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THE

SYDENHAM SOCIETY

INSTITUTED

MDCCCXLIII

LONDON

MDCCCXr.VII.

MICROSCOPICAL RESEARCHES

INTO THE

ACCORDANCE IN THE STRUCTURE AND GROWTH

OF

ANIMALS AND PLANTS.

TRANSLATED FROM THE GERMAN

O]

DR. TH. SCHWANN

PROFESSOR IN THE UNIVERSITY OF LOCVAIN,
ETC. ETC.

BY

HENRY SMITH

FELLOW OF THE ROYAL COLLEGE OF SURGEONS OF ENGLAND,
SURGEON TO THE ROYAL GENERAL DISPENSARY, ALDERSGATE STREET.

LONDON

PRINTED FOR TUfi SYDENHAM S0CIE1 »

M DCCCXLV1 I .

C. AND J. ADLARD, PRINTERS,

BARTHOLOMEW CT.OSK.

TRANSLATOR'S PREFACE.

Any attempt on my part by way of introduction or com-
mendation of Professor Schwann's work, must, I feel, be
altogether misplaced and unnecessary. The treatise has now
been seven years before the public, has been most acutely in-
vestigated by those best competent to test its value, and the
first physiologists of our day have judged the discoveries
which it unfolds as worthv to be ranked amongst the most
important steps by which the science of physiology has ever
been advanced. The Royal Society of London has evinced
its sense of the great merit of the work by awarding to its
Author the Copley Medal for the year 1845. The exten-
sive reputation and fully-acknowledged value of the original
work, then, forbid my presuming that any one of my readers
can be altogether unacquainted with it and the general -
outlines of the Cell-Theory ; I may, however, I trust, be
permitted to add a few words respecting the edition which is
now presented to the Subscribers of the Sydenham Society.

In the first place, I desire to tender my most unfeigned
and unreserved apologies to the Council and Subscribers of
the Society for the delay which has occurred in the issuing
of this translation, and to assure the latter body that their

b

vi TRANSLATOR'S PREFACE.

Council is in no degree responsible for its tardy appearance ;
when, nearly three years since, the Council did me the honour
to accept an offer on my part to present to the Society a
translation of Professor Schwann's treatise, I fully hoped to
have proceeded with so pleasing a labour without interruption
or hinderance ; but various unforeseen circumstances, both of a
professional and domestic nature, have occurred to prevent the
accomplishment of my object until the present moment.

I am greatly indebted to the Author for the labour which
he has expended in revising his work for this translation.
Amongst the most important advantages which this edition
has derived from his revision, I may mention the addition of
many notes illustrative of the text, and the amalgamation of
the two papers on Cartilage and Ossification, which, as they
were originally written and printed at a considerable interval
of time, led to some difficulty in the comprehension of the
Author's precise views on that subject ; and that circum-
stance is also to be received as explanatory of the appearance
of some of the delineations of Cartilage in Plate III. It was
originally intended to have added notes, which should bring
down the history of the subject to the period of publication,
but it was found that they would form a mass of material
almost as large as the original text, and the idea was therefore
abandoned.

In order that the reader might be in possession of the whole
of the evidence upon which the Cell-Theory was originally
based, I have appended a translation of Dr. Schleiden's
Monograph so frequently referred to by our Author.

TRANSLATOR'S PREFACE.

Vll

It is to be feared that many of my readers may consider
an apology to be necessary on my part fur the style of the
translation, and think that I might have followed the German
less closely with advantage ; the nature of the subject, how-
ever, inyolying as it does such very minute descriptions, and
the constant repetition of the same terms, added to the im-
possibility of doing justice to the Author's close deductions in
any other form than a literal translation, necessitated a much
more rigid adherence to the original text, than I should have
thought requisite under any other circumstances.

The Plates have been most faithfully copied from the
originals by Mr. Henry Adlard.

HENRY SMITH.

Henrietta Street, Cavendish Square :

November 30///. J 84 7.

AUTHOR'S PREFACE.

It is one of the essential advantages of the present age,
that the bond of nnion connecting the different branches of
natural science is daily becoming more intimate, and it is to
the contributions which they reciprocally afford each other that
we are indebted for a great portion of the progress which the
physical sciences have lately made. This circumstance there-
fore renders it so much the more remarkable, that, notwith-
standing the many efforts of distinguished men, the anatomy
and physiology of animals and plants should remain almost
isolated, though advancing side by side, and that the conclu-
sions deducible from the one department should admit only of
a remote and extremely cautious application to the other. Of
late, the two sciences have for the first time begun to be more
and more intimately allied. The object of the present treatise
is to prove the most intimate connexion of the two kingdoms
of organic nature, from the similarity in the laws of develop-
ment of the elementary parts of animals and plants.

The principal result of this investigation is, that one com-
mon principle of development forms the basis for every sepa-
rate elementary particle of all organised bodies, just as all
crystals, notwithstanding the diversity of their figures, are
formed according to similar laws. I have endeavoured to
explain the design of such a comparison more fully in the
commencement of the third section of this treatise, and will
now lay before the reader those data which are of most im-
portance in an historical point of view in reference to the
development of this idea.

x AUTHOR'S PREFACE.

As soon as the microscope was applied to the investigation
of the structure of plants, the great simplicity of their struc-
ture, as compared with that of animals, necessarily attracted
attention. Whilst plants appeared to be composed entirely
of cells, the elementary particles of animals exhibited the
greatest variety, and for the most part presented nothing at
all in common with cells. This, harmonised with the opinion
long since current, that the growth of animals, whose tissues
are furnished with vessels, differed essentially from that of
vegetables. An independent vitality was ascribed to the
elementary particles of vegetables growing without vessels,
they were regarded to a certain extent as individuals, which
composed the entire plant ; whilst, on the other hand, no such
a view was taken of the elementary parts of animals. An
essential difference both in the mode and in the fundamental
powers of growth was thus maintained.

It soon, however, appeared that animal tissues do also
occur which grow without vessels ; for instance, in the forma-
tion of the ovum, and the earlier stages of development of the
embryo previous to the formation of the blood ; and, secondly,
certain tissues of the adult, the epidermis for example. "With
respect to the ovum, which manifested indubitable proofs of
an actual vitality, all physiologists were agreed in ascribing to
it a so-called plant-like growth. This resemblance to the plant
had reference to a growth of the conspicuous parts of the ovum
without vessels, and was in no way connected with the form
and mode of growth of the elementary particles. No one,
however, considered that the analogy of the ovum entitled him
to infer the operation of a plant-like growth of the elementary
particles in the non-vascular tissues of the matured animal ;
on the" contrary, the opinion rather gained ground, that these
tissues originated and grew by means of a secretion from the
surface of the organised tissues. Such was supposed to be
the case with the epithelium, the crystalline lens, &c. This

AUTHOR'S PREFACE. xi

opinion still maintained its ground, even when the structiuc
of the tissues became more accurately known. Nor did the
plant-like growth of the component parts of the ovum abolish
the assumed essential difference of the growth of the vascular
tissues.

A very important advance was made in the year 1837,
when an actual growth of the elementary particles of epithe-
lium was proved to take place without vessels. Henle (Sym-
bols ad anatomiam vill. intest. Berol. 1837) showed that the
cells in the superficial layers of epithelium are much more ex-
panded than those in the deeper strata, a fact which leaves
scarcely any doubt as to their true plant-like (i. e. non-vascular)
growth. Henle1 says (1. c. p. 9), " Hoc in loco (in planta
pedis) cellularum (retis Malpighii) diametrum extrorsum
augeri, saepius repetita observatione pro re certa affirmare
audeo. Quas retis cellulas non minus in fcetu suillo sensim
increscentes transire in cellulas epidermidis, nunquam non
inveni." Purkinje and Raschkow (Meletem. circa mammal,
dentium evol. Vratisl. 1835) had made the following obser-
vations upon the development of the epidermis : " In primis
evolutionis periodis — squamulae — epithelii nondum ita con-
formatae sunt ut in ilia periodo, quae partui praecedit, scd
parenchyma plantarum cellulis simillimum ostendunt, cum
quaeque squamula, qua3 postea talis apparet, tunc temporis
tanquam cellula polyedrica e membrana tenacissima const an s
globosamque guttulam continens in conspectum veniat. Pressu
applicato rumpebantur istse cellules atque lymphaticum liquo-
rem effundebant, quae cellulas, procedente evolutione, verisimile
complanatae in illas polyedricas squamas ruutantur." Ilenle,
when quoting this passage, adds (1. c. p. 9) : " Haec ilia mini
vero sola compressio in causa esse possit, ut parva cellula

1 Henle's observations are detailed at page 7G of this treatise. The research'
Turpin and Dumortier could not be quoted, as I only became acquainted with them
at the conclusion of my work.

XII

AUTHOR'S PREFACE.

in tantam laminam extendatur, nondum satis mihi constat :
certe principio increscere volumen cellulse, nescio an imbibitione,
constabit, nisi spes fallit, promotis disquisitionibus." The
cantion with which Henle (and, indeed, every good physiolo-
gist) expresses himself in this passage with reference to the
true growth of non-vascular tissues, is the best illustration of
the state of the question. There is another observation of
Henle's, which is opposed to the epithelium being regarded as
a lifeless substance secreted from the organised tissue; I allude
to the passage (1. c. p. 22 et seq.) where he proves that the
vibratile cilia, whose motion it is so difficult to explain by
physical laws, stand upon little cylinders which are merely a
modification of the epithelium.

Turpin (Annal. des Sciences natur. vii, p. 207) showed that
the corpuscles, which Donne had found in vaginal discharges,
and regarded as cast-off epithelium, were organised cells, and
were in general oblong, and either pointed at one or both
ends, or. altogether irregular in figure, and that a new gene-
ration of spherical vesicles1 took place in their interior. He
then remarks (1. c. p. 210) : " On ne peut s'empecher, apres
avoir bien etudie les vesicules dont est formee la couche de
mucus produite par la membrane muqueuse vaginale, d'y voir
un tissu cellulaire bien organise et compose comme tous les
tissus cellulaires vegetaux, d'un agglomerat, par simple conti-
guite, de vesicules distinctes et vivant individuellement chacune
pour leur propre compte au depens de Feau muqueuse, qui les
baigne de toutes parts." Turpin then compares this tissue of
animal cells, presented under the appearance of mucus, with
what he calls " suppurations vegetales, excretions muqueuses,
qui semblent suinter sous forme de gouttelettes, de la surface
des tissus vifs," and which is generally comprised under the

1 May there not have been some confusion here with the nuclei of the epithelium-
cells ? At present, as far as regards Mammalia at least, we know of no formation
of cells within cells in the epithelium.

AUTHOR'S PREFACE. xiii

name of cambium j and finally adds (1. c. p. 212), " En etendant
la comparaison entre deux choses si comparables, on trouve
que la forme variable des vesicules du tissu cellulaire du mucus
de la membrane vaginale, leur allongement en pointc, leur
flaccidite, toujours entretenue par l'humidite constante qui
baigne les tissus animaux, et le developpement dans lcurinterieur,
soit des granules, soit des vesicules spheriques, sont toutes
clioses qui s'observent egalement dans la composition de tous
les tissus cellulaires vegetaux mous et aqueux, et que Pon
designe par le nom de palpe ou de paren chyme dans certaines
tiges ou feuilles grasses et dans certains fruits murs ou blettes."
In the same year, Dumortier communicated researches into
the development of the ova of snails. (Annal. des Sciences
natur. viii, p. 129.) He observed, that in the mucus-globule,
present in these ova, and from which the embryo is developed,
there are generated cells, in the interior of which, secondary
cells are formed, and so on, and that this tissue of cells be-
comes transformed into the liver, whilst the other tissues origi-
nate from a gelatinous mass, which exhibits myriads of points.
In his conclusions, he says (1. c. p. 163), " En examinant
devolution des Mollusques, nous avons demontre que les tissus
animaux, quoique formes originairement de meme par la solidi-
fication des surfaces, se developpent de differentes manieres :
le tissu cellulaire par des productions medianes, le tissu dermo-
musculaire par un feutre de canalicules centripetes. Ainsi, chez
les animaux, les tissus ne se forment pas au depens les uns des
autres ; il n'y existe pas un tissu generateur unique, mais bi n
plusieurs tissus originairement distincts. — Les belles observa-
tions de M. Mirbel ont prouve que chez les vegetaux il existe
un seul tissu originel, le tissu cellulaire, qui par une suite de
metamorphoses, se transforme en tissu vasculaire. Par con-
sequent, le regne vegetal est caracterise par Funite originel, et
le regne animal par la pluralite originelle des tissus." \ an-
beneden and Windischmann give a difl^rcnt explanation to

xiv AUTHOR'S PREFACE.

these observations of Duraortier, in as much as they regard the
tissue consisting of cells as the yelk and not the liver. (Bulletin
de TAcad. royale de Bruxelles, torn, v, No. 5.)

Other instances of the resemblance in form between different
animal tissues and those of vegetables had already been
repeatedly pointed out. Thus it was frequently said, in refer-
ence to thickly-crowded animal cells, or even mere globules,
that they presented an appearance resembling vegetable cellu-
lar-tissue ; and Valentin (Nov. Act. N. C. xviii, P. 1, 96),
after describing the nucleus of the epidermal cells, states that
it reminded him of the nucleus which occurs in the vegetable
kingdom, in the cells of the epidermis, the pistil, &c. Nothing,
however, resulted from such comparisons, because they were
mere similarities in figure, between structures which present
the greatest variety of form.

Schleiden instituted researches into the mode of development
of vegetable cells, which illustrated the process most excellently.
This admirable work appeared subsequently in the second part
of Muller's Archiv for 1838. He found, that in the forma-
tion of vegetable cells, small, sharply-defined granules are first
generated in a granulous substance, and around them the cell-
nuclei (cytoblasts) are formed, which appear like granulous
coagulations around the granules. The cytoblasts grow for a
certain time, and then a minute transparent vesicle rises upon
them, the young ceil, so that, in the first instance, it is placed
upon the cytoblast, like a watch-glass upon a watch. It then
becomes expanded by growth. Schleiden communicated the
results of his investigations to me, previous to their publication
in October, 1837. The resemblance in form, which the chorda
dorsalis, to which J. Miiller had already drawn attention, and
the branchial cartilage of the tadpole present to vegetable cells,
had previously struck me, but nothing resulted from it. The
discoveries of Schleiden, however, led to more extended re-
searches in another direction.

AUTHOR'S PREFACE. xv

In the above-mentioned investigations of Henle, Turpin, and
Dumortier, the resemblance whicli the animal tissues examined
(epithelium and the liver or yelk of snails) bore to plants, lay,
in the first place, in the circumstance, that their elementary
particles grew without vessels, and in part, free in a fluid, or
even inclosed in another cell ; and in the second place, in that
these elementary particles exhibiting a non-vascular growth,
were furnished with a peculiar wall, like the cells of plants.
When this coincidence was furnished, we were entitled to
arrange these cells as near to the vegetable cells as the different
kinds of animal cells, for instance, germinal vesicles, blood-cor-
puscles, and fat-cells, stand together, when regarded as different
species comprised under the natural-history idea of cells.

The state of the matter, therefore, when I commenced my
researches was as follows : The elementary particles of or-
ganised bodies presented the greatest variety of form; there
was a resemblance between many of them, and, according to
the greater or lesser degree of similarity, a group of fibres, of
cells, of globules, and so on, might be distinguished, and
in each of these divisions again there were different forms.
As the cells taken collectively differed from the fibres, so also,
only in a less degree, must the separate kinds of cells differ from
each other, and the different kinds of fibres from each other.
All those forms seemed to have nothing else in common, save
that they grew by the addition of new molecules between those
already existing, that they were living elements. So long as
the epithelium-cells were regarded as a secretion of the
organised substance, they could never, in that sense, be classed
with the living elementary particles. There seemed to be no
general rule with respect to the mode in which the molecules
were joined together to form the living particles ; here they
united into one kind of cells, there into another, and at a third
spot into a fibre, and so on. The principle of development ap-
peared to be altogether different for such particles as differed

xvi AUTHOR'S PREFACE.

in their physiological signification ; and the diversity in the
laws which it was necessary to assume in the development of
a cell and a fibre, was also, only in a less degree, necessarily
assumed between the different kinds of cells and the different
sorts of fibres. Cells, fibres, &c. were therefore merely natural-
history ideas, and no conclusion could be drawn from the mode
of development of one kind of cell as to that of any other kind ;
and, in fact, no such deductions were made, although we were ac-
quainted with some important points in the process of develop-
ment of certain kinds of cells ; for example, the blood-corpuscle
(see p. 67 of this Treatise), and the ovum (see the Supplement,
p. 217). Although the investigations quoted above determined
the important fact of the non-vascular growth, they did not
thereby effect any change in our views. The idea of proving
the similarity of the principle of development for elementary
particles which were physiologically different, by a comparison
of animal cells with those of vegetables, was not contained in
those researches, and with these, therefore, the investigators
before mentioned might well come to a stand-still.

The discoveries of Schleiden made us more accurately ac-
quainted with the process of development in the cells of plants.
This process contained sufficient characteristic data to render
a comparison of the animal cells in reference to a similar
principle of development practicable. In this sense I com-
pared the cells of cartilage and of the chorda dorsalis with
vegetable cells, and found the most complete accordance. The
discovery, upon which my inquiry was based, immediately lay
in the perception of the principle contained in the proposition,
that two elementary particles, physiologically different, may
be developed in the same manner. For it follows, from the
foregoing, that if we maintain the accordance of two kinds of
cells in this sense, we are compelled to assume the same princi-
ple of development for all elementary particles, however dis-
similar they may be, because the distinction between the other

AUTHOR'S PREFACE. xvii

particles and a cell differs only in degree from that which exists
between two cells ; so also the principle of development in the
latter can only then be similar, when it repeats itself in the
rest of the elementary particles. 1 therefore quickly asserted
this position also, so soon as I was convinced of the accordance
between the cells of cartilage and those of plants in this sense.
It now became easy to accommodate the principle which I
had laid down to the rest of the tissues, since the principle
itself had already made me acquainted with the law of their
development. Actual observation also completely confirmed
the conclusion which had been drawn with respect to the rest
of the tissues. It was not absolutely necessary that this
principle should recur in the elementary particles of vascular
tissues ; for since no independent vitality of the elements,
and therefore no diversity in the fundamental powers of
growth, was assumed in their case, so, without prejudice to
the principle, might they be subject to entirely different laws
of development. But slight as was the probability at the
commencement, that the principle could be carried out with
respect to them, observation soon showed that vessels do not
establish any essential difference in growth, but merely occa-
sion some distinctions, which may be explained as the con-
sequences of a more minute distribution of the nutrient fluid;
of the change of material facilitated both by that means and
by the circulation ; and of a greater capacity of imbibition
in the animal substance. Thus was the proposition firmly
established by observation, that there is one common principle
of development for the elementary particles of all organised
bodies. It had already indeed been long known that all
tissues were formed from a grauulous mass; but that these
granules bore some direct relation to the subsequent ele-
mentary particles, and what that relation might be was known
in respect to but a few of the particles, and in them the mode
of development appeared to differ so much, that unity neither

xviii AUTHOR'S PREFACE.

was nor could be recognised in it ; for the conformity of the
principle of development consists chiefly in the similar origin of
these granules themselves, and this circumstance was not known,
indeed the term granules or granulous mass was sometimes
used to denote the entire cells, sometimes the nuclei, and some-
times granulous substances which form to a certain extent
as chemical precipitates, and have no direct connexion with
the elementary cells of organised bodies.

I communicated a preliminary review of the results gained,
and which already comprehended most of the tissues, in the
beginning of the year 1838, in Froriep's ' Notizen/ Nos. 91,
103, and 112. The detailed description required a longer
time ; the first two portions of the present Treatise were placed
before the Academy of Paris in August and December, 1838.
J. Miiller and Henle have already applied the theory to the
most important pathological processes, and it now only requires
to be extended to comparative anatomy, particularly amongst
the lower animals.

At the conclusion of the Treatise I have attempted a theory
of organisms, and for that purpose have excluded everything
theoretical from the work itself, in order that facts might not
be confused with hypothetical matter. The theory has at
least this advantage, that by its aid any one may form a pre-
cise idea for himself of the organic processes, which may con-
duct to new researches; such a theory may therefore be of
use, even if assumed to be decidedly false. It contains the
principles of the organic phenomena, both of the healthy and
diseased organism. It was my intention to have added an
application of the theory to the several organic processes ; but
circumstances compelled me to bring the work to a conclusion.
Perhaps at some future time I may find opportunity to fill up
the deficiency.

Berlin, March 1839.

CONTENTS.

Introduction

r a ( ; k

SECTION I.

On the Structure and Growth of the Chorda Dorsalis and Cartilage

1. Chorda dorsalis .

2. Cartilage .....

10
ih.
15

continuous tissues

SECTION II.

On Cells as the Basis of all Tissues of the Animal Body .
First division. On the ovum and germinal memhrane
Second division. Permanent tissues of the animal body

Class I. Isolated, independent cells

1. Lymph-corpuscles

2. Blood-corpuscles

3. Mucus-corpuscles

4. Pus-corpuscles .
Class II. Independent cells united into

1. Epithelium

2. The pigmentum nigrum

3. Nails .

4. Hoofs

5. Feathers

6. The crystalline lens
Tissues, in which the cell-walls have coalesced with each other

or with the intercellular substance

1. Cartilage and bone ....

2. The teeth .....
Fibre-cells, or tissues, which originate from cells that l)ecom(
elongated into bundles of fibres

1. Areolar tissue

2. Fibrous tissue

3. Elastic tissue

Class III.

Class IV.

36
40
64

67
ib.
ib.
70
71
73
ib.
77
80
81
82
87

96
97

ib.

110

ib.

123

124

xx CONTENTS.

PAGE

Class V. Tissues generated from cells, the walls and cavities of which coalesce

together . . . . . 129

1. Muscle ...... 130

2. Nerves . . . . . 141

3. Capillary vessels . . . . .154

SECTION III.

