
justly recognised as classical.

What interests us chiefly in the work of this embryological period is,
of course, the relation of embryology to comparative anatomy and to pure
morphology. The embryologists were not slow to see that their work threw
much light upon questions of homology, and upon the problem of the unity
of plan. Von Baer, we have seen, recognised this clearly in 1828;
Rathke, in one of his most brilliant papers, the
_Anatomisch-philosophische Untersuchungen ueber den Kiemenapparat und das
Zungenbein_ (Riga and Dorpat, 1832), used the facts of development with
great effect to show the homology of the gill-arches and hyoid
throughout the vertebrate series; Johannes Mueller made great use of
embryology in his classical _Vergleichende Anatomie der Myxinoiden_ (i.
Theil, 1836), and, according to his pupil Reichert, firmly held the
opinion that embryology was the final court of appeal in disputed points
of comparative anatomy;[198] Reichert himself in a book of 1838
(_Vergleichende Entwickelungsgeschichte des Kopfes der nackten
Amphibien_) discussed the two different methods of arriving at the
"Type"--the anatomical method of comparing adults, and the embryological
method of comparing embryogenies. Of the embryological method, he says,
"Its aim is to distinguish during the formation of the organism the
originally given, the essence of the type, and to classify and interpret
what is added or altered in the further course of development.
Embryologists watch the gradual building up of the organism from its
foundations, and distinguish the fundament, the primordial form, the
type, from the individual developments; they reach thus, following
Nature in a certain measure, the essential structure of the organism,
and demonstrate the laws that manifest themselves during embryogeny" (p.
vi.). The embryologists, influenced in this greatly by von Baer,
gradually felt their way to substituting for the "Archetype" of pure
morphology what one may perhaps best call the _embryological archetype_.
How the transition was made we can best see by following out the course
of discovery in one particular line. We choose for this purpose the
development of the skull, a subject which excited much interest at this
time and upon which much quite fundamental work was done, particularly
by Rathke and Reichert.

Following up his discovery of gill-slits and arches in the embryos of
birds and mammals, Rathke in two papers of 1832[199] and 1833[200] worked
out the detailed homologies of the gill-arches in the higher
Vertebrates. He describes how in the embryo of the Blenny there is a
short, thick arch between the first gill-slit and the mouth. A furrow
appears down the middle of the arch dividing it incompletely into two.
In the anterior halves a cartilaginous rod is developed which is
connected with the skull; these rods become on either side the lower jaw
and "quadrate." In the posterior halves two similar rods are formed
which develop into the hyoid. The hyoid is at first connected with the
skull, but afterwards frees itself and becomes slung to the "quadrate."
From the hinder edge of the hyoid arch grows out the membranous
operculum, in which develop later the opercular bones and branchiostegal
rays. The upper jaw is an independent outgrowth of the serous layer.

The serial homology of the lower jaw and quadrate with the hyoid and
with the true gill-arches was thus established in fish, and Rathke had
little difficulty in demonstrating a similar origin of lower jaw and
hyoid in the embryos of higher Vertebrates. He could even, as we have
noted before, find the homologue of the operculum in a flap which grows
out from the hyoid arch in the embryo of birds.

But Rathke could not altogether shake himself free from the
transcendental notion of the homology of jaws with ribs, and this led
him to draw a certain distinction between the first two and the
remaining gill-arches, by which the homology of the former with the ribs
was asserted and the homology of the latter denied. He thought he could
show that the skeletal structures (lower jaw, "quadrate," and hyoid) of
the first two arches were formed in the serous layer, just like true
ribs, and like them in close connection with the vertebral skeletal
axis. The other, "true," gill-arches appeared to him to be formed in the
mucous layer, in the lining of the alimentary canal. They had no direct
connection with the vertebral column, and seemed therefore to belong to
what Carus[201] had called the visceral or splanchno-skeleton. He did not,
however, let this distinction hinder him from asserting the substantial
homology of all the gill-arches _inter se_, the first two included.