Review of the previous researches — The formative process of Cells — The

Cell-theory . . . . . 161

Survey of Cell-life . . . . . .168

Theorv of the cells . . . . . . 186

Supplement (vide page 46) on the signification of the germinal memhrane . 217
Remarks upon a statement put forth by Professor Valentin, respecting pre-
vious researches on the subject of this work . .219
Explanation of the Plates . . . . , 225

CONTRIBUTIONS TO PHYTOGENESIS, by Dr. M. J. Schleiden . 229

Description of Plates to do. . . . . . 265

I

MICROSCOPICAL RESEARCHES,

&c. &c.

INTRODUCTION.

Although plants present so great a variety of external form,
yet they are no less remarkable for the simplicity of their
internal structure. This extraordinary diversity in figure is
produced solely by different modes of junction of simple ele-
: mentary structures, which, though they present various modi-
fications, are yet throughout essentially the same, namely, cells.
The entire class of the Cellular plants consists only of cells ;
many of them are formed solely of homogeneous cells strung
together, some of even a single cell. In like manner, the Vas-
cular plants, in their earliest condition, consist merely of simple
cells ; and the pollen-granule, which, according to Schleiden's
discovery, is the basis of the new plant, is in its essential parts
only a cell. In perfectly- developed vascular plants the struc-
ture is more complex, so that not long since, their elementary
tissues were distinguished as cellular and fibrous tissue, and
vessels or spiral-tubes. Researches on the structure, and par-
ticularly on the development of these tissues, have, however,
shown that these fibres and spiral-tubes are but elongated cells,
and the spiral-fibres only spiral-shaped depositions upon the
internal surface of the cells. Thus the vascular plants consist
likewise of cells, some of which only have advanced to a higher
degree of development. The lactiferous vessels are the only
structure not as yet reduced to cells ; but further observations
are required with respect to their development. According to
Unger (Aphorismen zur Anatomie und Physiol, der Pflanzen,

D. H. HILL LIBRARY1

North Carolina State College

2 INTRODUCTION.

Wien, 1838, p. 14,) they in like manner consist of cells, the
partition- walls of which become obliterated.

Animals, which present a much greater variety of external
form than is found in the vegetable kingdom, exhibit also, and
especially the higher classes in the perfectly-developed condition,
a much more complex structure in their individual tissues.
How broad is the distinction between a muscle and a nerve,
between the latter and cellular tissue, (which agrees only in
name with that of plants,) or elastic or horny tissue, and so
on. When, however, we turn to the history of the development
of these tissues, it appears, that all their manifold forms originate
likewise only from cells, indeed from cells which are entirely
analogous to those of vegetables, and which exhibit the most
remarkable accordance with them in some of the vital pheno-
mena which they manifest. The design of the present treatise
is to prove this by a series of observations.

It is, however, necessary to give some account of the vital
phenomena of vegetable cells. Each cell is, within certain
limits, an Individual, an independent Whole. The vital phe-
nomena of one are repeated, entirely or in part, in all the rest.
These Individuals, however, are not ranged side by side as a
mere Aggregate, but so operate together, in a manner unknown
to us, as to produce an harmonious Whole. The processes
which go forward in the vegetable cells, may be reduced to the
following heads : 1, the production of new cells; 2, the expan-
sion of existing cells; 3, the transformation of the cell-contents,
and the thickening of the cell-wall ; 4, the secretion and ab-
sorption carried on by cells.

The excellent researches of Schleiden, which throw so much
light upon this subject, form the principal basis for my more
minute observations on these separate vital phenomena. (See his
" Beitrage zur Phytogenesis," in Midler's Archiv, 1838, p. 137,
plates 3 and 4.)i

First, of the production of new cells. According to Schleiden,
in Phamogamous plants, this process always (except as regards
the cells of the Cambium,) takes place within the already ma-
ture cells, and in a most remarkable manner from out of the
well-known cell-nucleus. On account of the importance of the

1 [A translation of this paper forms part of this volume. — Trans.]

INTRODUCTION. 3

latter in reference to animal organization, I here introduce an
abridgment of Schleiden's description of it. A delineation is
given in plate I, fig. 1, a, a, taken from the onion. This struc-
ture— named bv R. Brown, Areola or cell-nucleus, by Sclilciden,
Cvtoblast — varies in its outline between oval and circular, ac-
cording as the solid which it forms passes from the lenticular
into the perfectly spheroidal figure. Its colour is mostly yel-
lowish, sometimes, however, passing into an almost silvery
white ; and in consequence of its transparency, often scarcely
distinguishable. It is coloured by iodine, according to its
various modifications, from a pale yellow to the darkest brown.
Its size varies considerably, according to its age, and according
to the plants, and the different parts of a plant in which it is
found, from 0*0001 to 0-0022 Paris inch. Its internal struc-
ture is granular, without, however, the granules, of which it
consists, being very clearly distinct from each other. Its
consistence is very variable, from such a degree of softness as
that it almost dissolves in water, to a firmness which bears
a considerable pressure of the compressorium without altera-
tion of form. In addition to these peculiarities of the cvto-
blast, already made known by Brown and Meyen, Schleiden has
discovered in its interior a small corpuscle (see plate I, fig. 1, ftj
which, in the fully-developed cvtoblast, looks like a thick ring,
or a thick -walled hollow globule. It appears, however, to pre-
sent a different appearance in different cytoblasts. Sometimes
only the external sharply-defined circle of this ring can be dis-
tinguished, with a dark point in the centre, — occasionally, and
indeed most frequently, only a sharply circumscribed spot. In
other instances this spot is very small, and sometimes cannot
be recognized at all. As it will frequently be necessary to speak
of this body in the following treatise, I will for brevity's sake
name it the "nucleolus" (Kernkorperchen, "nucleus-corpuscle .")
I According to Schleiden, sometimes two, more rarely three, or,
as he has personally informed me, even four such nucleoli occur
in the cytoblast. Their size is very various, ranging from the
semi-diameter of the cytoblast to the most minute point.

The following is Schleiden' s description of the origin of the
cells from the cytoblast. So soon as the cytoblasts have attained
their full size, a delicate transparent vesicle, the young cell,
rises upon their surface, and is placed upon the flat cvtoblast

4 INTRODUCTION.

like a watch-glass upon a watch. It is at this time so delicate
that it dissolves in distilled water in a few minutes. It gradu-
ally expands, becomes more consistent, and at length so large,
that the cytoblast appears only as a small body inclosed in one
of the side walls. The portion of the cell-wall which covers the
cytoblast on the inner side, is, however, extremely delicate and
gelatinous, and only in rare instances to be observed; it soon
undergoes absorption together with the cytoblast, which like-
wise becomes absorbed in the fully-developed cell. The cyto-
blasts are formed free within a cell, in a mass of mucus-granules,
and the young cells lie also free in the parent cell, and assume,
as they become flattened against each other, the polyhedral
form. Subsequently the parent cell becomes absorbed. (See a
delineation of young cells within parent cells, plate I, fig. 2,
b, b, b.) It cannot at present be stated with certainty that the
formation of new cells always takes place from a cystoblast, and
always within the existing cells, for the Cryptogamia have not
as yet been examined in this respect, nor has Schleiden yet ex-
pressed his views in reference to the Cambium. Moreover,
according to Mirbel, a formation of new cells on the outside of
the previous ones takes place in the intercellular canals and on
the surface of the plant in the Phanerogamia. (See Mirbel on
" Marchantia," in Annales du Musee, 1, 55 ; and the counter-
observations of Schleiden, Miiller's Archiv, 1838, p. 161.) A
mode of formation of new cells, different from the above de-
scribed, is exhibited in the multiplication of cells by division of
the existing ones ; in this case partition-walls grow across the
old cell, if, as Schleiden supposes, this be not an illusion, inas-
much as the young cells might escape observation in conse-
quence of their transparency, and at a later stage, their line
of contact would be regarded as the partition wall of the parent
cell.

The expansion of the cell when formed, is, either regular on ;
all sides, in which case it remains globular, or it becomes poly-
hedral from flattening against the neighbouring cells, or it is irre-
gular from the cell growing more vigorously in one or in several
directions. What was formerly called the fibrous tissue, which
contains remarkably elongated cells, is formed in this manner.
These fibres also become branched, when different points of
the cell-wall expand in different directions. This expansion of

INTRODUCTION. 5

the cell-wall cannot be explained as a merely mechanical effect,
which would continually tend to render the cell-membrane
thinner. It is often even combined with a thickening of the
cell-wall, and is probably effected by that process of nutrition
called intus-susception. (See Hugo Mold's " Erlauterung und
Vertheidigung meiner Ansicht von der Structur der Pflanzen-
substanzen/' Tubingen, 1836.) The flattening of the cells may
also be ascribed to the same cause.

With regard to the changes which the cell-contents and cell-
wall undergo during vegetation, I only take into consideration
the thickening of the latter, as I have but a few isolated obser-
vations upon the transformations of the contents of animal cells,
which however indicate analogous changes to those of plants.
The thickening of the cell-walls takes place, either by the depo-
sition from the original wall, of substances differing from, or
more rarely, homogeneous with it, upon the internal surface of
the cell, or by an actual thickening of the substance of the cell-
wall. The first-mentioned form of deposition occurs in strata,
at least this may be distinctly seen in many situations. (See
Meyen's Pflanzen-Physiologie. Bd. 1, tab. I, fig. 4.) Very
frequently, — according to Valentin, universally, — these deposi-
tions take place in spiral lines ; this is very distinct, for example,
in the spiral canals and spiral cells. The thickening of the cell-
membrane itself, although more rare, appears still in some in-
stances indubitable, for instance, in the pollen-tubes, (e. g.
Phormium tenax.) Probably that extremely remarkable phe-
nomenon of the motion of the fluid, which has now been ob-
served in a great many cells of plants, is connected with the
transformation of the cell-contents. In the Charse, in which it
is most distinct, a spiral motion ma}' also be recognized in it.
But, for the most part, the currents intersect each other in the
most complex manner.

Absorption and Secretion may be classed as external ope-
rations of the vegetable cells. The disappearance of the parent
cells in which young ones have formed, or of the cell-nucleus
and of other structures, affords sufficient examples of absorption.
Secretion is exhibited in the exudation of resin in the intercel-
lular canals, and of a fluid containing sugar by the nectar-
glands, &c. &c.

In all these processes each cell remains distinct, and main-

6 INTRODUCTION.

tains an independent existence. Examples, however, also occur
in plants, where the cells coalesce, and this not merely with
regard to their walls, but the cavities also. Schleiden has found
that in the Cacti, the thickened walls of several cells unite to
form a homogeneous substance, in which only the remains of
the cell-cavities can be distinguished. PI. I, fig. 3, represents
such a blending of the cell-walls observed by Schleiden. The
entire figure is a parent cell, with thickened walls, in which
four young cells have formed, the walls of which are likewise
thickened and have coalesced with each other, as well as with
those of the parent cell ; so that only the four cavities remain
with their nuclei in a homogeneous substance. The spiral ves-
sels, and, according to Unger, the lactiferous vessels also, afford
examples of the union of the cavities of several cells by the
absorption of the partition walls.

After these preliminary remarks we pass on to animals. The
similarity between some individual animal and vegetable tissues
has already been frequently pointed out. Justly enough, how-
ever, nothing has been inferred from such individual points of
resemblance. Every cell is not an analogous structure to a ve-
getable cell ; and as to the polyhedral form, seeing that it neces-
sarily belongs to all cells when closely compacted, it obviously
is no mark of similarity further than in the circumstance of
densely crowded arrangement. An analogy between the cells of
animal tissues and the same elementary structure in vegetables
can only be drawn with certainty in one of the following ways :
either, 1st, by showing that a great portion of the animal tissues
originates from, or consists of cells, each of which must have
its particular wall, in which case it becomes probable that these
cells correspond to the cellular elementary structure univer-
sally present in plants ; or, 2dly, by proving, with regard to
any one animal tissue consisting of cells, that, in addition
to its cellular structure, similar forces to those of vegetable
cells are in operation in its component cells ; or, since this is im-
possible directly, that the phenomena by which the activity of
these powers or forces manifests itself, namely, nutrition and
growth, proceed in the same or a similar manner in them as in
the cells of plants. I reflected upon the matter in this point of
view in the previous summer, when, in the course of my re-

INTRODUCTION. 7

searches upon the terminations of the nerves in the tail of the
Larvce of frogs (Medic. Zeitung, 1837), I not only saw the beau-
tiful cellular structure of the Chorda Dorsalis in these larvae, but
also discovered the nuclei in the cells. J. Miillcr had already
proved that the chorda dorsalis in fishes consists of separate cells,
provided with distinct walls, and closely packed together like
the pigment of the Choroid. The nuclei, which in their form
are so similar to the usual flat nuclei of the vegetable cells that
they might be mistaken for them, thus furnished an additional
point of resemblance. As however the importance of these
nuclei was not known, and since most of the cells of mature
plants exhibit no nuclei, the fact led to no farther result.
J. Midler had proved, with regard to the cartilage-corpuscles
discovered by Purkinje and Deutsch in several kinds of cartilage,
from their gradual transition into larger cells, that they were
hollow, thus in a more extended sense of the word, cells ; and
Miescher also distinguishes an especial class of spongy cartilages
of a cellular structure. Nuclei were likewise known in the
cartilage-corpuscles. Miiller, and subsequently Meckauer,
having observed the projection of the cartilage-corpuscles at
the edge of a preparation, it became very probable that at least
some of them must be considered as cells in the restricted sense
of the word, or as cavities inclosed by a membrane. Gurlt also,
when describing one form of permanent cartilage, calls them
vesicles. I next succeeded in actually observing the proper wall
of the cartilage-corpuscles, first in the branchial cartilages of
the frog's larvse, and subsequently also in the fish, and also the
accordance of all cartilage-corpuscles, and by this means in
proving a cellular structure, in the restricted sense of the word,
in all cartilages. During the growth of some of the cartilage-
cells, a thickening of the cell-walls might also be perceived.
Thus was the similarity in the process of vegetation of animal
and vegetable cells still further developed. Dr. Schleiden oppor-
tunelv communicated to me at this time his excellent researches
upon the origin of new cells in plants, from the nuclei within
the parent-cell. The previously enigmatical contents of the cells
in the branchial cartilages of the frog's larvse thus became
clear to me; I now recognized in them young cells, provided
with a nucleus. Meckauer and Arnold had already found fat-
vesicles in the cartilage-corpuscles. As I soon afterwards sue-

8 INTRODUCTION.

ceeded in rendering the origin of young cells from nuclei
within the parent-cells in the branchial cartilages very pro-
bable, the matter was decided. Cells presented themselves in
the animal body having a nucleus, which in its position with
regard to the cell, its form and modifications, accorded with
the cytoblast of vegetable cells, a thickening of the cell-wall
took place, and the formation of young cells within the parent-
cell from a similar cytoblast, and the growth of these without
vascular connexion was proved. This accordance was still
farther shown by many details; and thus, so far as con-
cerned these individual tissues, the desired evidence, that these
cells correspond to the elementary cells of vegetables was fur-
nished. I soon conjectured that the cellular formation might
be a widely extended, perhaps a universal principle for the
formation of organic substances. Many cells, some having
nuclei, were already known ; for example, in the ovum, epi-
thelium, blood-corpuscles, pigment, &c. &c. It was an easy
step in the argument to comprise these recognized cells under
one point of view ; to compare the blood-corpuscles, for example,
with the cells of epithelium, and to consider these, as likewise
the cells of cartilages and vegetables, as corresponding with each
other, and as realizations of that common principle. This was
the more probable, as many points of agreement in the progress
of development of these cells were already known. C. H. Sclmltz
had already proved the preexistence of the nuclei of the blood-
corpuscles, the formation of the vesicle around the same, and
the gradual expansion of this vesicle. Henle had observed the
gradual increase in size of the epidermal cells from the under
layers of the epidermis, towards the upper ones. The growth
of the germinal vesicle, observed by Purkinje, served also at first
as an example of the growth of one cell within another, although
it afterwards became more probable that it had not the signi-
fication of a cell, but of a cell-nucleus, and thus furnished proof
that everything having the cellular form does not necessarily
correspond to the cells of plants. A precise term for these
cells, which correspond to those of plants, should be adopted ;
either elementary cells, or vegetative cells (vegetations-zellen) .
By still further examination, I constantly found this principle
of cellular formation more fully realized. The germinal mem-
brane was soon discovered to be composed entirely of cells, and

INTRODUCTION. 9

shortly afterwards cell-nuclei, and subsequently also cells were
found in all tissues of the animal body at their origin ; so that
all tissues consist of cells, or are formed by various modes, from
cells. The other proof of the analogy between animal and vege-
table cells was thus afforded.

I shall follow the same course in communicating the separate
observations, and shall speak, therefore, in the next place of the
structure and growth of the chorda dorsalis and cartilage, and
in the second section treat of the germinal membrane and the
remaining tissues.

10 STRUCTURE AND GROWTH

SECTION I.

ON THE STRUCTURE AND GROWTH OF THE CHORDA DORSALIS

AND CARTILAGE.

1. Chorda Dor salts.

The Chorda Dorsalis in the larva? of frogs and fishes lies
in, or in some instances, under the bodies of the vertebras, and
is continued behind the coccyx, through the whole length of
the tail. It is inclosed by a firm sheath, and forms a spindle-
like, consistent, gelatiniform, transparent cord, which is thick-
est at the commencement of the tail, and thence gradually
diminishes in each direction, both towards the skull and the
point of the tail. It cannot well be separated entire in re-
centlv killed animals, but is best obtained from them in the
form of delicate transverse sections. If the animal be placed
in water for twenty-four hours or longer after death, and the
tail then severed from the body at their point of junction, the
chorda dorsalis may be entirely pressed out, by gently scraping
along its course from the point of the tail, or from the head,
towards the wound. As this does not succeed if the animal be
allowed to lie out of water for the same period after death, the
easier separableness appears to depend upon a penetration of the
water between the chorda dorsalis and its sheath ; the firmer con-
nexion of it in the fresh condition, however, only upon a more
close contact, or wedging in of the chorda dorsalis, and not upon
a vascular connexion, for I do not suppose that it contains any
vessels. Microscopically examined, it exhibits, as J. Miiller
has discovered in fishes, a cellular structure in its interior, sur-
rounded externally by a proportionately thin cortical substance
(rinde), which is beset with scattered granules. The interior
exactly resembles the parenchymatous cellular tissue of plants.
(See plate I, fig. 4.) It is readily seen, especially at the point
of contact of three cells, that each one is surrounded bv its
own proper membrane. The cells vary much in size, being

OF THE CHORDA DORSALIS. 1 1

usually largest in the centre, and becoming somewhat smaller
towards the outside. They have an irregular polyhedral shape,
mostly with spherical surfaces, which are sometimes convex
towards the outside, sometimes towards the cavity of the cell.
Their walls are very thin, colourless, smooth, aud almost
completely transparent, firm, and slightly extensible. They
dissolve readily in caustic potash. The rudiments of the
chorda dorsalis in the conical interstices of the vertebne of
cartilaginous fishes are not dissolved by dilute or concentrated
acetic acid. The chorda dorsalis of fishes according to J. Miiller
does not become converted into gelatine after long boiling.
The cells of the chorda dorsalis of frog's larvae contain in their
interior a colourless, homogeneous, transparent fluid, which
does not become cloudy at a boiling heat ; the slight clouding
observed in the chorda dorsalis after boiling, appears to be
situated more in the cell-walls, which afterwards appear mi-
nutely granulated.

In the larva of Pelobates fuscus another formation occurs,
inasmuch as by far the greater proportion of these cells contain
a very distinct nucleus. It has the appearance of a somewhat
yellowish-coloured small disc, of a roundish oval form, rather
smaller than a blood-corpuscle of the frog, and almost as flat.
(See plate I, fig. 4 a, where it is represented from the chorda dor-
salis of Cyprinus erythrophthalmus.) In frog's larvae the nucleus
is nearly twice as large. It has a sharp, dark margin, and ap-
pears minutely granulated. In this little disc may be seen one,
rarely two, and very seldom three dark, sharply circumscribed
spots. It thus entirely resembles, both as a whole as well as in
its modifications, the cytoblast of vegetable cells with its nu-
cleolus, and microscopically, cannot at all be distinguished from
it. Compare plate I, fig. 4 «, with plate I, fig. 1 a. But it also
corresponds with it in its position in the cell. In very many
cells, the vertical wall of which is viewed from above, it may
be seen that the nucleus lies close on the inner surface of the
wall of the cell, or even embedded in the wall. It appears
then, as in plate I, fig. 1 a, only still somewhat flatter. I
have not, however, succeeded in observing that a lamella of
the cell-wall passes over its internal surface, which is also but
rarely seen in plants. If the external minutely granulated
cortical substance of the chorda dorsalis of Pelobates fuscus

12 STRUCTURE AND GROWTH

be more accurately examined, it is found that the granules are
oval, and furnished with a nucleolus, and that, with the excep-
tion of their being only about half as large, they entirely re-
semble the cell-nuclei. This cortical substance is not sharply
separated from the proper tissue of the chorda dorsalis ; and as
the cells of the latter suddenly diminish very much towards the
cortical substance, I think that these granules upon the latter
are the cytoblasts of flattened cells which form it. Sometimes,
although but indistinctly even with a very favorable light, very
fine lines may be perceived in the intermediate spaces between
these granules, where the cells come in contact, as in the
common tabular (or scaly) epithelium. In the chorda dorsalis
of the larva of Rana esculenta, where the nuclei in the cells
are not distinct, these nuclei in the cortical substance are not
seen ; the tabular structure, however, is evident in them. One
must be very cautious in denying the presence of the cyto-
blasts, when they are not immediately recognizable. They
may in animals, as in plants, attain such a degree of trans-
parency, as renders them very difficult of observation. Thus,
I could not for a long time detect them in the rudiment of the
chorda dorsalis, which is found in the conical intermediate
spaces of the vertebrae, in a large Carp, until on a very clear
day they appeared very pale but quite recognizable, and of pre-
cisely the form above described. They were somewhat more
distinct in the Pike and Cyprinus erythrophthalmus. The
delineation, plate I, fig. 4, is taken from the latter. They
are however smaller in these fishes than in frog's larvae.

To return to the larva of Pelobates fuscus. Here the cells
of the chorda dorsalis lie so close to each other, that the walls
of the two neighbouring cells are in immediate contact. Even
when three or more cells are in contact, they are generally so
close, that only the contiguous walls are observable. Some-
times, however, in such instances, a small intermediate space
remains, which is larger than could be filled up by the unthick-
ened cell-wall ; and there is then seen, as in plants, a species
(apparent or real?) of intercellular substance, or an intercel-
lular canal. With regard to this latter (intercellular canal), occa-
sionally, though rarely, in such an instance of close contiguity
of three cells, upon making a transverse section, the cell- walls are
observed sharply bounded, as well towards the cell as externally,

OF THE CHORDA DORSALIS. 13

and between the cells a small triangular interstice is seen, which
is filled by a transparent fluid (not by air), or at least by a sub-
stance which refracts the light in a different manner from the
cell-walls, just as it is represented in plate I, fig. 1 c, from the
onion.

Young cells, which float free, form within the cells of the
chorda dorsalis, as in plants. They are, however, in the larvae
of the frog so transparent, that very favorable light and good
instruments are required to see them. The number of cells,
also, in which new ones are formed in the larvae is not great, at
least in such as are to be had in the latter part of autumn. Id
the above-mentioned species of Cvprinus, and also in other
fishes, they are, however, easy to be seen, and in greater number.
Vesicles of very various sizes may be perceived in the cavities
of many of these cells, and also in those of the larvse of the
frog, though they are more difficult of observation in the latter ;
a single one of these vesicles sometimes fills the greater part
of the cavity; and occasionally several lie in one cell. (PI. I,
fig. 4, b, b, c.) They are commonly quite round; but not
unfrequently two are in contact, and flattened against each
other. That they lie free in the cell, follows from the fact,
that they may be isolated without rupture. If, for instance,
a small portion of the chorda dorsalis be torn into minute
pieces, and a thin plate of glass with water be placed upon
them, by moving this lightly backwards and forwards a few
times, some such isolated vesicles may often be brought into
the field of vision. They may then be made to roll about, and
thus demonstrate their globular form. I have taken great
pains to discover a nucleus in their walls, but without suc-
cess. The young cells of the chorda dorsalis, also, in the larvse
so often mentioned, have often the appearance, so long as
they are not isolated, of possessing a nucleus : but one may
readily be deceived here, since such a nucleus may belonsr to
a cell lying above or below them. Caution must also be
used, not to confound a globular epithelial cell, which may have
slipped into the chorda dorsalis in making the transverse sec-
tion, with the true cells of that structure. I have not as vet
been able, with certainty, to observe any nucleus, at least not
of the characteristic form, in isolated young cells of the chorda
dorsalis. In rare instances, a very small corpuscle, (d, d, of

14 STRUCTURE AND GROWTH

the figure,) lay in the inner surface of the young cell. It must
remain a question whether the nucleus is really wanting, or
whether it is only not visible in consequence of its translucency,
or whether these corpuscles are developed into the nucleus.
The chorda dorsalis accords with the vegetable cells, at least in
this respect, that young cells are formed within the old ones.

With regard to the thickening of the cell walls ; these ap-
pear to remain always simple (unchanged) in the chorda dorsalis
of the larva of the frog. But in the fully developed osseous
fishes, in Cyprinus, for example, a thickening is exhibited in
those cells which lie near the axis of the conical interspaces
of the vertebrae. The cell-cavities always become smaller in
consequence of this thickening of the walls. The thickened
walls, or the intermediate substance between the cell cavities
consist of closely cohering longitudinal fibres, between which
very fine transverse fibres are also sometimes seen. The longi-
tudinal fibres run uninterruptedly past several cells ; and the
primitive membrane of each cell can no longer be distinguished.

To sum up the researches upon the chorda dorsalis in a few
words ; it may be said to consist of polyhedral cells, which have,
in or on the internal surface of their walls, a structure, according
in its form and position with the nucleus of the cells of plants,
namely, an oval flat disc containing one, two, or more rarely
three nucleoli. The cells usually lie in close contact with each
other; but sometimes at points where three or more cells meet
together, a sort of intercellular substance, or an intercellular
passage is seen. Young cells, which are at first round, and float
free, are formed within parent cells. Nuclei of the charac-
teristic form, are not distinctly observed in these young cells,
but sometimes a small globule lies upon their inner surface. In
those cells which undergo farther development, the cell-mem-
brane ceases to exist as a distinct structure, and the interme-
diate substance between the cell cavities consists for the most
part of longitudinal fibres.

With the exception of the formation of these fibres, into the
origin of which I have not yet examined, and the absence of the
nucleus in the young cells, these cells entirely accord with the
vegetable cells. It must remain undecided whether the nu-
cleus is really wanting in these young cells, as it is not yet
proved to exist in all plants, (for example in many acotyledo-

OF CARTILAGE. 15

nous plants,) or whether the little corpuscle, which presents
itself on the inner surface of some young cells, is the nucleus
which grows with the cell, as it is observed to do in some other
animal cells ; or whether the nucleus in the young cells is in-
visible in consequence of its transluccncy, since even fully-deve-
loped cells are met with, in which, although certainly present,
it is, in consequence of its transparency, barely visible.

2. Cartilage.

The accordance of the structure of cartilage with the tissue of
plants is of more importance in reference to animal organization.
We have here to do not only with a more widely extended animal
tissue, but also with one which, at least, in its subsequent stages
of development, contains vessels, and therefore bears more
decidedly the character of an animal tissue. The simplest form of
cartilage is exhibited in the cartilages of the branchial rays of
fishes. If, for example, a branchial ray of Cyprinus erythroph-
thalmus be loosened from the branchial arch, and the mucous
membrane be removed by gentle scraping, the cartilage remain-
ing presents the appearance of a little rod, which diminishes
from the point of its insertion on the branchial arch towards its
free end, its sides being somewhat compressed, and exhibiting
on their margins some blunt prominences. The structure of
this cartilage is very simple. At the point it perfectly resembles,
in its whole appearance, the parenchymatous cellular tissue of
plants. (See pi. I, fig. 5, from the above-mentioned Cyp.
eryth.) Little polyhedral cell-cavities with rounded corners are
seen lying closely together. The cell-cavities are separated
from each other by extremely thin partition walls. The cell-
contents are transparent, and a small pale round nucleus (a) may
be seen in some cells when in the recent state, in others only
after the action of water upon them. The structure of the
lateral prominences of the cartilage is similar to that at the
point, only that the cells are somewhat extended in length.
Advancing from that point towards the middle, or still better from
the point towards the root of the branchial ray, the partition
walls of the cell-cavities are observed to become gradually
thicker; and the cavities are here somewhat smaller. (PI. I,
fig. 6.) On the thickened cell-walls it may now also be seen

16 STRUCTURE AND GROWTH

that the intermediate substance of the cell-cavities is not a
simple structure, but one composed of the walls peculiar to the
contiguous cells : that is to say, each cell is surrounded with a
thick ring, its peculiar wall, the external outline of which is
more or less distinct. In the preparation from which the deli-
neation is taken, it was in some parts quite as distinct as the
internal. Between two cells these external outlines blend into
one line, but separate again when the contact of the cell-walls
ceases \ there is thus often left between the cell- walls a three
or four-cornered intermediate space (c), filled with a kind of
intercellular substance. No other structure, no deposition of
strata, or distinction between primary cell-membrane and se-
condary deposit can be observed in the thickened cell-walls.
The cell-contents also remain clear after the thickening of the
walls. At the base of the branchial ray, it is scarcely possible
to distinguish between the different cells-walls, and the cartilage
presents the appearance of a homogeneous substance, in which
separate small cavities only are seen. (PL I, fig. 7.) Around
some few only of the cell-cavities, a trace of the peculiar cell-
walls may be seen in the form of a ring. This ring is usually
somewhat thin, so that the entire intermediate substance of the
cell-cavities cannot be formed of the cell- walls ; but the inter-
cellular substance, which was so small in quantity in the centre
of the branchial ray, here contributes essentially to the forma-
tion of the cartilaginous substance, and often completely pre-
vents the immediate contact of the cell-walls. This intercel-
lular substance appears, however, to be homogeneous with that
of the cell- walls, and in most situations coalesces with them.
The cell- cavities, which are here transparent and without gra-
nulous contents, are now the cartilage-corpuscles.

The process of formation of this cartilage is as follows. It
consists originally of cells, which lie in very close contact, but
every one of which has its special, very thin cell- membrane.
This follows, firstly, from the complete accordance in appear-
ance, of cartilage in its earliest stage, with vegetable cellular
tissue; secondly, from the presence of the nucleus in the
young cells of cartilage, a structure which, as will subsequently
be seen, occurs in almost all the cells proved to exist in other
tissues; thirdly, from the fact, that a separation of the cell-
walls is often distinctly perceptible in instances where they are

OF CARTILAGE. i;

thickened. These cell-walls lie either in close contact, or have
only a trace of intercellular substance between them, or there
is sufficient of that material to entirely prevent the contact of
the different cells. Their walls, which are originally formed
of a very thin membrane, become thickened. The cavities of
the cells with thickened walls which are seen in the centre of
the branchial ray, are smaller than those of the cells which lie
nearer the surface, the walls of which arc less dense ; but,
whether this is produced by a thickening of the cell-wall taking
place from without inwards, or whether rather the cells were not
smaller in their original formation, is a matter of uncertainty.
No deposition of strata, nor any distinction from the primordial
cell -membrane, can be recognized in these thickenings of the
walls. The condensed cell-walls at length coalesce either with
each other, or with the intercellular substance, to form one
homogeneous mass, in which only the cell-cavities remain per-
ceptible, presenting the appearance of small distinct excavations
filled with a transparent substance ; these cell-cavities are the
cartilage-corpuscles.

In the foregoing description no error can arise from the
great variety in form which the cartilage-corpuscles frequently
present ; for, on examining the branchial rays of a very large
pike, the gradual transition may be traced, from the thin-
walled almost globular cells to the most varied forms, in which
the remains of the cell-cavities are so much extended in length
as to give to the cartilage almost a fibrous appearance.

The same extremely simple process of formation (modified,
however, in some important respects) is presented in all carti-
lages. These modifications, the fundamental type of which is
already pointed out in the cartilages of the branchial rays of
fishes above described, depend chiefly upon the share relatively
contributed by the thickened cell-walls, or the intercellular sub-
stance, to form the intermediate substance of the cell-cavities,
or cartilage-corpuscles. We have seen that this intermediate
substance was formed almost entirely of the thickened cell-
walls, with but a minimum amount of intercellular substance,
in the centre of the branchial rays of fishes, whilst at their base,
that is, in the earliest formed cartilage, the intercellular sub-
stance preponderated, and the less dense cell-walls contributed
less to the formation of the true substance of the cartilage.

2

is STRUCTURE AND GROWTH

Tlie walls of the cells appear to contribute little or nothing to
the formation of the substance of most of the ossifying carti-
lages,— those of the higher animals for example.

The cartilages of the branchial arches of the tadpole, like
those of the branchial rays of fishes, consist of cells, which are,
however, much larger than those of the fish, though smaller
than the cells of the chorda dorsalis, with which they have, in
every other respect, much similarity. The partition-walls of
the cells are thicker than in the chorda dorsalis, but they may
still be termed thin when compared with the cell-cavities. (See
pi. Til, fig. 1, which exhibits branchial cartilage from the young
larva of Pelobates fuscus.) The cartilage intended to be used for
investigation must be taken quite fresh from the living animal ;
for the structures become very indistinct if it be allowed to lie
in water for any time after death, even though it be entire.
After stripping off the mucous membrane, the cellular structure
is readily recognized by the aid of the microscope. The cells
vary much in size, and are more or less flattened against one
another. The wall of each separate cell may be distinctly seen
in the majority of instances, and its thickness might even be
measured ; that we cannot trace it so evidently in the smallest
cells is probably referrible to the extreme thinness of their
sides. The walls of the cells are for the most part in contact,
but intercellular substance may be seen in many situations,
and especially where several cells are contiguous. The surface
of the cartilage, which is represented on the left and lower
margin of the figure, (pi. Ill, fig. 1,) is formed in the first place
of intercellular substance, which, in as much as the cells ori-
ginate in it, may be called Cytoblastema.

This cartilage may, therefore, be described as consisting of
intercellular substance, or cytoblastema, in which great num-
bers of cells are seen. The cell-contents are generally clear as
water ; but in the younger and smaller ones (for example, c}) the
contained matter is less pellucid, and somewhat granulous. Each
cell contains a spherical granulous nucleus, which lies upon the
inner surface of the wall, and which again encloses a nucleolus.
The size of the nucleus is not precisely alike in all the cells : it is
somewhat larger in the large ones, but its size bears no proportion
to the increased bulk of the cell ; and again, the smaller cells
are not much larger than the nucleus which they contain.

OF CARTILAGE. 19

Nuclei, around which no cells have yet commenced to be de-
veloped, may be observed in the cytoblastema between the
cells in some situations ; for example, a and b. These like-
wise contain a nucleolus, and arc somewhat less than the nuclei
in the smaller cells.

The above observations furnish us with a complete repre-
sentation of the development of cartilage-cells, and show the
accordance of that process with the development of vegetable-
cells, inasmuch as they exhibit the simultaneous presence in the
cytoblastema both of simple nuclei, and of cells containing a
nucleus of similar shape and size upon the inner surface of
their walls, and which may be observed in all stages of tran-
sition, from such as are scarcely larger than the nucleus they
contain, to such as are many times its size. Simple nuclei are
first present, developed in the cytoblastema. When these have
arrived at a certain size, the cell is formed around and closelv-
encompassing them. The cell gradually expands, whilst the
nucleus remains lying on a point of the inner surface of its
wall. The nucleus, also, increases somewhat in size, but not in
proportion to the expansion of the cell. Now these three hy-
potheses may be assumed from the above facts ; either the cell
is first developed, and the nucleus subsequently, or both arc
developed simultaneously, or the nucleus is first developed,
and then the cell around it. The first supposition, that the
cells are developed earlier than the nuclei, is not possible, since
in that case cells would be found at a certain period of deve-
lopment without nuclei. The simultaneous development of a
cell, together with its nucleus, as two distinguishable struc-
tures, is equally impossible, for then we should observe a stage
of development, at which as yet the cell and nucleus had not
reached the size of the ordinary nucleus. In order to explain
the above observations, we must, therefore, have recourse to
the third supposition, viz. that the nucleus is first developed
and then the cell around it.

The form of the young cells depends upon the space allotted
them for expansion. They are, therefore, cither round or angular,
according as the neighbouring cells permit of, or limit their re-
gular expansion. Two or more cells are often developed close to-
gether in one intercellular space, and thus compress those already
formed, and the intercellular substance on the outside of them ;

20 STRUCTURE AND GROWTH

tins f;ict explains the common appearance of two or four cells
tying together in a group, being separated from one another
by thin Avails, whilst between such groups and the neighbour-
in"- cells we see much more intercellular substance.

The cells at first appear finely granulated, and not so trans-
parent as in the more fully developed condition. The thick-
ening of the cell-membrane takes place simultaneously with
its expansion. One of the cells in pi. Ill, fig. 1, exhibits two
nuclei, one of which, like those of all the other cells, has but ;
one nucleolus, the other having two. It may be conjectured, '
that this second nucleus is destined to the formation of a young
cell within the larger one.

In the intercellular substance at e in the same figure (pi. Ill,
fig. 1,) may be seen a small corpuscle, surrounded by a granu- i
lous and indistinctlv circumscribed mass, the rest of the inter- \
cellular substance being smooth and homogeneous. This is,
perhaps, a nucleus in the act of formation, the nucleolus of
which is already developed ; and when the granulous mass sur- 1
rounding that structure has obtained a defined external boun- j
darv, it will form a nucleus. If such be the case, we have \
here an instance of accordance of the development of the germ
itself with the formation of the nucleus of vegetable- cells ob- '
served bv Schleiden.

On examining the cartilage of the branchial arches of the
tadpole in the more completely developed state, (pi. I, fig. 8,)
we find the cells generally lying in groups, so that two, three,
or four lie close together, separated from other groups by
thicker partition walls. The special walls of the individual
cells are less distinct, but at several spots where three or more
cells are in contact, for example, at a, the separation of the
walls may yet be seen, and a trace of intercellular substance is
also present ; the latter, however, is almost homogeneous with the
cell-walls. It may also be observed that the cell-walls are thicker,
in these situations than they are represented in pi. Ill, fig. 1.
Some parallel lines may be seen at various spots in these con-
densed cell- walls, and the thickening might, in such instances,
be supposed to be really produced by a stratified deposition of
the substance upon the internal surface of the cell-wall. But
at the same time it must be remembered, that every partition-
wall between two cells must consist of two layers, each of which

OF CARTILAGE. 21

corresponds to the wall of the corresponding cell. This appear-
ance of strata, however, is observed only in the thick walls
between two groups of cells, and as these groups probably ori-
ginate by the formation of two or four cells within a parent
cell, each half of the partition-wall between two groups must
(presuming such to be the mode of their formation) consist
again of two layers, the one of which corresponds to the wall
of the parent cell, the other to that of the secondary cell, so
that each partition-wall of two groups must consist of four
layers. Although it does, indeed, appear that even a greater
number of layers or strata are present, yet I must at the same
time remark, that these observations are by no means suffi-
ciently conclusive for the proof of a fact so important in refer-
ence to the process of nutrition, and that I am so far from re-
garding them as evidence of a stratified deposition of the sub-
stance, as not to hold such a thing to be even probable. The
appearance is probably an optical deception. As before stated,
no distinction was found between primary cell-membrane and
secondary thickening in the cartilages of the branchial rays of
fishes, but it seemed that the cell-membrane had actually be-
come thickened ; neither is there any such distinction to be
observed in the branchial cartilages of the tadpole.

If the above described groups be assumed to have had their
origin by the formation of secondary cells within a primary
parent one, in that case, secondary cells which completely fill
the parent one have not been developed in all the primary cells,
for isolated cells occur in the branchial cartilages of Pelobates
fuscus, which are somewhat larger than the secondary ones,
but smaller than the other primary cells, and remarkable also, as
will be seen immediately, from their contents.

The cells of the branchial cartilages of the larva of Pelobates
fuscus just mentioned, contain within them one or more nuclei.
(PI. I, fig. 8, d.) These nuclei, which may be easily isolated,
are either slightly oval, or perfectly globular, more or less
granulous and yellowish, and apparently hollow. They contain
one or two very distinct, round, dark nucleoli, which lie in
their interior either close upon the wall, or very near to it.
The nuclei (a portion of them at least) appear to lie free in
the cell-cavity, for they may readily be isolated. The above
mentioned primary cells of the larva of Pelobates fuscus in

22 STRUCTURE AND GROWTH

which none of these secondary cells, completely filling the
parent one have been developed, contain very commonly
several such nuclei, and also one or more young cells. PL I,
fig. 8, fft represents such young cells from the branchial carti-
lages of the larva of Rana esculenta. They are round vesicles
containing a nucleus identical in form and size with those
which lie free, but which is situated upon the internal surface
of the wall, and never in the centre of the cell. This nucleus
is never wanting in the young cells. The cells, however, vary
much in size, some being scarcely larger than the nucleus they
contain, others twice or thrice as large. From one to three
such young cells, in various stages of development, are com-
monly found within the primary one, where they sometimes
become flattened from want of space. As the figure represents,
most of the secondary cells contain these young ones, and but
few of them only simple nuclei (such as have no cell around them),
in some of the young cells, indeed, a second somewhat paler
nucleus appears. These young cells lie free within the primary
cell, and may be isolated in the same manner as was described
with regard to those of the chorda dorsalis. They appear in the
first instance to be perfectly transparent ; but gradually obtain
a granulous yellowish aspect, and it is remarkable, that the
earliest formation of this yellowish deposit takes place generally
if not constantly, in the neighbourhood of the nucleus.

It will thus be seen that these young cells, (fat cells ?) which
are formed within the true cartilage-cells, furnish us with a
series of observations as regards their development, similar to
that observed in the formation of the cartilage-cells themselves:
namely, simple nuclei, cells closely encompassing those nuclei,
and all the stages of transition up to the largest cells ; but
never have we met with these young cells without nuclei. So
that the same conclusions might be arrived at with respect to
the mode of their development, as were before with regard to
that of the cartilage- cells, namely, that the nuclei are first
formed, and around them the cells, precisely as in plants. The
nucleus in these young cells, however, does not appear to in-
crease in growth after the cell has once formed around it.
The accordance in form between these and the young cells of
vegetables is shown by comparing Plate I, fig. 8, with fig. 2, b.

The nucleus of the true cartilage-cells like that of vegetable-

OF CARTILAGE. 23

cells is subsequently absorbed. After the cartilages of the
branchial rays of fishes have been exposed to the action of water,
it is only in the young cells that the nuclei are visible ; they
are much more rarely seen in those cells of which the walls are
already very much thickened. In many cells of the branchial
cartilages of the tadpole, a small nucleus with a ragged outline
may be observed, which is probably the cytoblast of the cell
in the act of undergoing absorption. These cytoblasts (nuclei)
of the true cartilage-cells always lie in the cell-cavity, even
when its wall is thickened, and it is impossible to distinguish
whether they lie free or are still connected with the cell-wall.
A twofold explanation is here possible : either the cytoblast
separates from the wall after the formation of the cell-membrane
is perfected, and falls free into the cavity (as occurs in plants),
and at such period a secondary deposition of substance upon
the cell- wall first commences ; or the thickening of the wall is
due to an actual increase of the original cell-membrane, and in
that manner the nucleus is pushed inwards, and may remain
in connexion with the wall. If a secondary deposition of
substance took place before the nucleus was disengaged from
the cell-membrane, that body must be enclosed in the wall,
and would not lie in the cell-cavity. As both these expla-
nations are possible, it will be seen that no conclusion can be
drawn from the position of the nucleus, as to whether the
thickening of the cell-wall be a secondary deposition, or an
actual growth of the cell-membrane. Sometimes a carti-
lage-cell presents more than one nucleus; when in such a
case the original nucleus of the cell is absorbed, all those
observed are probably the germs of new cells, which have not
as yet commenced their development. The same fact is fre-
quently observed in plants. The nuclei in the branchial
cartilages of the tadpole have for the most part the same size j
some, however, which are probably not as yet perfectly formed,
are smaller than others. It also often occurs that a nucleus
is seen expanded to three or four times the usual size ; such
instances might be mistaken for young cells without nuclei,
but they may be readily recognized by their general aspect.
They are more transparent and delicate, and exhibit one or
two nucleoli, which are easily detected j when two are present
they are widely separated from one another. According to

24 STRUCTURE AND GROWTH

Schleiden, a similar enlargement of the nucleus also occurs in
plants, thus affording a remarkable accordance in what seems
a very unimportant circumstance. It appears to be a kind of
abortion ; for I have never yet seen a cell formed around such
a nucleus.