Rathke's discoveries relative to the development of the jaws, the hyoid
and the operculum, enabled him to make short work of the homologies
proposed for them by the transcendentalists. He could prove from
embryology that the jaws were not the equivalent of limbs, as so many
Okenians believed. He could reject, with a mere reference to the facts
of development, Geoffroy's comparison of the hyoid and the
branchiostegal rays in fish with sternum and ribs. He could show the
emptiness of the attempts made by Carus, Treviranus, de Blainville and
Geoffroy, to establish by anatomical comparison the homologies of the
opercular bones, for he could show that these bones were peculiar to
fish, and were scarcely indicated, and that only temporarily, in the
development of other Vertebrates.[202] He did not, however, himself
realise the relation of the ear-ossicles to the gill-arches, though he
knew that Spix and Geoffroy were quite wrong in homologising them with
the opercular bones in fish. He described, it is true, the development
of the external meatus of the ear and the Eustachian tube from the slit
which appears between the first and the second arch, as Huschke had done
before him; he described, in confirmation of Meckel, the "Meckelian
process" of the hammer running down inside the lower jaw; but the
discovery of the true homologies of the ear-ossicles was not made until
a year or two later by Reichert.

In his further study of the development of _Blennius viviparus_, Rathke
observed some important facts about the development of the vertebral
column and skull. He found that the vertebral centra were first formed
as rings in the chorda-sheath, which give off neural and haemal
processes. The vertebra later ossifies from four centres. The chorda
(notochord) is prolonged some little way into the head, and the base of
the cranium is formed by the expanded sheath, which reaches forward in
front of the end of the notochord. This cranial basis shows a division
into three segments, in which Rathke was inclined to see an indication
of three cranial vertebrae. (It turned out that this division into three
segments did not really exist, and Rathke later acknowledged that he had
made an error of observation.) The side walls of the skull grow out from
this base and form a fibrous capsule for the brain. The cranial section
of the chorda itself shows no sign of segmentation; but later on the
cranial portion of the chorda-sheath ossifies, like the vertebrae, from
several centres. The vomer, which, in the classical form of the
vertebral theory of the skull, was the centrum of the fourth, or
foremost, cranial vertebra, does not, according to Rathke, develop in
continuity with the cranial basis and the chorda sheath, but develops
separately in the facial region.

Von Baer, like Rathke at this time, was also to some extent a believer
in the vertebral theory of the skull. In his second volume (1834, pub.
1837) he holds that the development of the skull, as the sum of the
anterior vertebral arches, is in general the same as that of the other
neural arches, and is modified only by the great bulk of the brain
(_Entwickelungsgeschichte_, ii., p. 99). He had, however, some doubts as
to the entire correctness of the vertebral theory, doubts suggested by a
study of the developing skull. "In the course of the formation of the
head in the higher animals, something additional is introduced which
does not originally belong to the cranial vertebrae. At first we see the
vertebration in the hinder region of the skull very clearly. Afterwards
it becomes suddenly indistinct, as if some new formation overlaid it"
(i., p. 194).

Even more clearly is his doubt expressed in his paper on _Cyprinus_.
"Upon the formation of the vertebral column only this need be said, that
at this stage the notochord is very clearly seen, and the upper and
lower arches and spinous processes are visible right to the end of the
tail, but the separation into vertebrae ceases abruptly where the back
passes into the head. I do not hesitate to assert _that bony fish, too,
have at this stage an unsegmented cartilaginous cranium_ (as
cartilaginous fish have all their life), the prominences and hollows of
which constitute its only resemblance with the vertebral type" (1835, p.
19).

A convinced supporter of the vertebral theory was Johannes Mueller, who,
in his classical memoir on the Myxinoids,[203] discussed at some length
the relation between the development of the vertebrae and the development
of the skull. His memoir is principally devoted to comparative anatomy,
but in treating of the skeleton he pays much attention to development.
He describes the formation of the vertebrae in elasmobranch embryos; for
the facts regarding other Vertebrates he relies largely on work by
Rathke (_Blennius_, 1833) and Duges (1834). He recognises as the basis
of his comparisons the homology of the notochord in all vertebrate
embryos with the persistent notochord which forms the chief part or the
whole of the vertebral column in the Cyclostomes. The notochord
possesses an inner and an outer sheath and the outer sheath is
continuous with the _basis cranii_ (p. 92). It is in the outer sheath
that the vertebrae develop--from four separate pieces, in fish at least,
plus an additional element which helps to form the centrum. The skull of
Vertebrates consists, according to Mueller, of three vertebrae, whose
centra are the basioccipital, the basisphenoid and the presphenoid.
Other bones besides those belonging to the vertebrae are present, but
this formation out of three vertebrae gives the essential schema for the
skull. Now the brain capsule, like the sheath of the spinal cord, is a
development from the outer sheath of the notochord. If the skull
consists of vertebrae we should expect the centra of the skull-vertebrae
to develop in the outer sheath at the sides of the cranial section of
the notochord as two separate halves, just as do the bodies of the
vertebrae; we should expect further the cartilaginous side-walls of the
cranium to develop in the membranous brain-sheath just as the neural
arches develop in the membranous sheath of the spinal column. In
Rathke's discovery (!) of a segmentation of the _basis cranii_ into
three parts, and of the isolated formation of the vomer, Mueller sees a
confirmation of his view that the skull is composed of three and not
four vertebrae. But there is nothing in Rathke's observations to support
the idea that the centra of the cranial vertebrae are formed from
separate halves. Mueller has to be content with a reference to the state
of things in _Ammocoetes_ (which, by the way, he did not know to be the
young of _Petromyzon_). In the simple skull of _Ammocoetes_ the base is
formed chiefly by two cartilaginous bars lying more or less parallel
with the longitudinal axis of the skull and embracing with their hinder
ends the cranial portion of the notochord.