The cranial cartilages of the tadpole (Plate I, fig. 9) are dis-
tinguished from the branchial by the smaller size of the cell-
cavities, and the increased strength of the firm intermediate
substance. The walls of the separate cells cannot now be
traced, they appear to have coalesced with the intercellular
substance, which is present in greater quantity. The cells lie
in groups of two or four together, and it is very probable, that
in this cartilage, each group is formed of cells, which have
been developed in a parent cell ; for some may be seen, for
example at c, which do not as yet quite fill the original cell.
Such an instance, however, is rarely so very distinct as not to
admit of a doubt. There is a very striking similarity between
the group «, fig. 9, and fig. 3, which represents four young
vegetable cells developed in a parent cell, and the thickened
walls of which have coalesced with one another and with those
of the parent cell, so that the four cavities onlv remain in an
homogeneous substance. That portion of the cell-cavities
which is still visible is filled with a granulous yellowish sub-
stance, in which lie one or more nuclei, or young cells provided
with a nucleus : these remains of the cell-cavities are the car-
tilage-corpuscles discovered by Purkinje.

The intercellular substance is universally much more pro-
minent in the cartilages of mammalia than it is in those
hitherto described, and in them it forms the principal part of
the firm mass of the cartilage. There is not, however, any
essential difference either between the structure of the several
kinds of cartilage of mammalia, or between these and the car-
tilage of lower animals, the only distinction being that it is a
little more difficult to prove the existence of the special walls
of the cartilage-cells in the former.

The intercellular substance in some cartilages of mammalia
is at first so soft, that the cells fall apart under slight pressure,
and float free in the fluid. If, for example, a thin lamella be
cut off from the cartilage at the angle of the lower jaw of a
foetal pig of three and a half inches in length (a period when

OF CARTILAGE. 25

the cartilage is about to become, but is not as yet, ossified), and
placed under the compressorinm, the cells will be seen to lie so
closely in it, that the space occupied by them may be estimated
at three fourths, and that of the intercellular substance at one
fourth of the whole volume. Many of the cells which have
become separated by the process of cutting, float already in the
fluid; and on slightly compressing the preparation many more
become loose, and flow out in streams from the intercellular
substance into the surrounding fluid. The intercellular sub-
stance is too soft to prevent the separation, but at a subsequent
period of development this cannot be effected. According to
Meckauer the cartilage-corpuscles may also be isolated by boil-
ing. I once succeeded in crushing one of these young carti-
lage-cells while still in connexion with the preparation. The
first effect of the compressorium was to produce an extension
of breadth ; it then suddenly shrank together, whilst a clear
fluid streamed out, thus proving the contents of the cell to be
fluid and transparent. Now, inasmuch as these cells present in
different instances a more or less granulous appearance, it fol-
lows that the cells of ossifying cartilage must have a peculiar
investing membrane, which is granulous, and thus that they
are actual elementary cells, in our sense of the word, and nei-
ther mere excavations in the substance, nor perfectly solid
corpuscles. The appearance of the cells which float about en-
tirely accords also with this view, for while their contents seem
to be clear, the cells look granulated. All of them contain a very
beautiful oval or circular, not flattened cell-nucleus, situate
upon the internal surface of the wall, and this nucleus en-
closes one or two very distinct nucleoli ; in short, they in every
respect accord with the elementary cells of most of the other
tissues. By the aid of acetic acid we may also frequently suc-
ceed in rendering the cell-walls visible upon a thin lamella of
cartilage, and as the cell-contents are at the same time dis-
solved by the acid, it has the additional advantage of bringing
the nucleus into view, which is sometimes indistinct in conse-
quence of the granulous nature of the contents. Plate III, fig.
2, exhibits a portion of cartilage so treated with acetic acid ;
it is taken from the as yet unossificd portion of the ilium of an
embryo pig of five inches in length. The cell- walls, with their
double outlines, may be seen, and both the illuminated and

D. KL HILL UBRARY

2(J STRUCTURE AND GROWTH

dark side in the thickness of the walls distinguished. The
delineation, at the same time, proves how important a share is
taken by the intercellular substance in the formation of the
firm structure of cartilage.

The cartilages of the foetus do not altogether accord in chemi-
cal constitution with those of the adult, since we can obtain from
them by boiling but a small quantity of a gelatinous substance,
and that only with great difficulty, and they afford no true
gelatine (capable of forming a jelly). I boiled some unossi-
fied cartilages, consisting of apophyses of the femur and carti-
laginous portions of the scapulae, taken from several embryo
pigs, measuring three and a half inches in length. After
twelve hours' boiling, they entirely crumbled into very small
scales, which gave a variegated appearance to water in which
they were stirred about, and appeared under the microscope
extremely thin and granulous. The fluid, when filtered and
evaporated almost to dryness, did not coagulate. Alcohol pro-
duced a copious precipitate, wTiich was dried, afterwards dis-
solved in boiling water, and then evaporated almost to dryness;
still no coagulation took place. Alum, however, clouded the
fluid, and acetic acid had the same effect, but in a much
slighter degree. As the quantity of cartilage made use of in
the foregoing experiment was too small, I made a further in-
vestigation with cartilage which had already become ossified,
from the same embryos, namely, the frontal and parietal bones,
scapulae, humerus, femur, and some ribs. The unossified parts
were removed as cleanly as possible from all the bones. The
earthy matter was withdrawn by hydrochloric acid; the carti-
lages were then washed with water, and boiled for twenty-four
hours. Under this process they fell to pieces very slowly ,
meanwhile numerous little glittering scales appeared in the
fluid, which, after being dried, resembled very minute fish-
scales, and exhibited a beautiful play of colours. They were,
perhaps, the lamellae described by Deutsch, which surround the
minute medullary canaliculi. The form of most of the pieces
of cartilage remained perfectly recognizable, and was but
slightly altered. They looked of a yellowish-white colour, and
not at all gelatinous, as substances usually do when about to
be transformed into gelatine. The fluid was filtered from these
little scales and pieces of cartilage, and then evaporated almost

OF CARTILAGE. 27

to dryness. It did not exhibit any trace of coagulation after
standing twenty-four hours. After being dried, it was again
dissolved in boiling water, on which occasion, however, a por-
tion remained undissolved. It was, therefore, filtered ; the
fluid was copiously precipitated by alum, and the precipitate
was, for the most part, although not entirely, dissolved, on the
addition of alum in excess. Acetic acid likewise rendered the
fluid very turbid, and an excess of acid did not entirely remove
the cloudiness. It was copiously precipitated by tincture of
gall-nuts, and acetic acid removed this precipitate again, leaving
a very slight turbidness. (Acetic acid likewise completely dis-
solves the precipitate obtained from glue by tincture of gall-
nuts, therefore glue, when dissolved in acetic acid, will not be
precipitated by the tincture.) According to these reactions,
the gelatinous substance obtained appears to be chondrin, not-
withstanding that it wras obtained from ossified cartilage. The
question, therefore, arises — does the cartilaginous substance
which is connected with earthy matter in the foetus really
yield chondrin instead of the gelatine of bone, or wras there
much unossified cartilage still contained in what appeared to
be ossified, and was that the sole source of the chondrin ? The
point is, at all events, worthy of renewed investigation. It is
surprising that the foetal cartilages should exhibit so great a
resistance to the action of boiling water, and that although
they yield a small quantity of a gelatinous substance, they do not
afford any which has the property of gelatinizing.

The formative processes of cartilage hitherto described,
proceed, as it appears, without the presence of vessels in the
structure ; such at least is the case in thin cartilages, to which
probably the fluid parts of the blood can penetrate from the
vessels of the neighbouring tissues. In the branchial rays of
the fish, for example, I could not find any space in which ves-
sels could have existed ; throughout the structure masses of
cartilage and cartilage-corpuscles were to be seen, but no canals
which could have been traversed by vessels.

The manner in which ossification proceeds now becomes an
interesting object of inquiry. The investigation is best pur-
sued by making very fine sections with a razor, from the half-
ossified cartilages of the extremities, vertebrae, or coccyx, of
the larva of Pelobatcs fuscus. The little cartilage-cells, which

28 STRUCTURE AND GROWTH

arc not enclosed one within another, and are for the most part
furnished with a nucleus, are readily recognized in the true
cartilaginous substance of the unossified cartilages. I am not
prepared to state whether this substance is formed by thicken-
in"- of the cell-walls, or by the intercellular substance. The
earthy matter is first deposited in the true cartilaginous sub-
Btance. It first appears in the form of isolated, extremely
minute granules, by which an indistinct appearance of arched
stride is sometimes produced. At other points, these little gra-
nules of earthy matter lie collected together into larger irregu-
lar heaps. I do not know whether these little collections are
depositions of pure earthy matter which has not as yet united
with the cartilage, and therefore merely provisional deposits
which subsequently are distributed equally in the cartilaginous
substance (which is not probable), or whether this earthy
matter is already united with the cartilage, and that the regular
aspect which the structure presents when ossified may be ac-
counted for by the gradual union of the earthy matter with it
after the same mode. I saw no such deposition of earthy matter
in heaps in the incompletely ossified parietal bones of the same
larva, but the whole cartilaginous substance contained it equably
distributed without any perceptible granules. In both instances,
however, when dilute hydrochloric acid is applied to the object
under the microscope, the boundary denoting the solution of the
earthy matter, and the consequent transparency of the cartilage,
may be distinctly seen advancing in the form of a sharply-defined
line from the edge of the preparation towards the interior, proving
that, in the cartilages first mentioned, there was earthy matter
equably united with the substance, in addition to the heaps and
isolated granulous deposits. For this boundary line cannot be
produced by the mere progressive imbibition of the acid with-
out a solution of the earthy salts ; at least neither an unossi-
fied cartilage, nor one from which the earthy matter had been
previously withdrawn and the acid again washed from it, ex-
hibited the phenomenon of such a line advancing towards the
interior. During the early period of ossification, when this
line arrives at a cell-cavity, it becomes indented proportionally
to the size of the cavitv, because it does not come in contact
with any earthy matter there ; the cell-cavities, in the first
instance, being free from earthy salts. The reverse, however, is

OF CARTILAGE. 2!)

the case in the more completely ossified parts ; there the cell-
cavity remains behind, forming a dark indentation in the
line, which as it advances renders the tissue transparent, and
leaves the cavity a black spot, from which dark fibres, simi-
lar to those of the corpuscles of bone, issne in a stellated form.
Shortly afterwards the fibres disappear, then the corpuscle gra-
dually diminishes, and at last vanishes also, leaving a pale spot.
Such an appearance could not be due to an air-bubble in the
cell-cavity ; for in that case, I think, the course of its exit
might be followed. It is probably a more compact mass of
earthy matter, which does not become dissolved so quickly as
that contained in the substance of the cartilage. After this
has become impregnated with earthy matter, the cell-cavities are
also filled, and when so filled they are the osseous corpuscles.
Similar observations might be instituted on the ossified carti-
lages of mammalia, in which the identity of osseous and carti-
lage-corpuscles was rendered more certain by Miescher's re-
searches. The next question which presents itself concerns
the nature of those minute fibres which proceed in a stel-
lated form from the osseous corpuscles. After the earthy mat-
ter has been withdrawn the corpuscles may still be seen, though
rendered very pale by that process ; the fibres, however, are
not at all visible, although a formation corresponding to them
is certainly present in the cartilaginous substance, and their
extraordinary minuteness sufficiently explains the invisibility.
The same formation might also exist before ossification, but
be invisible from the like cause. As these fibres and the
cell- cavities become filled with earth v matter simultaneouslv,
and at a later period than the cartilaginous substance, and
since they contain the earthy salts in a more compact and
less easily soluble mass, it is probable that they are hollow
tubes, that is, canaliculi which proceed from the cell-cavities,
spreading out into the cartilaginous substance. According,
therefore, to the view which we take respecting the cartilage-
corpuscles, according as we consider them to be the cavities of
cells, the walls of which have become thickened and blended, not
only with one another but with the intercellular substance, so
as to form the cartilaginous substance; or as we take them
for the entire cells, and the intermediate substance of the
cell-cavities as only intercellular substance, so must these tubes

30 STRUCTURE AND GROWTH

be viewed either as canaliculi which penetrate from the cell-cavity
into the thickened cell-walls, or as hollow prolongations of the
cells into the intercellular substance. In the first case, they
might be compared to the porous canals of vegetable cells ; in
the second, they would correspond with prolongations of
cells, such as we shall often again meet with in the progress of
this work. Meanwhile, for an example of those cells which
are extended out on all sides into canals, and which I have
called stellated cells, the reader is referred to plate II, figs. 8
and 9, where those transformations are delineated from pigment-
cells. I decidedly give the preference to the latter explanation
of the canaliculi, because they pass through the entire thick-
ness of the firm cartilaginous substance, a fact which, in order
to be consistent with the first view, requires for its explanation
that the substance between the cell-cavities should be formed
of the thickened cell-walls, which is certainly not the case
in the cartilages of mammalia, as is seen in plate III, fig. 2.
The osseous corpuscles, with their canaliculi, would therefore
be the cartilage- cells transformed into stellated cells, and filled
with earthy matter. We shall return to this metamorphosis
of round into stellated cells when treating of the pigment. The
resemblance between stellated pigment-cells and osseous cor-
puscles is sometimes very striking, as is shown, for example,
by the pigment-cell which lies to the extreme right in plate II,
fig. 9. The compact bony substance is intercellular substance ;
it is, however, probable that the walls of the stellated osseous
cells form some, if only a very small part, of it.

When ossification takes place, the earthy matter is first de-
posited in this intercellular substance, and probably at a sub-
sequent period also in the cell-cavities. The deposition often
causes the substance to assume a darkish granulous appearance
in the first instance, which it afterwards loses, becoming more
equally dark. If we assume, what is extremely probable, that
the earthy matter is contained in bones in combination with
the cartilaginous substance, in a manner analogous to a che-
mical union, and not in the form of minutely-divided granules,
the mode in which the union with the earthy salts takes place
may then be explained in two ways : either the earthy matter
combines with a particle of cartilaginous substance in such a
manner that each smallest atom receives in the first instance a

OF CARTILAGE. 31

minimum of salts, and gradually more and more, until the whole
portion of cartilage obtains its due quantity ; or, the earthy
matter unites at first with some only of the smallest atoms of
the cartilage, combining, however, with these to the full propor-
tion which their capacity of saturation requires ; the remaining
atoms then gradually and successively receive their due portion
of the salts, so that each atom does not chemically combine with
them until it can become completely saturated. The latter
explanation, from the analogy with inorganic combinations, and
from the above-mentioned granulous appearance which cartilage
exhibits when undergoing ossification, appears to me by far the
more probable. For, according to the first view, the medullary ca-
naliculi, in the neighbourhood of which the deposition of earthy
matter first commences, ought to be surrounded, not by a gra-
nulous appearance, but by a dark shadow which should gradu-
ally fade away to a pale edge.

I conceive the formation of the medullarv canaliculi in ossi-
fying cartilage to be similar to that of the capillary vessels,
which will be examined hereafter. We shall return to them
again, as also to the origin of the concentric laminas of bone.

We will now briefly sum up the observations upon cartilage,
and refer to the phenomena of vegetable life, wdiich either accord
with or are dissimilar to them. Cartilage originates from cells,
every one of which has its special, and, in the first instance,
very thin wall ; precisely like those of vegetables. These cells
either lie closely together, and on that account are flattened
against one another, like those of plants (see pi. I, figs. 5 and 6),
or, there is intercellular substance present, and this again either
in so very small a quantity as to be visible only in situations
where three or four cells are in contact (see fig. 6, c), or in
so much greater quantity, as to prevent the contiguity of the
different cell-walls (pi. I, fig. 7 ; and pi. Ill, fig. 1.) Most
of the cells, at their earliest period of development (and per-
haps constantly) contain a nucleus, that is, a round or oval, and
sometimes hollow corpuscle (pi. I, fig. 5, a ; and pi. Ill, figs. 1
and 2), which again generally encloses one or two nucleoli.
The cartilage-cells originate in the first place by the formation
of the nucleus in the cvtoblastema, around which the cell is
afterwards formed, so that the latter at first closely encompasses
the nucleus. The nucleus advances slightly in growth after the

32 STRUCTURE AND GROWTH

formation of the cell, but in a much lower proportion. It is
subsequently absorbed; frequently, however, not before ossifi-
cation. This is precisely what occurs in vegetables. The walls
of the cartilage-cells become thickened (compare figs. 6 and 7
with fig. 5), which is also the case with many vegetable-cells.
No distinction, however, between primary cell-membrane and
secondary deposit can be observed in cartilage- cells, and such a
deposition in strata as is often distinctly seen in thickened cells
of plants cannot be made out here with sufficient certainty.
The cell-nucleus in the meantime, when not absorbed, remains
lying upon the inside of the thickened wall. An instance of
actual thickening of the cell-membrane without a stratified
deposit, does not, however, appear to be wanting in plants,
e. g. the pollen-tube of Phormium tenax. (See the Introduction.)
But it seems, that a thickening of the walls of the cartilage-
cells does not take place universally, it does not for instance in
the ossifying cartilages ; the true cartilage substance may also
be formed entirely, or at least chiefly of the intercellular sub-
stance. The condensed cell-walls subsequently coalesce with
one another, or with the intercellular substance, so that at last
only the cell-cavities remain in an homogeneous substance.
Whether the walls of those cartilage-cells which do not undergo
any thickening become blended with the intercellular substance
or not, remains uncertain. An analogous instance of coalescenec
of the cell-walls is afforded by vegetables, for Schleiden has ob-
served such a blending in the layer of bark which lies im-
mediately beneath the cuticle of the Cacti.

The cartilage-cells often contain either simple nuclei (i. e.
without cells around them), or young cells with such nuclei.
These young cells are formed free within the parent-cell,
without vascular connexion. Their nucleus is first formed, and
afterwards the cell around it, just as in the true cartilage- cell.
This is one of the most important instances of accordance be-
tween animal and vegetable cells, for the latter, according to
Schleiden, are developed in like manner from the nucleus, and
likewise within a parent-cell. (See the Introduction.) We may
therefore confidently compare the nucleus of these young cells,
as also that of the true cartilage-cell, to the cytoblast of vege-
table cells. Their shape and the eccentric position of their
nucleus, placed as it is upon the internal surface of the cell- wall,

OF CARTILAGE.

also accord with the young colls of plants. Compare plate I,
fig. 8, ff, with fig. 2. The form of the nucleus likewise corre-
sponds with that of many vegetable cells. In these young cells
of cartilage, it is presented to the observer as a small oval or
perfectly spherical corpuscle, having, in many instances, a
granulous and somewhat yellowish appearance, and containing
one or two nucleoli. (Compare this with the description of the
nucleus of vegetable cells in the Introduction.) The nucleus of
the cartilage-cell appears to be hollow, a fact which has not
been observed with regard to the cytoblast of vegetable cells,1
and the nucleoli lie close upon, or in the neighbourhood of the
internal surface of its wall, whilst, according to Schleiden, thc\
lie deep in the cytoblast of vegetable cells.

The cartilage-cells, when once formed, appear to be endued
with the capacity to grow throughout the entire mass of the
structure. The case is different with regard to the formation
of new cells. This takes place in certain situations only, on
the surface of the cartilage, for instance, or between the last
formed cells. We have alread}^ seen that in the branchial rays
of fishes, the least developed cells lay at the point, and
lateral margins. The little rod, which the branchial ra\
represents, does not increase in size by the formation of new
cells between the original ones throughout its entire length,
but its extension in the longitudinal direction is produced
by the development of new cells in the neighbourhood of
the point, and it increases in breadth by the same process
going on in the neighbourhood of the side Avails. It is a
familiar fact, that the cylindrical bones grow chiefly upon the
surface and at the end of the shaft. The formation of new
cartilage-cells usually takes place only in the neighbourhood of
the surface which is in contact with the organized substance,
(I refer throughout this passage to that period alone, at which
the cartilage docs not contain any vessels of its own,) but it
is not exclusively confined to that situation, it may also
proceed in the intercellular substance between the last-formed
cells.

At the period of ossification, the earthy matter is first de-
posited in the cell-walls, or in the true cartilage-substance, the

1 In a letter which I have received from Schleiden, he informs me t lint lie has

also found hollow nuclei in plants.

• >

• >

34 STRUCTURE AND GROWTH

remains of the cell- cavities also become filled with it at a
subsequent period, and at the same time the stellated canali-
culi issuing from them make their appearance. The formation
of these canaliculi probably takes place by the transformation
of round cartilage-cells into a stellated form, after the manner
of the pigment-cells at plate II, figs. 8 and 9.

The above detailed investigation of the chorda dorsalis and
cartilage, has conducted us to this result, — that the most impor-
tant phenomena of their structure and development accord with
corresponding processes in plants, that some anomalies and
differences may indeed still remain unexplained, but that
they are not of sufficient importance to disturb the main con-
clusion, viz. that these tissues originate from cells, which
must be considered to correspond in every respect to the
elementary cells of vegetables. Thus then are we furnished
with the first of the proofs required in the Introduction ; that
is to say, we have shown with regard to a certain tissue, that
it not only originates from cells, but that these cells in the I
process of their development manifest phenomena analogous to
those of the cells of plants. We have now thrown down a
grand barrier of separation between the animal and vegetable
kingdoms, viz. diversity of structure. We have become ac-
quainted with the signification of the individual parts of the ani-
mal tissues as compared with the vegetable cells, and know that
cells, cell-membrane, cell-contents, nuclei, and nucleoli in the
former are in every respect analogous to the parts having
similar names in the cells of plants. We have already observed
several modifications both of the nucleus and cell. The former
presented itself as a corpuscle having either an oval or circular
outline, spherical in figure, or very much flattened, sometimes
hollow, and often scarcely perceptible, in consequence of its
transparency, but generally granulous and yellowish, and con-
taining in its interior from one to three nucleoli. This
nucleus lay within, and fast adhering to the w* all of the cell,
but never in its centre. The fundamental form of the cell
appeared to be that of a round vesicle, but we have also ob- j
served the flattening of the cells against one another, the
presence of intercellular substance between them in greater
or less quantity, and lastly, the thickening of the cell-walls.

OF CARTILAGE. 35

We have seen the generation of cells within cells, and the
formation both of the young cells in cartilage, and of the
true cartilage-cells themselves, was proved to take place
around the nucleus, in the same manner as that described
by Schleiden in vegetable cells. The other proof for the
accordance of animal and vegetable structure (see Introduction,
p. 6) yet remains to be supplied, viz. that most or all animal
tissues are developed from cells. If this proof only were
furnished, the analogy of such cells to the elementary cells of
plants would at once become extremely probable ; we may now
assert that analogy so much the more firmly, since the cells
of two distinct tissues have been proved in detail to correspond
with those of plants.

SECTION II.

ON CELLS AS THE BASIS OF ALL TISSUES OF THE ANIMAL BODY,

The young cells contained within the cartilage-cells (see
plate I, fig. 8, ff) may be regarded as the elementary form
of the tissues previously considered, and may be described as
round cells having a characteristic nucleus, firmly attached to
the internal surface of the wall. As the above were proved to
correspond with the vegetable cells, it follows, that it is only
necessary to trace back the elementary structure of the rest
of the tissues to the same formation,' in order to show their
analogy also with the cells of plants. In some tissues this
proof is easy, and immediately afforded ; in others, however, it
is obtained with much difficulty, and it would frequently be
altogether impossible to demonstrate the cellular nature of
some, if the connexion between the different steps in this
investigation were lost sight of. The difficulty arises from the
following circumstances: 1st. The minuteness of the cells ; in
consequence of which it is not only necessary to use a power
magnifying from 400 to 500 diameters, but it is also frequently,
indeed generally found impossible to press out their contents.
2dly. The delicate nature of the cell-membrane. When this has
a certain density, its external as well as internal outline may
be recognized, and the distinction between it and the cell-con-
tents may thus be placed beyond a doubt. But if the cell-mem-
brane be very delicate, the two outlines meet together in one line,
and this may readily be regarded as the boundary line of a
globule, not enclosed by a special enveloping membrane. 3dly.
The similar power of refraction possessed by the cell-wall and
cell-contents, in consequence of which the internal outline
of the former cannot be observed. 4thly. The granulous nature of |
the cell-membrane, which when the contents are also granulous,
cannot be distinguished from them. Lastlv, the variety of

ON CELLS, ETC. 37

form presented by the cells, for they may be flattened even to
the total disappearance of the cavity, or elongated into cylinders
and fibres. From these circumstances, many of the cells which
now come before us for consideration, have been described as
mere globules, or granules, terms which do not express their
true signification, and even when they were spoken of as cells,
or cells furnished with a nucleus, the description rested only
upon a slight analogy, since but very i'cw of them (for example,
the pigment-cells), were proved to be actually hollow cells.
But — as the precise signification of the nucleus is unknown, and
as the cell-membrane is not proved to be anything essential to
those cells (and this follows from their accordance with vege-
table cells), upon the analogy with which the proof of the
cellular nature of the rest of the globules provided with a
nucleus will be based, — there is no contradiction involved in the
supposition that a nucleus may be contained in a solid globule
as well as in a cell.

From the difficulties of this investigation above detailed, it
will be seen that a given object may really be a cell, when even
the common characteristics of that structure, namely, the per-
ceptibility of the cell-membrane, and the flowing out of the cell-
contents, cannot be brought under observation. The possibility
that an object may be a cell, does not, however, advance us
much ; the presence of positive characteristics is necessary in
order to enable us to regard it as such. In many instances
these difficulties do not present themselves, and the cellular
nature of the object is immediately recognized ; in others, the
impediments are not so great but that the distinction between
cell-membrane and cell-contents is at least indicated, and in
such cases other circumstances may advance that supposition
to a certainty. The most important and abundant proof as to
the existence of a cell is the presence or absence of the nucleus.
Its sharp outline and dark colour render it in most instances
easily perceptible ; its characteristic figure, especially when it
encloses nucleoli, and remarkable position in the globule under
examination, (being within it, but eccentrical, and separated
from the surface only by the thickness of the assumed cell-wall,)
all combine to prove it the cell-nucleus, and render its analogy
with the nucleus of the young cells contained in cartilage, and
with those of vegetables, as also the analogy between tli-

38 ON CELLS AS THE BASIS

globules under examination, in which it lies, and those cells,
consequently the existence of a spherical cell-membrane in the
globules, extremely probable. More than nine tenths of the
globules in question present such a nucleus ; in many the
special cell-membrane is indubitable, in most it is more or
less distinct. Under such circumstances, we may be permitted
to conclude that all those globules which present a nucleus of
the characteristic form and position, have also a cell-membrane,
although, from the causes before specified, it may not be per-
ceptible. The different tissues will also afford us many instances
of other circumstances which tend to prove the existence of
an actual cell-membrane. An example of what is referred to
would be afforded by an instance, in which a certain corpuscle
(furnished with a nucleus), about the cellular nature of which
a doubt existed, could be proved to be only a stage of deve-
lopment, or modification in form, of an indubitable cell. The
cell-nuclei and their distance from each other when scattered
in a tissue, also serve as indications, when the outlines of the
cells have to be sought for. They likewise serve to guide
conjecture as to the earlier existence of separate cells, in
instances where they have coalesced in the progress of develop-
ment. When a globule does not exhibit a nucleus during
any one of the stages of its development, it is either not a cell,
or may at least be preliminarily rejected, if there be no other
circumstances to prove it such. Fortunately, these cells devoid
of nuclei are rare.

In addition, however, to the cellular nature of the elementary
structures of animal tissues, there are yet other points of
accordance between them and the cells of plants, which may
generally be shown in the progress of their development, and
which give increased weight to the evidence tending to prove
that these elementary structures are cells. The exceedingly
frequent, if not absolutely universal presence of the nucleus,
even in the latest formed cells, proves its great importance for
their existence. We cannot, it is true, at present assert
that, with regard to all cells furnished with a nucleus, the
latter is universally the primary and the cell the secondary
formation, that is to say, that in every instance the cell is
formed around the previously existing nucleus. It is probable,
however, that such is the case generally, for we not only meet

OF ALL ANIMAL TISSUES. 39

with separate nuclei in most of the tissues, distinct from those
which have cells around them, but we also find that the
younger the cells are, the smaller they are in proportion to
the nucleus. The ultimate destiny also of the nucleus is
similar to that of the vegetable cells. As in the last named,
so in most animal cells it is subsequently absorbed, and remains
as a permanent structure in some few only. In plants, ac-
cording to Schleiden, the young cells are always developed
within parent cells, and we have also seen such a development of
new cells within those already farmed in the chorda dorsalis
and cartilage. If, however, any doubt existed as to whether
the primary cells of these tissues were formed within previously
existing parent cells, none such can arise in reference to many
of the tissues next to be considered. We shall indeed fre-
quently meet with a formation of young cells within older
ones, but it is not the rule, and does not occur at all with
regard to many of them.

The following admits of universal application to the forma-
tion of cells ; there is, in the first instance, a structureless1
substance present, which is sometimes quite fluid, at others
more or less gelatinous. This substance possesses within
itself, in a greater or lesser measure according to its
chemical qualities and the degree of its vitality, a capacity to
occasion the production of cells. When this takes place the
nucleus usually appears to be formed first, and then the cell
around it. The formation of cells bears the same relation to
organic nature that crystallization does to inorganic. The
cell, when once formed, continues to grow by its own individual
powers, but is at the same time directed by the influence of
the entire organism in such manner, as the design of the
whole requires. This is the fundamental phenomenon of all
animal and vegetable vegetation. It is alike equally consistent
with those instances in which young cells are formed within
parent cells, as with those in which the formation goes on

1 [Strukturlos. — I have ventured to translate this word as ahove, although 1 am
aware it is open to ohjection. The idea intended to he conveyed hy the author is
that of a suhstance in which no definite structure can he detected. As the word
will be frequently used in the following pages, the reader is requested to assign this
signification to it invariably. — Trans.]

40 THE OVUM AND

outside of them. The generation of the cells takes place in a
fluid, or in a structureless substance in both cases. We will
name this substance in which the cells are formed, cell-germi-
nating material (ZellenkeimstofF), or cytoblastema. It may
be figuratively, but only figuratively, compared to the mother-lye
from which crystals are deposited.

We shall refer to this point at greater length hereafter, and
only anticipate our subject with this result of the investigation,
in order to facilitate the comprehension of what follows.

In the previous section of- this work we have discussed in
detail the course of development of some of the animal cells,
having taken the chorda dorsalis and cartilage for our examples.
We are now required to prove, as far as is possible, that all
the tissues either originate from, or consist of cells. We
separate this investigation into two divisions. The first treats
of the Ovum and Germinal membrane, in so far as they form
the common basis of all the subsequent tissues. The second
division embraces the permanent tissues of the animal body,
with the omission of the two already described.

FIRST DIVISION.

On the Ovum and Germinal Membrane.

The ovum of Mammalia lies, as is known, within the Graafian
vesicle. I have not made any investigation as to whether that
vesicle may be considered to have the signification of a cell.
It is indeed a cell in the general sense of the word, being a
cavity in the substance of the ovary, it has even a special
membrane ; but as we here only receive the word cell as sig-
nifying an elementary part of animals and plants, it becomes
necessary to inquire whether this membrane may not be a
secondary formation resulting from the junction of other struc-
tures which are elementary. The history of the development
of the Graafian vesicle must show whether that be the case or
whether it originate by the mere growth of a cell furnished
with a structureless cell-membrane, which cell inav formerly

GERMINAL MEMBRANE. 41

perhaps, have had a nucleus.1 Within this vesicle lies the
ovum or vesicle of Baer, embedded in a layer of granules.
When these granules are examined with a magnifying power
of 450, they are readily recognized to be cells, that is, round
vesicles containing a nucleus, which is situated upon the
internal surface of the wall. The nucleus being granulous
and darker than the rest of the object falls under observation
first. It encloses one or two nucleoli. The cell surrounding
it varies in size, being in the average about half as large again
in diameter, but some are much larger. The cells arc for the
most part extremely delicate, and round, when separated from
one another. When in connexion, they often flatten against
one another, and assume a polyhedral form. In addition to
these cells, isolated nuclei appear also to be present within the
Graafian vesicle, perhaps as the germs of new cells. The pro-
duction of these cells proceeds according to the fundamental
law mentioned at page 39, within the fluid of the Graafian
vesicle, that being their germinative material or cytoblastema.
Whether this fluid is to be regarded as cell-contents, and the
cells produced in it as being formed within a parent cell, must
depend upon the solution of the question, as to whether the
Graafian vesicle be an elementary cell or not : but the dcci-
sion of this point is not essential, for the rule that cells
originate within others is not universal. When the inde-
pendent vitality of cells is borne in mind, we can readily
conceive how these, when they (after the bursting of the
vesicle) arrive with the ovum in the uterus, may be further
developed into other structures (the chorion according to
Krause.) Within this granulous or rather cellular disc then
the ovum or vesicle of Baer lies embedded, (see the represen-
tation, plate II, fig. 1, taken from Krause.) The first object
which attracts observation is the dark spherical yelk, surrounded
by a transparent space, (zona pcllucida of Baer, chorion of
Wagner.) Krause found (Midler's Archiv, 1837, p. 27) that
the yelk is surrounded by a peculiar membrane, d (vitelline
membrane), and that the transparent space is enclosed externally

1 According to the researches of Martin Hairy (Phil. Trans. Part II, lS.'iH, p. 305,
&c), both cases appear to occur, so that a cell composed of a structureless mem-
brane is first formed, (the ovisac of Barry,) and subsequently an external vascular
covering of cellular tissue. On the relation of tills follicle to the mode of develop-
ment of the ovary itself, sec Valentin in Muller's Archiv, 183s, p. 020.

42 THE OVUM AND

by a very delicate pellicle, the albumen-membrane, b, also that
the transparent substance itself (albumen) is sufficiently fluid to
permit of such a degree of displacement of the yelk as to allow
of its coming into contact even with the albumen-membrane.
Although I have never yet succeeded in observing this pellicle,
and though in my researches the transparent membrane, on
the bursting of the yelk, always tore with smooth edges like a
solid substance, yet the observations of the respected discoverer
are too precise to admit of a doubt upon it. It is also sup-
ported by the analogy of most of the ova of other classes of
animals, in which chorion and vitelline membrane may gene-
rally be distinguished, notwithstanding that they sometimes lie
close upon each other. The albumen-membrane has probably
the signification of a cell-membrane, in which case the albumen
will be the cell- contents, and the yelk a young cell. Accord-
ing to Wharton Jones, the transparent areola (zona pellucida)
of the ovum, or the albuminous layer in the fecundated
ovum of mammalia, becomes considerably expanded in the
tubes, a fact which would be readily explained by the inherent
energy of the albumen-membrane when regarded as a cell.
In such case, however, the mode of formation of the albumen
would be very different from the corresponding process in the
bird's egg, where, according to Purkinje, it is secreted by the
oviduct, and a membrane (chorion) is formed around it sub-
sequently, which cannot therefore have the signification of a
cell-membrane, and is moreover not simple in structure, but
composed of fibres. Meanwhile an investigation might be
made, as to whether the albumen in the egg may not also be
first surrounded and formed by an equally thin pellicle,
around which a secondary external membrane may subsequently
be produced. According to Purkinje, however, this is not
the case, and I could not discover any such pellicle upon the
inner surface of the shell-membrane of the excluded egg. I
have not made any inquiry as to whether the chorion of fishes
is a cell-membrane or not. It is covered internally with a
very beautiful epithelium, which is made up of more or less
flat hexagonal cells, each of which has its nucleus.

Within the transparent areola, or, according to Krause, the
albuminous layer, lies the vesicle of Baer, or the yelk ; which,
from Krause's statement, is enclosed by a peculiar structureless

GERMINAL MEMBRANE. 48

membrane, the double outline of which he recognised, (plate
II, fig. I, d.) It is thus highly probable that the yelk of the
mammalian ovum is a cell. Even if, as Wagner intimates, the
vitelline membrane in other animals should sometimes be
formed only secondarily within the chorion, it would not
materially interfere with our purpose, since in that case the
chorion would be the cell-membrane. The ovum universally
possesses an external closed membrane (whether it be chorion
or vitelline membrane), which is structureless, and not gene-
rated from other elementary structures, and therefore is the
ovum always a cell. The yelk-cell encloses the vitelline sub-
stance as its cell-contents, and upon its internal surface lies
the germinal vesicle, or vesicle of Purkinje, (fig. 1, f.)
This, as is known, is a very transparent thin-walled vesicle,
containing a pellucid fluid, according to Wagner coagulable by
spirits of wine. It encloses almost universally (Wagner cites
but very few exceptions) upon the internal surface of its wall,
a corpuscle, called by the discoverer, R. Wagner, germinal spot,
or germinal disc, (fig. 1, g.) In mammalia it is generally flat.
In many instances several of these spots are present, their
number, however, is said by Wagner to bear proportion to the
age of the ovum, they being fewer and much more firmly
attached to the wall of the germinal vesicle in young ova. I have
frequently observed in osseous fishes (where they are often pre-
sent in such numbers as to prevent the fluid in the vesicle from
being seen) that when one of these corpuscles, after the bursting
of the germ-vesicle, passed through a narrow space, it first
became considerably elongated, and then drawn out in the
centre to a thin thread, which soon broke. The two ends
afterwards retracted, and thus two round globules were pro-
duced from one corpuscle, in a similar manner to what we may
observe in the drops of fat upon soup. They appear, therefore,
to be composed of a tenacious substance which is not misciblc
with water. Purkinje states that the germinal vesicle in birds
is firmly fixed to the vitelline membrane, but Baer and
Wagner describe it as lying in the centre of the yelk at first,
and rising to the surface at a subsequent period.

The decision of the question, as to the precise signification
of the germinal vesicle, now becomes of great importance. Is
it a young cell generated within the yelk-ccll, or is it the

44 THE OVUM AND

•

nucleus of the yelk-cell ? If the former, it is in all probability
the most essential rudiment of the embryo ; but if it be the
nucleus of the yelk-cell its importance vanishes with the forma-
tion of the yelk-cell, and according to the analogy of most
cell-nuclei, it must either become absorbed altogether at a
subsequent period, or continue for a time simply rudimentary,
without forming any important new structure. The follow-
ing is the ordinary career of a simple cell : a nucleus is
present in the first instance ; around it a cell is formed j the
nucleus at first often increases in size as the cell grows, but
their growth is by no means proportionate, that of the cell
being much more rapid ; the cell-contents are at first transpa-
rent ; a firm precipitate or new formation next commences in
the cell, and this occurs immediately around the nucleus,
which is at first enclosed by it ; the nucleus then either
becomes entirely absorbed, or continues only rudimentary and
(with the following exception) I have never observed it to
give origin to any other essential formation. One or more
oil-globules once appeared to me to be formed during the ab-
sorption of the nucleus in the adipose cells within the cranial
cavity of a young carp. The importance of the decision of
this question in reference to the germinal vesicle thus becomes
very obvious. Unfortunately, however, neither the observa-
tions upon the subsequent relations of the germ-vesicle, nor
those on the origination of the ovum, are sufficiently extensive
or certain for the purpose.

We shall next proceed to analyse both views of the question
more minutely, and afterwards compare them with the obser-
vations. If the germ-vesicle be a young cell, in the first place,
it is absolutely necessary that the yelk-cell should first exist,
and that the germ-vesicle should afterwards be developed within
it ; 2dly, the germ-vesicle must not be connected with the
vitelline-membrane, but must be developed free at some chosen
spot within the cavity of the yelk ; 3dly, the germ-vesicle
may be regarded either as a cell without a nucleus, and in
that case the spots of Wagner belong to the cell-contents, or
Wagner's spot, wdien it is single, is the nucleus; when there
arc several present, the others either differ essentially from one
particular spot, and pertain to the cell-contents, or they are
nuclei of young cells afterwards to be developed within the

GERMINAL MEMBRANE. 45

•

germ-vesicle. Before the spot can be considered to be the nu-
cleus, it is necessary that it should, in the first instance al
least, be connected with the wall of the vesicle. If, however,
the germinal vesicle be the nucleus of the yelk-cell, it is
essential, in the first place, that it should, in all probability, be
present before the yelk-cell ; at all events, that in proportion
as the ovum is younger, should the vesicle be larger in relation
to the cell ; 2dly, it must, at first, lie upon the vitellinc-
membrane, and be more or less intimatelv connected with it :
3dly, the germinal-vesicle, when regarded as a nucleus, either
has no nucleoli, or Wagner's spots are to be considered to re-
present them ; in the first case they form the contents of the
nucleus. In the enumeration of these points, no regard is
had to the relations of the germ-vesicle subsequent to impreg-
nation, because it is desirable to determine its ultimate destiny,
to a certain extent a priori, from its signification, and thus t>>
be enabled at the least to afford a guide to the much more
difficult observation of the fecundated ovum. If the researches
were complete, the distinctions above cited would be sufficienl
for the correct determination of the question at issue, the
decision of the first point indeed would of itself be ample
evidence.

When we take into consideration the first point raised on
either side, we should be compelled to decide in favour of the
latter view, and regard the germ-vesicle as a nucleus, if it were
proved to be first present, and also that the yelk-cell is formed
around it as a simple cell, narrowly encompassing it in the
first instance, and becoming gradually expanded. In the next
place, it is certain that at an early period the germ-vesicle
is much larger in proportion to the yelk-cell, and that it
at first grows pari passu with the yelk-cell, but that subse-
quently the latter increases in size in a much greater ratio,
whilst the vesicle remains stationary; and these are preciselv
the relations in which the vesicle should stand in order to be
regarded as a nucleus. But these facts are not entirely irre-
concilable with the first view. A young cell, the germ-vesicle,
might be imagined to form within the yelk-cell at a very early
period of its growth, which young cell might at first increase
in size more rapidly than the original one, but cease to do so
earlier, whilst the parent-cell might continue to be developed

46 THE OVUM AND

•

in size. Such a circumstance is, however, very rare, and the
weight of evidence before us is much in favour of the second
view ; but in order to determine this point, it is necessary to .
inquire whether the vesicle exist before the cell. That such
is the case is not yet proved, although Baer and Purkinje sup-
pose it to be so, and an observation of Wagner's favours the
supposition. (Prodromus Physiologias Generationis, p. 9, fig.
xviii, a.) He found the posterior extremity of the oviduct of
Acheta campestris full of germinal vesicles, which became gra-
dually expanded in their progress through the oviduct. The
oviduct becomes dilated in its further course ; globules are
observed in it, which. Wagner regards as yelk-globules, and
between them lie the germ- vesicles; then " each vesicle becomes
surrounded by its yelk and chorion, and thus the individual
ova become separated." He does not state, however, in what
manner the vitelline-membrane is produced. Is it formed as
a cell, at first narrowly encompassing the germ-vesicle, and
then gradually expanding; or does it at the same time enclose
a quantity of the surrounding yelk -globules ? It is difficult
to conceive the latter mode of formation ; but if the former be
the correct one, the globules surrounding the germ-vesicles in
the oviduct cannot be yelk -globules. Fresh researches are
therefore necessary, which, if they should be confirmatory of
the first view, will also be decisive for considering the germ-
vesicle as a cell-nucleus.1

With regard to the second point, — namely, as to whether the
germ-vesicle be more or less intimately connected with the
membrane of the yelk-cell at an early period, or lie free within
it, — any evidence afforded by its solution would be comparatively
inconclusive. According to Baer and Wagner, the vesicle in
the first instance lies in the centre of the velk-cell, and onlv
rises to its wall at a later period. Baer quotes the ova of
frogs as examples in which it lies for a long time in the centre
of the yelk. The germ- vesicle is generally found on the wall
of the cell; and in birds, according to Purkinje, it is frequently
so intimately connected with it, that it tears in the attempt to

1 See the Supplement. The observations of Wagner upon the ova of insects
which are there quoted, and the recent researches of Barry on those of mammalia
and birds, (1. c. p. 308,) prove the germinal vesicle to be first formed, and then the
vitelline membrane round it.