These bars, declares Mueller, are clearly the still separate halves of
the _pars basilaris cranii_, and represent the divided centra of the two
hinder cranial vertebrae. To complete the parallel between the
development of the skull and of the vertebrae, it would have been
necessary to show that the side walls of the cranium developed in a
similar manner from separate pieces. Mueller could not prove this point
from the available embryological data, and indeed the facts which he did
use had to be twisted to suit his theory. A curious apparent
confirmation of his idea that the centra of the cranial vertebrae are
formed from separate halves was supplied in 1839 by Rathke's discovery
of the trabeculae in the embryonic skull of the adder.

The next big step in the study of the development of the skull was
taken by a pupil of Mueller, C. B. Reichert, who showed in his work
very distinct traces of his master's influence. Reichert's first and
most important contribution to the subject was his paper on the
metamorphosis of the gill, or, as he called them, the visceral arches
in Vertebrates,[204] particularly in the two higher classes. Reichert
describes the similar origin in embryo of bird and mammal (pig) of
three "visceral" arches. These arches stand in close relation to the
three cranial vertebrae which Reichert, like Mueller, distinguishes. He
makes the retrograde step of admitting only three aortic arches, and
he is not inclined to consider the three visceral arches as equivalent
to the gill-arches of fish--in his opinion they have more analogy with
ribs, though differing somewhat from ribs in their later
modifications. The visceral arches are processes of the visceral
plates (von Baer), which grow downwards and meet in the middle line,
leaving between one another and the undivided body wall three visceral
slits opening into the pharynx. The first visceral process is
different in shape from the others, for it sends forward, parallel
with the head and at right angles to its downward portion, an upper
portion in which later the upper jaw is formed. The other two
processes are straight. From the hinder edge of the second visceral
arch there develops, as Rathke had seen, a fold which is comparable
with the operculum of fish. The first slit develops externally into
the ear-passage, internally into the Eustachian tube, and in the
middle a partition forms the tympanic ring and tympanum. Inside each
of the visceral processes on either side a cartilaginous rod develops.
In the first process this rod shows three segments, of which the first
lies inside that portion of the process which is parallel with the
head. This upper segment forms the foundation for the bones of the
upper jaw. The lowest segment of the cartilaginous rod becomes
Meckel's cartilage, and on the outer side of this the bones of the
lower jaw are formed. The middle segment becomes in mammals the incus
(one of the ear-ossicles), and in birds the quadrate. Meckel's
cartilage, which was discovered by Meckel[205] in fish, amphibians and
birds, is a long strip of cartilage which runs from the ear-ossicle
known as the hammer in mammals,[206] to the inside of the mandible.
Reichert shows how this relation comes about. The hammer, according to
his observations on the embryo of the pig, is simply the proximal end
of Meckel's cartilage, which later becomes separated off from the long
distal portion (see Fig. 9). The third ear-ossicle of mammals, the
stapes, comes not from the first arch but from the second. The
cartilaginous rod of the second arch segments like the first into
three pieces. Of these the uppermost disappears, the middle one, which
lies close up to the labyrinth of the ear, becomes the stapes, and the
lowest becomes the anterior horn of the hyoid. The stapes forms a
close connection with the hammer and the incus. In birds, where there
is a single ear-ossicle, the columella, the middle piece of arch I
forms, as we have seen, the quadrate, by means of which the lower jaw
is joined to the skull. The proximal end of Meckel's cartilage, which
in mammals forms the hammer, here gives the articular surface between
the lower jaw and the quadrate. The columella is formed from the
middle piece of the three into which the cartilage of the second arch
segments. It is, therefore, the homologue of the stapes in mammals.
The third arch takes a varying share, together with the second, in the
formation of the hyoid apparatus.

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