GERMINAL MEMBRANE. 47

separate them. Although the position of the vesicle in the
middle of the yelk-cell affords evidence rather in favour of its
being regarded as a young cell, yet it is not altogether incon-
sistent with its character as a nucleus : for it is only during
the earliest formation of the cell that the nucleus is required
to be connected with it; it is frequently disconnected at a
later period, and lies loose in the cell. At that stage of deve-
lopment, however, in which the vitelline-membrane closely en-
compasses the germ-vesicle, it is impossible to decide whether
it lie in the middle or on the wall of the cell. This point,
therefore, is of more ideal than practical importance for the
prosecution of the investigation.

The third point relates to the signification which attaches
to the individual parts of the germ-vesicle. It may be hol-
low consistently with both views. Although we are not as yet
acquainted with any hollow nuclei in plants,1 we have never-
theless found nuclei in cartilages which were hollow, and de-
cidedly to be regarded as cytoblasts. The question now arises,
what are Wagner's spots or spot ? If the germ-vesicle be con-
sidered to be a young cell, one of them may be its nucleus, and
the rest cell-contents, or nuclei of young cells, which will be
developed afterwards; if it be regarded as nucleus, the spots
may either be nucleoli, or merely its contents. It is a fact in
favour of the former view, that only one spot is present in
most instances, the others being usually produced at a later
period. Wagner has sometimes observed one or more minute
points in this single spot, and has delineated them from Alcedo
hispida, Lepus cuniculus, Ovis aries, &c; I have also sometimes
met with small points of this kind which gave the spot, in some
degree, the appearance of a nucleus adhering to the wall of the
cell, and containing within it these little points as its nucleoli.
Meanwhile, their presence is too inconstant, and they are gene-
rally too indefinite, to permit of our attributing any importance
to them in the decision of the present question. The extra-
ordinary number in which they frequently occur is opposed to
their being regarded as nucleoli within the germ-vesicle, pre-
suming it to be a cell-nucleus, for in fishes they sometimes fill
the entire vesicle, at least, being closely crowded, they cover

1 <

See Note, p. :>.'{.

48 THE OVUM AND

the internal surface of it. Three is the largest number of
nucleoli which I have observed in other nuclei, and Schleiden
has in some very rare instances seen four in plants. If, how-
ever, thcv are onlv the contents of the nucleus, and not
nucleoli, it must be allowed that they differ very much from
the contents of almost all other nuclei, which are generally
yellowish, and made up of extremely minute granules. The
only exception which I have met with was that already men-
tioned respecting the nucleus of the adipose cells in the cranial
cavity of a young carp. This last point seems therefore
rather in favour of the germ -vesicle being regarded as a
young cell.1

"When the whole of the above detailed evidence is reflected
upon in connexion, it will be seen that it is as yet impossible
to decide the question as to whether the germinal vesicle be
cell or nucleus. The opinion that the vesicle is to be regarded
as a cell-nucleus, seems for the present to have the ascendancy,
inasmuch as the observations upon the first and most important
point, viz. the prior existence of the germ-vesicle to that of
the yelk-cell appear to be in favour of that view.2 The sub-

1 Since in vegetable cells the nucleolus is the primary formation, and the nucleus
a secondary one around it, and as the same has been shown to be most probably the
case in animal cells, (see page 20, on the production of the nucleus of cartilage-
cells,) so also in this case the signification to be assigned to "Wagner's spot depends
upon the history of the development of the germ-vesicle. The observations of
Wagner, quoted in the Supplement, show, however, that the single germinal spot of
the ova of insects is first formed, and the germinal vesicle afterwards around it.
The former must then be considered as nucleolus to the vesicle, which corresponds
to the nucleus. When several of Wagner's spots occur, their signification is totally
different from that of the first one, and they are to be regarded only as secondary
formations in the interior of the germ-vesicle. In fact, the younger the ova of fishes
and frogs, the fewer spots are observed in them.

2 The following is the probable course of formation of the ovum, according to the
researches now before us; the ovisac (Eisach, ovisac of Barry, internal mem-
brane of the Graafian vesicle) is first developed. In this (according to analogy
with Wagner's observations on the ova of insects) a germinal spot is generated, as
nucleolus to the ovum. Around that spot the germinal vesicle is formed as nucleus
to the ovum; and .round this again the ovum-cell (Eizelle.) Martin Barry, in-
deed, (1. c. p. 308,) conjectures that the germ-vesicle is formed previously to the
ovisac ; but my respected friend expresses himself with great caution on the ques-
tion ; and it would in fact be difficult to determine whether a given vesicle were a
germinal vesicle, around which no ovisac had as yet formed, or an ovisac within
which no c;enn-vesicle bad as vet formed. The occurrence also in the lower ani-

GERMINAL MEMBRANE. 49

sequent relations of the vesicle seem also to afford evidence in
its favour. The disc, for instance, is formed around it, and
this perhaps corresponds to the granulous precipitate which

mals of several ova in one ovicapsnle is difficult of explanation by Barry's view.
In the further investigation of this subject, attention must continue to be fixed
upon the possible, and even probable, existence of a nucleus to the ovicapsule.
Wagner saw certain follicles in the mole, in which he could not detect a trace of
any enclosed body.

Wagner expresses himself in his new work (Lehrbuch der Physiologic, Leipzig,
1839, p. 34) as being doubtful whether the vesicles met with in his observations
on the preformation of the germinal vesicle in the ova of insects, were actually
vesicles or not. The observations of Barry on the ova of mammalia and birds,
are, however, in favour of the explanation of the ovum of the insect originally
given by the first -named highly respected investigator, and therefore also of
that which represents the germ-vesicle as nucleus of the ovum-cell. It is
true it might be said, that, regarding the germ-vesicle as a cell, a second one,
the ovum-cell was formed around it; but as opposed to that view, it must
be remembered that no example of a second cell being formed around the first is
afforded amongst all the other cells which exhibit a nucleus of the decidedly cha-
racteristic form. The point in dispute, as to the interpretation to be placed upon
the germ-vesicle, loses, however, somewhat of its importance if the theory which I
shall propose (see the conclusion of the treatise) be received, inasmuch as I shall
there endeavour to prove the formation of the cell around the nucleus to be merely a
repetition of the process by which the nucleus is formed around the nucleolus, and
that the whole process of development of the cell may be reduced to a single or
many times repeated formation of strata. The germinal-vesicle accordingly is the
first stratum, or a cell of the first order ; the yelk-cell the second stratum, or a cell
of the second order. As above stated at page 47, a minute point was observed in
the germinal spot by Wagner, and subsequently by myself also ; and my respected
colleague Vanbeneden lately found germinal spots in the ova of certain polypes
(Genus Zoanthus), and also in ova of Anodonta, which had not as yet left. the ovary,
that appeared granulous, but at the same time seemed to be hollow, and some of
which distinctly contained a very small round corpuscle. This observation accords
most completely with the theory which regards the cells as produced by a stratified
formation. This small corpuscle, which may be called a secondary nucleolus, would
here be the primordial formation; the germinal spot would be the first stratum
around it, that having in this instance become developed into a vesicle, in a manner
likewise to be explained hereafter by the Cell-Theory ; the germinal vesicle would
be the second, and the yelk-cell the third stratum. The formation of even a fourth
stratum, the albumen membrane, around the yelk-cell, would involve nothing con-
tradictory to the theory ; but in such case we certainly could not avoid regarding it
as a second cell, which had become formed around a previously existing one: for
the yelk-cell cannot well be considered to be a nucleus. The mode of formation of
this albumen membrane must, however, in the first instance, be ascertained by in-
vestigation.

1

50 THE OVUM AND

usually takes place around the nucleus in other cells; and again,
the germ-vesicle disappears, precisely as the nucleus of other
cells is generally absorbed. There is then no evidence that the
fluid of the germinal vesicle exercises a fructifying influence ;
but if it be the cell-nucleus, it disappears, because it has com-
pleted its office, — the formation of the yelk-cell. The disc,
which has formed around it, becomes developed into the
germinal membrane, and it is uncertain whether the remains
of the germ-vesicle also take part in that formation.

We shall next proceed to the consideration of the other
contents which the yelk-cell includes in addition to the germ-
vesicle, making use of the bird's egg for the purpose. Setting
aside some points of distinction of slighter importance, the
globules, well known as present in the yelk of the hen's
egg when laid, may be divided into two principal classes : a, the
globules of the yelk-cavity ; and b, those of the true yelk-sub-
stance. The former (a) are not only present in the yelkrcavity,
but occur also in the canal leading from it to the germinal
membrane, and in the little prominence, called by Pander the
nucleus of the tread (Kern des Hahnentritts). When many
of them lie close together, they exhibit a white colour, whilst
the true yelk-globules in such circumstances appear yellow.
They may also be distinguished from the latter globules under
the microscope, (see pi. II, fig. 2.) They are perfectly round
globules, with quite smooth edges, each enclosing a smaller one,
which is also perfectly spherical, and looks like an oil-globule,
being rendered very distinct by its sharp outline.

The remaining space in the large globules is usually trans-
parent, and not graimlous. But some may be observed which
have granulous contents, and they then completely resemble
the true yelk- globules, except that the latter do not gene-
rally contain any smaller ones with such dark outlines. Some-
times also, the globules of the yelk-cavity contain two or more
such smaller ones. The common yelk-globules (b), that is,
those of the true yelk-substance, may be distinguished from the
above-described by the following characteristics : they are upon
the whole larger, they have all granulous contents, and, for the
most part, do not enclose any smaller globules. They are very sen-
sitive to the action of water, which causes them to fall to pieces,
and then the granules enclosed within them becoming free, give

GERMINAL MEMBRANE. 54

a milk-white colour to the fluid. These granules, which are
of various size, resemble milk-globules, and, as has been fre-
quently remarked by others, exhibit also like them a brisk
molecular motion. In consequence of the speedy action of
water upon these globules, they must be examined in albumen
or a weak solution of common salt, which preserves them better.
These fluids also do not impart a white colour to the surface of
a yelk which is opened in them, as water does. The globule,
when crushed under the compressorium, tears somewhat sud-
denly on one side, the other margins remaining smooth, and
then, without any increase of the pressure, a large quantity of
the globules contained in it flow slowly forth. This fact indicates
an external membrane belonging to the globules, but it must be a
very soft and delicate one. Baer, who distinguishes four kinds
of them, believes that he has also sometimes seen such a mem-
brane in the yelk-globules of immature ovarian eggs. The
yelk-globules when isolated are round, but, in their natural
position in the yelk, they flatten against one another into
angular shapes, in which manner the crystal-like bodies observed
by Purkinje in the boiled yelk are produced. These bodies
generally make up the whole of the true yelk-substance of a
fresh egg, so that, with the exception of the contents of the
yelk-globules, we do not usually meet with any free granulous
! substance in the yelk. The minutely granulous substance
i which is observed in addition to the yelk-globules, particularly
after the action of water upon them, appears in most instances,
and on the external layers of the yelk invariably, to be produced
solely by the destruction of the yelk-globules. In the vicinity
of the yelk-cavity of a boiled egg, however, we frequently rind
a coagulated substance composed of granules similar to those
contained in the yelk-globules, and which appears to be actually
free yelk substance not enclosed within globules.

It is necessary to examine the eggs while still contained in
the ovary, if we wish to become acquainted with the process of
formation of these two kinds of globules (those of the yelk-
cavity and yelk-substance), and the mode of production of the
yelk-cavity and its canal. The younger eggs, having a diameter
of one or two lines, have a grayish-white colour, but arc not
yellow ; if such an one be cut through the centre, under water,
it is found to contain a thick, semi-fluid, grayish-white mass,

52 THE OVUM AND

part of which flows slowly out. Around this mass lies a more
consistent, cohering, membrane- like stratum, which lines the
cavity of the little egg. When a portion of this mass is exa-
mined under the microscope, a great many round and very trans-
parent vesicles or cells are observed in it, each of which encloses
a dark corpuscle resembling an oil-globule. Many such globules
float about free, and in addition to them there is also a good
deal of minutely granulous substance present. In order, how-
ever, to examine this mass in a perfectly natural condition, the
use of water must be avoided ; one of the little eggs, of from
half a line to a line in diameter, should be placed upon the dry
object plate, and then pierced, a drop of its contents being
allowed to flow out. This drop will be found to consist entirely
of very pale cells, most variable in size, each one containing a
round globule, the size of which is about proportionate to that
of the cell. This globule or nucleus resembles an oil-globule,
in consequence of its dark outline, (see pi. II, fig. 3.) Many
of these cells with their nuclei are so small, that, when lying
close together, they might be regarded as a merely granulous
substance ; the cells may, howrever, be recognised with a fa-
vorable light. Some of the larger ones occasionally contain
two or three of the globules or nuclei before mentioned. The
contents of the cells are usually quite transparent, but some
isolated ones are seen, in which a minutely granulous precipi-
tate has formed. These cells are enclosed within the egg, in
a small quantity of transparent fluid. In order to explain the
somewhat variable appearance which the contents of the egg
assume after contact with water, a small one should be placed
upon a glass with a drop of that fluid, and some of its contents
pressed out whilst under the microscope. A quantity of these
cells will then be seen to burst quite suddenly in the water,
precisely like soap-bubbles in the air. In consequence of their
paleness, the fact of the bursting is rendered manifest, in the
first instance, only by the sudden motion of the nucleus, which,
together with some minutely granulous substance, remains
behind. If these cells were solid, although ever so soft, this
sudden bursting would not be possible. They are therefore
true cells. I cannot say whether the globule enclosed in them
is to be regarded as the nucleus. Although it resembles an
oil-globule, it does not appear to be fat ; for if acetic acid be

GERMINAL MEMBRANE. 53

applied to a drop of the contents of the egg, it does not appear
to act materially upon the cells, and the contained corpuscle
becomes paler and somewhat swollen, which could not well
take place if it were fat. These cells, then, arc the earlier
stage of development of the subsequent globules of the yelk-
cavity. The larger ones already resemble them perfectly.
These globules of the yelk-cavity are therefore likewise cells.
Their nucleus-globule (Kernkugel) is acted on by acetic acid
precisely in the same way as it was in the earlier condition.
It does not lie centrally in the cell, but on the internal surface
of the wall, as is seen when the cells are caused to roll under
the microscope. When at rest, however, they are generally so
placed that the nucleus-globule occupies the most depending
point (because probably it is the heaviest portion of the cell), and,
on that account, it then appears to lie in the centre of the cell.
The yelk in the first instance contains only the yelk-cavity,
with its cells ; the proper yelk-substance with its globules not
being as yet formed. The colour of these young eggs is there-
fore also white, like the contents of the yelk-cavity.

The membrane-like layer which surrounds the above-described
contents of the egg, may be completely separated from the parts
which surround it externally with facility, after the egg has
been divided through the centre. It is not connected with
them, and appears, to the unaided eye at least, to be pretty
smooth on its external surface ; it is not possible to trace it
towards the interior. Its structure is peculiar. Purkinje, who
discovered it, describes it as consisting of globules, which re-
semble in form and size, but are more transparent than the
blood.-corpuscles. When spread out upon a plate of glass, and
examined with the microscope, it is seen to consist of two parts,
an internal minutely granulous stratum, and an external layer
of cells. Numerous little granules are observed in the internal
stratum, which resemble the nuclei of the above-described cells
of the yelk-cavity in their earliest stage, and I conjecture that
the cells of the yelk-cavity are formed from this stratum, so
that in fact it still pertains to the yelk-cavity. The external
layer consists of small round granulous cells, each of which
contains a nucleus, which again in many instances encloses
one or two nucleoli. Two or three such lavers of cells lie one
above another. These layers of cells are surrounded externally

54 THE OVUM AND

by a very transparent, perfectly structureless membrane, which
represents a closed cell-membrane, having as little connexion
with the ovary as with the layers of cells, and which is deno-
minated vitelline membrane. It is as readily separated from
the ovary as from the layer of cells, the latter, therefore, cannot
be merely its epithelium.

If we now proceed to examine larger eggs from the ovary,
such, for instance, as have attained a diameter of half an inch
or more, and are alreadv -yellow-coloured, on their being divided
across the centre under water, a white substance, the yelk-
cavity, will be found in their interior. This cavity contains
those cells, now in a higher stage of development, which in the
first instance alone formed the contents of the egg. Around
these a stratum of yellow substance, the proper yelk-substance,
appears, and round this again lies the layer of cells. Globules
may be recognised in the proper yelk-substance with the aid of
the microscope, as in the same substance of the mature yelk.
These globules, then, have been formed between the yelk-cavity
and the layer of cells. The question, however, arises how
this has been effected ? The following may be supposed to be
the mode of their production : — the innermost portion of the
yelk, the yelk-cavity, is the part which is first formed, the
innermost yelk-globules are therefore also the oldest, and the
formation of the new yelk-globules takes place externally upon
the internal surface of the layer of cells. If a small portion
of the layer of cells be so placed under the microscope that the
inner surface becomes turned towards the eye, and a spot be
sought for at which a thin layer of velk-substance is attached
to it, it will be seen that the yelk-globules do actually become
smaller in the proximity of the layer of cells, whilst in other re-
spects they retain their general appearance. The smallest of them,
which lie immediately upon the inner surface of the layer of
cells, are even smaller than the cells of the layer itself. It is
therefore extremely probable, that the formation of new yelk-
globules takes place on the inner surface of the layer of cells,
and that the globules then expand to their normal size some-
what quickly, for the stratum of small ones is but thin. Mean-
while new ones continue to form externally, until the yelk has
reached its normal size. The formation of the canal leading
from the yelk-cavity to the germinal vesicle may also be ex-

GERMINAL MEMBRANE. 55

plained in the same manner ; for instance, no formation of yelk-
globules can go on at that point at which the germ-vesicle and
the stratum for the germinal membrane are in connexion with
the layer of cells, but at that spot there must be a gap in each
stratum of yelk-globules, which by the increasing thickness of
the yelk-substance becomes a canal, necessarily conducting from
the velk-cavitv towards the germinal membrane, and into which
cells from the yelk-cavity become crowded. Now are these
globules of the proper yelk-substance cells ? I cannot prove
decisively that they are so j the following arguments, however,
render it probable : 1st, because Baer believes that he observed
an external membrane in some of them ; 2dly, because, when
ruptured at a particular spot by the compressorium, they at
once pour out a large portion of their contents without the
pressure being increased ; 3dly, because, notwithstanding that
they lie close together in the yelk and flatten against one
another, they do not run together; 4thly, because they so
closely resemble some of the cells of the yelk-cavity which are
furnished with granulous contents j Sthly, because they, like •
cells, appear to have an independent growth. These reasons
are sufficiently strong to render it probable that the yelk-
globules have a cellular structure, though they cannot be received
as decisive of the point. However, inasmuch as they all form
the contents of a larger cell, it is not absolutely necessary for
our purpose that they should be distinctly proved to be cells.
Both the indubitable cells of the yelk-cavity, and those proble-
matical ones of the proper yelk-substance, have an independent
growth in a fluid, and within another cell. They are cells
within cells. For although the formation of new cells takes
place only at the outside, yet they are still separated from the
organized substance, not only by the cell-membrane of the
entire ovum, but also by the layer of cells which is situated
immediately beneath it. We here, then, meet with an
instance of just such a formation and independent growth
of cells within a fluid as was expressed by the fundamental
phenomenon previously laid down. It is a point open to in-
vestigation, whether the cleaving of the yelk described by
Baer, Rusconi, and others, in the development of the lower
animals, the ova of frogs for example, may not also depend
upon a process of cell-formation, two cells being developed

56 THE OVUM AND

within the yelk in the first instance, and in each of these again
two new ones, and so on. •

We next proceed to consider the changes undergone by the
external layer of cells furnished with nuclei. In eggs which
have a diameter of a line, this entire membrane, if it may be so
called, appears to be made up merely of cells. In such as have
reached a higher stage of development, such as have a diameter of
upwards of half an inch, for instance, it consists of two strata,
the external of which is granulous, and no longer exhibits cells ;
the internal, however, is composed of cells, which are flat,
hexagonal, but also granulous, and bear the relation of a cover-
ing of epithelium to the outer one. The external stratum
passes away over the germinal vesicle and the foundation of the
germinal membrane, so that these structures may easily be re-
moved from its inner surface without injury to it. The internal
cellular stratum, on the contrary, is interrupted at the spot where
the germinal vesicle lies. I have not traced the mode of formation
of this external granulous stratum through all its details ; I sup-
pose it to be produced by a blending of the outer cells, which com-
posed the original membrane when it was made up entirely of
cells. As the period approaches at which the egg leaves the ovary,
the epithelium-like stratum of cells gradually disappears, and the
granulous membrane alone remains. It does not exhibit any
disposition to unite with the structureless external membrane
of the egg, even in eggs which are almost sufficiently mature for
extrusion. If such an egg be cut open under water, and the
investment derived from the ovary be drawn off, this granulous
membrane frequently remains tying upon the yelk, whilst the
structureless membrane follows the above-mentioned investment,
and may readily be proved to be connected with it, when they
are folded so that the inner surface forms a sharp edge. By
the aid of the compressorium this structureless membrane may
then be seen, projecting out from the border of the preparation.
It often separates in large pieces during this manipulation, so
that it has likewise no connexion with the parts pertaining to
the ovary. If the signification of vitelline membrane is to be
assigned to this structure, a blending between it and the granu-
lous stratum must take place in the oviduct, in order to form
the subsequent vitelline membrane of the extruded egg.

We now pass on to that portion of the egg from which the

GERMINAL MEMBRANE. 57

embryo is first formed, the germinal membrane. It represents,
as is known, a round, white, little disc, somewhat above a line
in breadth, which lies between the vitelline membrane and the
yelk-substance. This little disc, in a fresh-laid lien's egg, con-
sists of globules, which are of unequal size in different parts of
the germinal membrane. When examined with the microscope,
they appear much darker than the yelk-globules, (see plate II,
fig. 4.) They lie in close contact, so that they flatten against
one another to an hexagonal form. The boundaries of the dis-
tinct globules may be clearly distinguished, even when in con-
nexion. They may also be readily isolated from one another,
and are then round. They contain many smaller round gra-
nules of various size, with very dark outlines, which float about
singly when the globules are burst by pressure. Although these
granules, in most instances, completely fill the globules, yet some
globules may be observed where that is not the case, and where
a portion of the globule is transparent, and free from granules,
(a b, of the above figure.) I thought that I distinctly saw a double
external outline on one of these globules («), which would be
evidence of the presence of a cell-membrane. In most in-
stances, however, this is not distinct, and my principal reason
for concluding that they are cells, is, that it is so extremely
probable that they are developed to form the indubitable cells
of the incubated germinal membrane. I have not, however,
fully investigated this process, and only communicate my ob-
servations on the point, incomplete as they are. If the unin-
cubated germinal membrane be folded in such a manner that
its external surface form a sharp margin, that surface is found
to be tolerably even, dark, and composed immediately of the
globules of the germinal membrane already described ; the sur-
face of the germinal membrane of an egg which has been ex-
posed to brooding heat for four hours, presents a precisely
similar appearance. The same membrane, when examined also
upon its general surface, differs but very slightly in appearance
from one which has not undergone incubation. The globules
of which it consists merely appear to have more minutely
granulous contents. But if a germinal membrane after eight1

1 It is quite as impossible to define with any certainty a fixed time for a precise
stage of development of the elementary cells of the germinal membrane, as it is to
connect the formation of the area pellucida, the embryo, and its separate parts, with

58 THE OVUM AND

hours' incubation be folded in the same manner, its margin at
many points is found to be no longer dark and even, but to be
composed of extremely pale transparent cells, These cells pre-
sent every variety of size, some being as large and even larger
than the primitive globules of the germinal membrane. They
either project forward in the form of half-spheres, or the greater
portion of their spherical surface juts out in some instances, and-
they may be completely separated by pressure. They contain a
pellucid fluid, but no nucleus. The following fact shows them
to be cells ; some of them contain very minute, isolated, black
granules, which resemble the molecules described by Brown,
and exhibit molecular motion within the cell. This fact proves
that the contents of the cell must be fluid. A fluid which is
miscible with water cannot, however, preserve any definite form,
unless it be encompassed by a membrane. Such a structure must,
therefore, exist in this instance. It is not altogether easy to con-
vince one's self that these granules, exhibiting molecular motion,
do actually lie within the cells ; but it may be concluded from the
fact, that they do not flow away when the surrounding fluid is
allowed to escape, and that they are not moved beyond the
limits of the cell, but only to its wall and back again. Beneath
this stratum of cells lie the globules of the unincubated germinal
membrane, which, however, appear to have become still more
clear and minutely granulous than those of the membrane ex-
amined after four hours' incubation. In addition to these, se-
parate cell-nuclei may be observed, such as occur in the cells
of the serous layer at a subsequent period, and may be seen in
plate II, fig. 6. Still more internally than this layer, we meet
with perfectly dark globules. The serous and mucous layers of
the germinal membrane are perfectly formed in the e^g after
sixteen hours' incubation. If the membrane at that period be
folded so that its external surface may be seen, it will be found

any degree of certainty to any precise hour of incubation. The periods cited should
therefore only be taken as being near about the true determinations of the time.
The cells in the germinal membrane, before incubation even, do not appear to be al-
ways at the same stage of development ; thus, plate. II, fig. 4, c, and fig. 4, a, b, re-
presents cells from two different membranes. A great portion of the germinal mem-
brane from which c was taken consisted of such cells as that delineated, and I thought
I perceived molecular motion in the granules contained in some of them, which, if
correct, would clearly prove them to be cells.

GERMINAL MEMBRANE. 59

to be composed of cells, which project forwards in the form of
half-spheres, (plate II, fig. 5). A nucleus of the characteristic
form may be recognised in some of them. It lies upon the in-
ternal surface of the cell-wall, is round, and contains one or two
nucleoli. In most instances, however, no nucleus can be seen,
cither because none is present, or because it lies upon the
posterior side of the cell, in which position it cannot be per-
ceived, in consequence of the dark substance lying beneath it.
The cells also contain a transparent fluid, and some minute gra-
nules with molecular motion, which is evidence sufficient for
the existence of a peculiar cell-membrane. If, after the ger-
minal membrane has lain for a time in water, the mucous lavcr be
washed off, the general surface of these cells may be observed.
They are then seen to lie close together, and to flatten against
one another to hexagonal forms, (see plate II, fig. 6). They
contain a beautiful nucleus, which encloses one or two nu-
cleoli. They also present many minute granules, which ex-
hibit molecular motion. The cells may also be observed in the
recent germinal membrane, especially on its margin, at which
part it is more transparent, and there they project forward in
the form of large segments of a sphere. These cells then re-
present the serous layer of the germinal membrane, — which,
therefore, consists of round cells (their polyedrical form being
refcrrible solely to their lying so closely together), furnished
on the inner surface of their wall with the characteristic nucleus,
and containing a clear fluid, and some isolated smaller granules.
They might be conceived to be a mere covering of epithe-
lium to the serous layer. But if the serous layer be separated
after the blood has formed, for example, in an e^g which has
undergone forty-eight hours' incubation, the vascular layer re-
mains lying immediately upon this stratum of cells. Valentin
has already recognised these cell-nuclei, for he says, that
each of these layers of the germinal membrane consists of a
transparent vitreous jelly, but that they are to be distinguished
by the corpuscles which they contain. (Entwicklungsgcschiehtc,
page 287.) These corpuscles are the cell-nuclei, the trans-
parent substance in which they lie is composed of the cells, and
is gelatinous only in appearance. The cells have only a mi-
nimum of intercellular substance between them.
When, in the next place, we proceed to examine the mucous

60 THE OVUM AND

layer of the germinal membrane of an egg after sixteen hours'
incubation, we find it to be composed of globules, which vary-
greatly both in size and appearance, (see plate II, fig. 7.) The
large globules, which form the greater proportion, may be proved
to be cells, and Baer has already named them vesicles. The
molecular motion, which is frequently visible in isolated globules
within them, although much slighter in these instances than
in the cells of the serous layer, affords sufficient evidence of
their cellular character. They contain a transparent fluid and
granules of various kinds. One particular globule, having very
dark outlines, resembling those remarked in the cells of the
yelk-cavity, may be observed in almost every cell. Several of
the globules, and of all gradations of size, are frequently seen
in a cell. In addition to the above, a minutely granulated
substance is present in many of them. These cells lie some-
what loosely together in a structureless, tenacious, intercellular
substance, which is their cytoblastema, so that at this stage
they are but slightly flattened against one another. This in-
tercellular substance contains, in addition, perfectly dark glo-
bules and smaller granules, but I do not know what relation
they bear to the cells. A portion of them may, perhaps, be
nuclei of new cells. Yet I could not decide whether the one
dark globule, which is generally so very prominent in the cells
of the mucous layer, had actually the signification of a cell-
nucleus. It differs in form from the usual cell-nucleus very
materially. During the progressive development of the ger-
minal membrane, the quantity of intercellular substance, and
of those globules the cellular nature of which is not demon-
strable, diminishes very much, so that at a subsequent period
the cells lie close together, and present the appearance of ve-
getable cellular tissue. The description here given applies
only to the mucous layer on the outside of the area pellucida.
Within that the cells have quite a different appearance. They
are very much smaller, of pretty equal size, very transparent,
and contain no coarse granules, but only very small globules.
They do not appear to have any nucleus, and this fact dis-
tinguishes them from the cells of the serous layer, which pos-
sess a nucleus even within the area pellucida.

The first rudiments of the embryo appear to be formed from
the cells of the serous and mucous layers of the germinal mem-

GERMINAL MEMBRANE. 01

brane, that is, from such cells as are met with in the area
pellucida, so that the embryo is composed, partly of small
cells without nuclei, and partly of cells furnished with the cha-
racteristic nucleus. It presents, however, besides them, an
extraordinary quantity of simple cell-nuclei with nucleoli, around
which no cells have as yet formed.

I have made but few researches with respect to the structure
of the vascular layer, and from them, I could not (with the
exception of the vessels themselves and the blood) detect any
such essential difference between it and the mucous layer, as
was exhibited between the latter and the serous laver. As, how-
ever, the formation of the vessels themselves, although it ap-
pears to depend upon a production of cells, is not a process pe-
culiar to the germinal membrane, we shall defer it, to be re-
sumed at a subsequent stage of our investigation.

I have not ascertained the relation which these cells of the
layers of the germinal membrane have to the primitive globules
of the membrane before incubation, or within eight hours after
that process has commenced ; but inasmuch as it is probable
that at least one of those kinds of cells owes its origin to the
development of the primitive globules, we may be permitted to
suppose that those globules are likewise cells.

For the purpose of giving, in outline, a connected view of the
changes which the e^g undergoes, from its first formation up
to the period at wdiich the actual development of the embryo
commences, — in so far as the foregoing, more or less complete,
observations enable us to form a provisional conception of the
process of development, — we will proceed on the understanding,
that the germ-vesicle is the nucleus of the yelk-cell ; at the same
time, however, we expressly refer the reader to the more de-
tailed statement above furnished for the certainty both of this
and of every other separate point which occurs in the following
exposition. It is probable that the germ-vesicle is the first struc-
ture, and that the yelk-cell forms around it as its cell-nucleus.
Both advance in growth, the latter, however, much more rapidly
than the former. A precipitate, the commencement of the ger-
minal membrane, next forms around the germ-vesicle. Young
cells are simultaneously formed in the remaining space of the
yelk-cell, these are the cells of the subsequent yelk-cavity. Then
cells of another kind originate beneath the vitelline membrane,

62 THE OVUM AND

which are the subsequent cells of the proper yelk-substance.
They are formed round about the •neighbourhood of the vitelline
membrane, with the exception of that spot where the germ-
vesicle and the rudiments of the germinal membrane lie. These
cells expand very rapidly, while at the same time a new layer
is formed on the outside of them, and so on successively. In
this manner they surround the white cells of the yelk-cavity
with a layer of yellow cells, which is constantly increasing in
thickness ; as, however, a vacant space remains at the spot where
the germinal vesicle and germinal membrane are situated, by
the increasing thickness of the yelk-substance, the space be-
comes converted into a canal. The development of the vitelline
membrane proceeds continuously with these changes, in pro-
portion as the increasing contents require. When the yelk-
cell has attained its due size and the egg leaves the ovary, the
germ-vesicle, like most other cell-nuclei, disappears, and the now
more fully developed germinal membrane remains. It is made
up of globules, probably cells, having coarsely-granulated con-
tents. It grows during the process of incubation by the con-
tinual development of new cells. After sixteen hourV incuba-
tion, a distinction may be observed in the cells composing the
membrane. The more external ones form a layer, in which the
cells exhibit a nucleus of the characteristic form, and contain
a quantity of transparent fluid and minute isolated granules.
These cells are therefore clear, and firmly united together, and
have only a minimum of intercellular substance between them ;
they represent the serous layer of the germinal membrane. The
under stratum of the germinal membrane or mucous layer con-
tains cells of another kind ; they have no nucleus of the cha-
racteristic form, but contain one or more dark globules, and
frequently also some minutely granulous substance. These cells
lie loosely together in a larger quantity of intercellular sub-
stance, which contains smaller granules of different kinds, in
addition. When this division of the membrane into the 'two
layers is completed, and its superficies has become considerably
extended, and after a transparent spot, the area pellucida, has
formed in its centre— (the cells of the mucous layer in this
area being much smaller, but of pretty equal size, as com-
pared with one another, and having transparent contents with
very minute isolated granules),— the embryo is developed,

GERMINAL MEMBRANE. G3

as a portion of the germinal membrane separating from the
whole by a constriction. Both layers contribute to its forma-
tion, and it therefore consists of small transparent cells, some
of which (probably those pertaining to the mucous layer) con-
tain no nucleus, whilst others (those derived from the serous
layer) exhibit the characteristic cell-nucleus with its nucleoli.
In addition to these cells it contains a great many nuclei,
around which no cells have as yet formed. Between the
two layers of the germinal membrane other cells arise, which
may be regarded as representing a third layer, the vascular,
although they do not really form a connected independent
layer; of these we shall treat hereafter. These three layers
then, and pre-eminently the first two, form the mediate basis
of all the subsequent tissues.

The yelk is not a lifeless aliment for the embrvo, — as it is
when taken as food by the adult, to whose organism it is dead
and must be chemically dissolved, — but the cells of the yelk
take part in the vitality called forth by incubation. They
effect an alteration in their contents, wherebv the albumen
which they contain loses its property of coagulating, and the
granules become dissolved, in the same manner in which the
granules of starch dissolve in the cells of the vegetable embrvo.
In short, the yelk bears the same relation to the embryo as
regards its nutritive property, that the albumen bears to the
vegetable embryo.

In accordance with the analogy between the cells we are
treating of and those of vegetables, all the changes in the eggf
the growth of the germinal membrane, and even the first forma-
tion of the embryo, proceed entirely without vessels.

64 PERMANENT TISSUES OF

SECOND DIVISION.

Permanent Tissues of the Animal Body.

The foregoing investigation having taught us that the entire
ovum, from its first origin up to that period at which, by the
formation of the serous and mucous layers of the germinal mem-
brane, the foundation of all the subsequent tissues is laid, exhibits
simply a continual formation and more extended development of
cells, and having found the primordial substance of the tissues
itself to be composed of cells, we are now required to prove, that
the tissues do not only originate from cells in this general man-
ner, but that the special basis of each individual tissue is a matter
composed of cells, and that all tissues either consist entirely of or
are formed from cells which pass through a variety of transforma-
tions. These modifications, which some of the cells undergo in the
progress of their development to the subsequent tissues, are
very important, since thereby the cells not infrequently cease
to exist as separate independent structures. We have al-
ready (in the Introduction) seen such changes in plants, for
example, in the coalescence of the cell-walls observed by
Schleiden in the bark of the Cacti, and the blending of several
cells to form a tube in the spiral and lactiferous vessels. This
takes place to a much greater extent in animals, and, in general,
the higher the importance of a tissue is, the more do the cells
lose their individuality. We shall not, however, enumerate
these modifications here; we shall become acquainted with them
as the result of investigation of the separate tissues, and, at
the conclusion of the work, we shall combine them into a con-
nected representation of Cell-life. It is necessary, however, to
mention the most important of them at least preliminarily in
this place, in order to make a classification of the tissues.

Since all organic structure is primarily formed from cells,
the most scientific classification of general anatomy would
manifestly be one founded upon the more or less high de-
gree of development at which the cells must arrive, in order
to form a tissue. The complete retention, or relinquishment,

THE ANIMAL BODY. 65

to a greater or less extent, of their individuality by the cells,
should serve as the scale for their degree of development. We
give the name of independent cells to those in which the wall
remains distinguishable from the neighbouring structures
throughout the whole progress of its expansion. We apply the
term coalesced cells to those in which the wall blends, either
partially or entirely, with the neighbouring cells, or intercellular
substance, so as to form an homogeneous substance. The cell-
cavities, in such instances, are separated from one another only
by a single wall, as we have already observed in cartilage. This
is the first degree of coalescence ; the cacti present an example
of it in vegetables. The second, is that in which the walls of
several cells lying lengthwise together, coalesce with one another
at their points of contact, and the partition walls of the cell-
cavities become absorbed. In this way not only the walls but
the cavities of the cells also become united, as in the spiral and
lactiferous vessels in plants.

Upon these more or less important modifications of the
Cell -life the following classification of the tissues is based :
1st. Isolated, independent cells, which either exist in fluids, or
merely lie unconnected and moveable, beside each other. 2d.
Independent cells applied firmly together, so as to form a
coherent tissue, 3d. Tissues, in which the cell-walls (but not
the cell-cavities) have coalesced together, or with the intercel-
lular substance. Lastly, tissues in which both the walls and
cavities of many cells blend together. In addition to these,
however, there is yet another very natural section of the
tissues, namely, the fibre-cells, in which independent cells are
extended out on one or more sides into bundles of fibres. The
naturalness of this group will form my excuse for sacrificing
logical classification to it, and inserting it as the fourth class
(lth), consequently, that last mentioned, consisting of tissues,
in which the cell-walls and cell-cavities coalesce, becomes the
fifth (5th).

All tissues of the animal body may be comprised under these
five classes ; the classification, however, gives rise to some
difficulties. For instance, the fibres of cellular tissue and fat
must be placed in very different classes, so also the enamel of
the teeth and the proper dental substance. A second diffi-
culty arises from the fact, that transitions take place, the

5

66 PERMANENT TISSUES.

isolated cells, for example, passing over into those with
blended walls ; and again, a tissue which usually consists of
isolated cells, occasionally exhibits in different situations coa-
lesced cells. Such difficulties, however, present themselves in
all classifications of natural objects. Nature is very unwilling
to accommodate herself to our schemes. The object of her aim
is quite opposed to that of our intellect. She accords and ac-
commodates all contrarieties by gentle transitions : the intellect
disjoins, and seeks everywhere for strongly-marked contrasts.
If, however, regard be had to the most important structure
only in each individual tissue, — for example, in the nervous
system, to the nervous fibres and not to the ganglion-globules,
in cellular tissue, to its fibres and not to the fat, and so
forth, — and further, if we regard only that which is the general
rule as to these structures, all tissues may then be readily
brought under these five classes. With the desire of making
this work as complete as possible, I have applied this arrange-
ment to all the tissues in the way which has appeared to be
most probably correct, according to the investigations I have
hitherto made. Those researches are, however, far from com-
plete, and continued observations may perhaps render it
necessary, at some future time, to assign a different position
to some of the tissues. This may serve as a preliminary
sketch :

Class I. Isolated, independent cells. To this class the cells
in fluids pre-eminently belong ; Lymph- globules, Blood-
corpuscles, Mucus- and Pus -corpuscles, &c.

Class II. Independent cells united into continuous tissues.
Such as the Horny tissues and the Crystalline lens.

Class III. Cells, in which only the cell-walls have coalesced:
Cartilage, Bone, and the substantia propria (ivory) of the
Teeth.

Class IV. Fibre-cells : Cellular (areolar), Fibrous, and
Elastic tissue.

Class V. Cells, in which both the cell-walls and cell-
cavities have coalesced : Muscle, Nerve, Capillary
vessels.

ISOLATED INDEPENDENT CELLS. 67

CLASS I.

Isolated, independent Cells.

By the above term we understand cells which either float
free in fluids, or, at least, are moveable, though lying in close
contact. Such cells, therefore, possess the highest degree of
individuality. This class includes the cells of lymph, blood,
and the various secretions. The ovum might be placed at the
head of this class in a system of general anatomy ; but the
plan of the present work required that it should be discussed
previously.

1. Lymph-corpuscles. According to Vogel's description
(Physiologisch-pathologische Untersuchungen liber Eiter, &c.
Erlangen, 1838), the lymph-corpuscles appear to be cells,
although he does not express the fact in words. For ex-
ample, after the corpuscles have been exposed to the action of
acetic acid, a nucleus is brought into view, the production of
which I do not suppose to be referrible to a separation into
envelope and nucleus, but believe it to have been previously
formed, and rendered visible solely in consequence of the
greater degree of transparency acquired by the envelope, i. e.
the cell-membrane, and its contents, from the action of the
acid upon them. One of the nuclei, amongst the lymph-cor-
puscles, delineated in the above-mentioned work (fig. 4 /;)
appears to contain a nucleolus in its centre. I have not
made any researches myself upon this subject. The mode of
production of the lymph-corpuscles has not as yet undergone
investigation. They are probably formed in the lymph-
plasma, which serves as their cytoblastema, in accordance with
the general law before laid down. We cannot as yet decide
the question whether the nuclei are present before the cells,
and whether the latter are first formed around them ; perhaps
the small granules which Vogel delineates from lymph are
young nuclei.

2. Blood-corpuscles. C. H. Schultz was the first who proved

68

BLOOD-CORPUSCLES.

the blood-corpuscles to be vesicles.1 He relied especially upon
the manner in which they were acted on by water, whereby
they lose their colouring matter, swell, and become round, and
under which circumstances he frequently saw the nucleus roll
about within the round and very transparent vesicle. The last
fact would of itself be sufficiently conclusive. I have not as
yet observed this fact ; on the contrary, in most instances, the
nucleus decidedly adheres to the internal surface of the wall
of the vesicle, eccentrical as in all cells, though it may pro-
bably also sometimes become detached. The fact, however, of
the blood-corpuscles becoming swollen and round, renders their
cellular nature highly probable. If the envelope (hiille) of the
blood-corpuscle were not a flattened vesicle, it might indeed
lose its colour and swell in water, but it would retain its flat
form, like a sponge when filling with fluid. The circumstance
of the nucleus remaining on the wall during the swelling of
the blood-corpuscle in water is no accidental appearance; for
even in the round blood-corpuscles of a chick, forty-eight
hours after the commencement of incubation, when they were
not as yet flattened, I found that the nuclei, which were also
circular, were not placed in the centre, but lay eccentrical upon
the internal surface of the wall. The cellular nature of the
blood-corpuscle, and the signification of its separate parts
scarcely appear to admit of doubt when regarded in connexion
with the whole of this investigation. It is a flattened cell fur-
nished with a cell-nucleus, which is fixed to a spot on the in-
ternal surface of the cell-membrane. The size of the cell as
compared with the nucleus is not the same in all corpuscles ;
that of the nucleus is much more constant. The nucleus of
some blood-corpuscles of frogs which had swollen in water, also
appeared to me in some instances to be hollow. It also loses
its flatness in water, but retains its oval figure. I have

1 [This is clearly an oversight as Hewson not only demonstrated their vesicular
nature, and called them vesicles, but accurately described their becoming " changed
from a flat to a spherical shape," on the addition of water to the blood, and the falling
of the nucleus " from side to side in the hollow vesicle, like a pea in a bladder." See
'Philosophical Transactions,' 1773, vol. lxiii, Part II ; or, 'Experimental Inquiries,'
Part III, being ' a Description of the Red Particles of the Blood,' &c.,&c. (published
after his death), edited by Magnus Falconar, London, 1777 ; also the very valuable
republication of Hewson's Works by the Sydenham Society, edited by George Gulliver,
Esq., where the reader is particularly referred to pp. 220, 221. — Trans.]

BLOOD-CORPUSCLES. G9

never distinctly observed nucleoli in it ; occasionally only
I thought I perceived something of the kind, for instance, in
the blood-corpuscles of a salamander j it was not, however, snf-
ciently evident to permit of my asserting their presence. Cell-
contents must certainly exist ; for if the cell-walls lay imme-
diately upon one another, the corpuscle must be as much
thinner on the margins beside the nucleus as the thickness of
. the nucleus amounts to. If it be assumed that the cell-mem-
brane alongside the nucleus may be so much thicker as thereby
to produce the almost level side surfaces, the cell- membrane
must in such case have a thickness equal to the half of that
of the corpuscle ; but it would then be sufficiently thick to
allow of a double outline being distinguished when it was
swollen by water ; observation, however, does not detect any
such appearance. The red colouring matter forms the cell-
contents. It is difficult to decide whether the cell-membrane
and nucleus are also coloured, but it is in some degree pro-
bable that they are so, since otherwise the centre of the
corpuscle where the nucleus lies must appear white, whilst it
in fact exhibits a paler red colour. The colouring matter of
the blood-cells is not contained in granules, as it is in most
kinds of pigment, but in a state of solution. If the lymph-
corpuscles be cells, their transformation into the blood-corpus-
cles may at least be conjectured as taking place by their
becoming flattened and absorbing colouring matter. Those
blood-corpuscles in which the envelope (hiille) is smaller in
proportion to the nucleus, a fact often observed in the frog,
arc probably younger cells. I have made no observations
upon the formation of the blood-corpuscles in the germinal
membrane. According to C. II. Schultz (System dcr Cir-
kulation, p. 33), the blood- corpuscles in the chick are formed
round the yelk-globules. (?) The latter are first present, and
form the nucleus of the blood-corpuscles ; they become sur-
rounded with a delicate membrane. The vesicle then dilates,
and at length becomes flattened. This description accords
excellently with the fundamental laws previously developed,
and shows that as early as 183G Schultz had discovered the
pre-existence of the nucleus of the blood-corpuscle, the for-
mation of the blood- vesicle around it, and the gradual expansion
of that vesicle.

70 MUCUS-CORPUSCLES.

3. Mucus-corjmscles. The mucus corpuscles have already
beeu described as cells, iu consequence of their resemblance to
the cells of epithelium. They are round globules, enclosing a
nucleus, which is eccentrical. We already know this to be
the. elementary form of most animal and vegetable cells, and
the presence and characteristic position of the nucleus, there-
fore, warrant us in concluding that in this instance also the
globule is a cell, although an especial cell-membrane cannot
be distinguished. Guterbock discovered that the nucleus of
the mucus- corpuscle has the peculiar property of splitting into
two or three smaller corpuscles when acted upon by acetic
acid, and that the enclosing or cell-membrane is gradually
dissolved in the same acid. Vogel, indeed, attributes this pro-
perty to such mucus-corpuscles alone as have been secreted
by a morbid action, and to pus-corpuscles. But I have been
informed by Henle that the true mucus-corpuscles (of which,
according to him, only a very small quantity exist in healthy
mucus,) exhibit the same peculiarity, and that those which are
not affected by the acid are true epithelial cells. As I have
never observed any other cell-nuclei to be similarly acted on
by acetic acid, the fact marks the distinction between mucus
and pus-corpuscles and all other cells, and, according to Henle,
even the youngest epithelial cells do not possess this property,
so that the mucus-corpuscles differ distinctly from them. It
appears to be a characteristic of all cell-nuclei that they not
only are insoluble, but do not even become transparent in
dilute acetic acid. These, therefore, are peculiar cells, which
are formed in the fluid of mucus as their cytoblastema, in the
same manner as the yelk-cells in the fluid of the yelk-ball.
They become more abundant, when the cytoblastema obtains
a greater degree of " plasticity," as the result of irritation of
the mucous membrane; and as on the other hand the secretions
in the normal condition possess but a very small amount of
plastic force, and some — the urine and bile, for instance —
have not any ; we accordingly find in them but a very few
cells, or indeed none at all, save some cast-off epithelium. I
have not investigated the question whether the nucleus exist
before the cell in the mucus-corpuscles, or upon what the
division of these nuclei by means of acetic acid depends.

PUS-CORPUSCLES. 71

4. Pus-corpuscles. We are entitled to consider the pus-cor-
puscles as- cells, by the same arguments which we applied to
those of mucus. Vogel, indeed, regards them as identical
with those mucus-corpuscles which, according to his view, arc
morbidly secreted, but which Ilenle believes to be normal.
Thcv are similarly affected bv acetic acid, and cannot therefore
be young epithelial cells, in which, according to Ilenle, the
splitting of the nucleus does not take place under similar cir-
cumstances ; indeed, that property appears to be confined en-
tirely to the nuclei of the mucus and pus-corpuscles. Vogel
states that the nuclei of pus- corpuscles are concave. The pus-
corpuscles are thus peculiar cells which are formed in the
serum of pus, — i. e. in cytoblastema, exuded during inflamma-
tion, in increased quantity, and of anomalous composition, — •
precisely in the same manner that mucus-corpuscles originate
in mucus, and, indeed, as all cells form in their cytoblastema,
in accordance with the fundamental law already laid down.
According to the observations of H. Wood, they appear to be
earliest formed upon the surface of the granulations, and for
the reason that their cytoblastema, the pus-serum, is constantly
exuding freshest at that part, and therefore possesses in that
situation the greatest amount of plastic force, as we have
already observed in reference to the formation of new yelk-
cells on the outside and in the neighbourhood of the vitelline
membrane. It is, however, probable that the pus-cells pursue
an independent growth for a period, as we have seen to be the
case with respect to those yelk-cells which were far removed
from the vitelline membrane. It is also most likely that the
nuclei of the pus-cells are their first formed part, but I have no
investigations on the subject. The more healthy the pus, the
greater is its plastic force, and the greater the number of cells
which are formed in it, so that in healthy pus the quantity of
serum is very small in comparison with the number of cells.

I cannot state whether the oil-globules which are present in
certain secretion^, such as milk and chyle, are contained in
cells or not. I have not been able to detect anything indi-
cating that they arc so in milk; and, according to the theory
of the secretions, which will be communicated at a subsequent
stage of the work, there does not appear to be any necessity
why they should be so.

72 PUS-CORPUSCLES.

The low grade of development held by the class of cells now
under consideration, in which those elementary formations re-
tain their greatest degree of individuality, is indicated by the
fact that it presents so very few modifications. The mucus-,
pus-, and lymph-corpuscles are small round cells with a nucleus
attached to their walls. According to Henle, mucus- and
pus-corpuscles cannot be distinguished in any way from one
another, and those of lymph differ from them only inasmuch
as their nucleus is more round and granulous, and does not
crumble under the action of acetic acid. No difference exists
between them in the form of the entire cell. The blood-cor-
puscles present a higher degree of development in this class.
In them we not only find very characteristic cell-contents,
the red colouring matter, but the form of the cell also under-
goes an important alteration, inasmuch as it becomes flattened.
As this flattening takes place in cells which float free in a
fluid, it cannot be explained as the result of mechanical causes,
bat must manifestly be regarded as a peculiar stage of deve-
lopment of these cells. The nucleus is persistent in all these
cells, whilst in those more highly developed it usually disappears
at some subsequent period. Throughout this class the cyto-
blastema is a fluid ; and it is present in greater quantity than
we shall find to be the case in the next class. If the egg be
included in this class, we have yet another peculiarity in the
cells to be added to the above; viz. that not only have the
separate yelk-cells cell-contents consisting of distinct granules,
but that the development of the yelk-cells within the yelk
considered as one cell, is a formation of cells within cells, and
in some of these cells even a second enclosure takes place.
This peculiarity, however, is one which may almost be said to
stand in inverse ratio to the importance of the tissue. It
is most frequent, perhaps indeed universal, in vegetables,
occurs more rarely in animals, as in the egg} crystalline lens,
cartilage, and so on, and appears to be altogether absent in
the higher structures, as areolar tissue, muscle, &c. We
have already discussed the other peculiarities of the cells of the
egg. In the following class we shall not only find a greater
change in the form of the cells from flattening, but we shall
also become acquainted with many other different modifica-
tions of them.

INDEPENDENT CELLS, ETC. 73

CLASS II.

Independent Cells united 'into continuous Tissues.

This class presents us with the greatest similarity between
animal and vegetable structure, and, indeed, in so high a
degree, that even an experienced botanist cannot distinguish
some of the objects which belong to it from vegetable tissue.
Most animal cells may be distinguished from the mature vege-
table cells by their greater softness and delicacy ; but those
characteristics are in some measure wanting in this class, and it
would be very difficult to distinguish microscopically between a
thin laver cut off from the interior of the shaft of a feather and
a portion of vegetable tissue. We shall, therefore, take the
feather as our example, and endeavour to trace these cells,
which correspond in so striking a manner with vegetable tissue,
backwards to their primitive condition, explaining this transi-
tion by delineations, and in this way convince ourselves that,
in their early stage, they also accord with the primitive cells
of all "other tissues. The tissues comprised under the term
horny belong to this class, and the crystalline lens may also
be included in it. The cells of these tissues generally remain
independent, but more or less intimate blendings of the cell-walls
with one another also occur in this class. Horny tissue may be
reduced to two unessential subdivisions, viz. — 1. Its mem-
branous expansions, to which belong the Epithelium, in the
extended sense of the term (including the Epidermis), and the
Pigmentum nigrum, which must be enumerated here, in con-
sequence of its intimate alliance with the epithelium. 2. The
compact horny formations, including the Nails, Claws, Hair,
Feathers, &c.

1. Epithelium. — It is very difficult to determine what this
term ought to comprise. The cortical substance of the chorda
dorsalis, which is composed of flattened hexagonal cells (in the
larva of Rana esculcnta, for example), cannot be regarded as
epithelium, since it is made up of the same cells as those of the
interior of the chorda dorsalis : the sole dinerencc consisting in

74 EPITHELIUM.

their being flattened. The serous layer of the germinal mem-
brane also cannot well be considered to be epithelium, although
it has the same structure, and yet it is difficult to give a defini-
tion of it which shall not comprise these structures. We shall
not, however, enter upon this contention about mere terms, but
proceed to the consideration of the structure of the epithelium.
The simplest form of epithelium is that of the round cells
furnished with a nucleus which lies upon the inner surface of
their wall, and encloses one or two nucleoli. When in con-
nexion they assume a polyhedral form, but their free surface
usually projects in the form of a, segment of a sphere. Such
is the appearance presented by the epithelium in many situa-
tions ; I instance only that of the branchial rays of the fish
by way of illustration. The cells are usually smaller and more
granulous in mammalia ; but in the lower animals and in the
foetal stage of mammalia they are, in general, larger, smoother,
and sometimes so transparent as to be visible by a subdued
light only. I once had an excellent opportunity of observing
the epithelium upon the mucous membrane of the stomach of
a fcetal sheep, and its perfect resemblance to the parenchy-
matous cellular tissue of plants. A minutely granulous deposit
may often be observed in the interior of the transparent epi-
thelial cells ; in those of the branchial rays of the fish, for
instance, it appears to be formed in the neighbourhood of the
nucleus. According to Henle, two nuclei never occur in an
epithelial cell in mammalia ; but I have several times observed
that number in the external covering of the tadpole, and on one
occasion I remarked that a perfectly developed epithelial cell
furnished with a nucleus was enclosed within a larger cell.
Changes in form from this rudimentary globular shape occur in
the epithelial cells in two different manners ; they either become
flattened into tables, or prolonged into cylinders. The flattening
out into tables takes place in such a manner that the nucleus
forms the centre of one surface, as in the blood-corpuscle. I
have observed the stages of transition from the globular to the
tabular form in the epithelium of the external covering of the
tadpole, which occasionally presented hexagonal flat columns
or tables, the thickness of which was about equal to one third
of their breadth. The thickness is so very slight in proportion
to the breadth in the completely flattened epithelial cells, that

EPITHELIUM.

/ o

it is no longer possible to distinguish the two lamellae of the cell-
membrane. It often occurs that the tabular epithelial eel Is
are not regularly hexagonal, but represent flat elongated stripes,
a fact which has been observed by Ilenle in the epithelium of
the vessels.1 The cells which are prolonged into cylinders con-

1 During several years past I have occasionally observed an innermost apparently
structureless layer in different parts of the vessels, and as the elastic fibres of the
middle coat of arteries become gradually more and more minute towards the interior
of the vessel, and at length are scarcely perceptible, I regarded the Layer above de-
scribed as analogous to the middle arterial coat, in every respect but the possibility
of discovering fibres in it. I explained certain scattered spots which occurred in
it, by analogy with the middle and external coats of vessels. Lamellae, for instance,
were occasionally present, in which the elastic fibres had coalesced more or less
intimately, and only a trace of a fibrous arrangement remained. In such instances
there is seen a table composed of elastic tissue, perforated at different spots; I
regarded those spots as openings which might perhaps be filled with some foreign
substance. Purkinje and Rauschel (de Arter. et Venar. Structure) acknowledged
the accordance of this membrane with the middle arterial coat, but distinguished it
as a separate layer. Valentin denied that accordance, and described it as a peculiar
structureless membrane. Henle was the first to explain its true relations. By his
mode of scraping the internal surface of the vessels he obtained scales, which,
from our present more accurate knowledge, we now recognise as epithelium. They
were sometimes converted into lamella;. There cannot in fact be a doubt about the
correctness of this explanation, when the vessels of the foetus are examined. I
obtained by scraping, both from the larger veins and heart of a foetal pig, large
lamella; of the most beautiful epithelium, consisting of flat stripes, which were nearly
as long again as broad, and contained a very distinct and, in proportion to the size
of the scales, large nucleus, with one or two nucleoli. I could not succeed so well
in the few attempts which I made on arteries; probably the scales separate more
readily from one another in them, and can then no longer be distinguished from the
primitive cells of the elastic coat. The cells probably coalesce more or less in-
timately at a subsequent period, so as to form what is then a partially structureless
layer, and the nuclei also disappear in part. I now conjecture that the above-
described spots upon the inner coat may probably be persistent nuclei ; I have not,
however, made any new investigation upon the subject. With respect to the situation
in which the one or other form of epithelium occurs, I refer to Ilenle's very complete
treatise (Midler's Arehiv, 1838, Heft 1). In addition to the parts mentioned by
Ilenle, I have found epithelium upon the internal surface of the amnion in the foetus
of mammalia and man, where the hexagonal scales were very large and beautiful,
enclosing a very distinct nucleus and nucleolus. Amongst those in the foetal pig
were some larger round cells, furnished with a larger nucleus without a nucleolus.
The inner surface of the portion of the allantois projecting from the chorion in the
same foetus was also lined with tcssclated (tabular, scaly) epithelium consisting of
small scales. The external surface of the chorion was formed of cylindrical cells
closely packed together, and provided with a nucleus, being similar to the epithelial
cylinders of the intestinal mucous membrane discovered by Ilenle.

76 EPITHELIUM.

stitute the other modification in the form of the epithelial cells.
They were discovered by Henle in the intestinal mucus-mem-
brane. Thev likewise enclose the characteristic nucleus, and are
arranged with their longest sides in apposition. Their blunt
ends are turned outwards and free. The opposite end either
terminates abruptly also, as in the chorion, or proceeds to a
point. This tapering figure frequently commences at the upper
part, so that the cells then have the form of a pointed cone,
the base of which is turned towards the outside. Henle found
that the cilia stand upon the free surfaces of the epithelial j
cylinders in those membranes which present the phenomenon of
ciliary motion, a fact of itself sufficient to show that the epithe-
lium ought not to be regarded as a mere inanimate covering
to the organized structures.

With regard to the formation of the epithelial cells, Henle
has already proved the rete Malpighii to consist of round
nucleated cells, probably the young epidermal cells, and also
that the diameter of the cells increases towards the outside, so
that in the fcetal pig he was enabled to trace the gradual transi-
tion of the cells of the rete Malpighii into those of the epidermis.
(Symbolse ad anatomiam villor. intest., p. 5.) An actual growth
of the epithelial cells thus became very probable ; I have likewise
followed this process in the fcetal pig. The uppermost layer of
the epidermis is there formed of large, tabular, hexagonal cells,
furnished with a nucleus. Immediately beneath these lie
nucleated cells, which are already much smaller, and almost
round, so that the flattening must take place very rapidly. The
farther you proceed from the surface the smaller the cells be-
come, and the closer they encompass the nucleus. The size
of the nucleus also diminishes in some degree, but by no means
in the same proportion. In the lowest strata, the cells cannot
any longer be distinguished, but the nuclei lie close together,
with a small quantity of minutely granulous intermediate sub-
stance. It is, however, very difficult to obtain positive convic- ■
tion of this fact, for the stratum of nuclei is too firmlv connected !
with the cutis. We shall have an opportunity of observing
this relation of the nuclei more distinctly hereafter in the
feather. The mode of formation is probably this : cell-nuclei are
formed, in the first place, immediately upon the surface of the
cutis; and then around, and closely encompassing them, the cells.

PIGMENT.

/ •

The cells and the nuclei (the latter, however, in a much less
proportion) increase in size, and at length those in the upper-
most layers become flattened in such a manner that the nucleus
forms the centre of the table. This, then, is but a repetition
of the same course of development observed in most other cells -
Before I had proved the universal accordance between animal
and vegetable cells, Henle thought that the original increase
in volume of the epithelial cells might possibly be explained as
taking place by imbibition. (1. c., p. 9.) As, however, we
have observed this growth to be a phenomenon which occurs in
all animal cells — as we have seen the formation of cells around
the nuclei — as a chemical change in the cell-membrane may be
proved to take place during the expansion of many of the cells,
and as it frequently happens that not only does no thinning of
the cell-membrane occur during expansion, but that an actual
thickening takes place, all which are processes similar to those
of plants — we must ascribe a peculiar vitality to the animal as
well as to the vegetable cells, and explain this expansion of the
epithelial cells, like as we did that of plants, as a growth by
intussusception. The new epithelial cells, it is true, are formed
immediately upon the cutis only, where the greatest vital energy
prevails; but the cells expand independently, and grow by
intussusception. I have brought forward an instance in which
a young epithelial cell was formed within another in the tadpole.
But this is certainly a very rare circumstance, and the majority
of epithelial cells, in all the vertebrate animals, are certainly not
formed as cells within cells, but on the outside of the cells in
a minimum of cvtoblastema, which is exuded from the cutis. It
might be objected that this process of formation of the epi-
thelium could not be possible, for the reason that, if the
cells of the second stratum were twice as large as those of the
first, then, the whole layer of epidermis must be also twice as
large as the first. But this objection may be easily set aside
by the fact that the cells slide upon one another, and a double
or triple layer of cells may thus originate from one stratum
of nuclei.

2. The Pif/mentum nigrum. The pigment is familiarly known
as being usually contained in round or (in consequence of their
close apposition) hexagonal cells, in the form of innumerable

78 PIGMENT.

very minute granules, which exhibit a lively molecular motion.
This motion may sometimes be observed even within the cells,
so that the rest of their contents must be fluid. As it is also
known, that the pigment-granules may sometimes be pressed out
from the cells, no doubt can exist respecting the cellular nature
of these bodies, formerly called pigment-globules. The wall of
the pigment-cells exhibits a nucleus, which is already familiar to
some observers. It may be seen in the fetal condition of the
pigment cells of the choroid coat in mammalia, at different
points in that of the very young foetal pig for instance, quite
distinctly; and it occasions the well-known white spot in the
centre of the cells. It commonly contains one or two nucleoli.
It sometimes happens that no pigment-granules are deposited
around the nucleus, but that it is surrounded by a clear, trans-
parent areola.

Some pigment-cells undergo a most remarkable transforma-
tion, and one which acquires an especial importance, from the
fact that it serves as a type of formation for other more im-
portant classes of cells. This transformation consists in the
cells being elongated on three or more sides into hollow fibres.
These we shall name stellated cells. It has, indeed, been
necessary to allude to them already when treating of bone.
The characteristic contents of the pigment-cells render them
best adapted for an accurate examination of this type of for-
mation. The stellated pigment-cells, known under the name
pigment-ramifications, are best observed in the skin of the
tadpole. They exhibit varieties in form ; we select for our
description such of them as present the longest fibres. (See plate
II, fig. 9.) Their appearance is that of separate black spots,
from which slender black fibres issue on different sides. The
black spots represent the bodies of the cells filled with pigment ;
the fibres are the prolongations of the cells filled with the
same material. The separate pigment-granules may be dis-
tinguished in many situations. The body of the cell, which is
sharply defined on its exterior, sometimes presents a clearer
spot of a round or oval form, through which the cell-nucleus
glimmers, and in some few instances can be distinctly per-
ceived with its nucleolus. The diminution of the cell in various
directions, in order to pass over into a fibre, is so gradual that
there is no defined limit between them. The fibres pass be-

PIGMENT. 79

tween tlic cells of the epithelium, and are therefore frequently
curved : they arc in general thickest in the neighbourhood of
the cell, and diminish as they proceed from it ; but they some-
times also swell out slightly at some distance from the cell.
These fibres give oft* others at different points. The presence
of the cell-nuclenSj and the fact that all the stages of transition
from indubitable pigment-cells to these bodies may be demon-
strated, arc sufficient evidence that these black spots, with the
fibres proceeding from them, are actually cells, and that th
fibres are hollow prolongations of them filled with pigment.
These transitions are delineated in plate II, fig. 8, just as they
existed close together in another part of the tail of a tadpole :
a is an indubitable pigment-cell, scarcely differing from an
ordinary one ; it has also its nucleus. The only circumstance
which distinguishes the majority of the primitive cells of these
stellated pigment-cells from common pigment* cells is that they
are generally smaller, and more closely filled with pigment.
b is a smaller cell, which has commenced to taper ; and c is
distinctly elongated into a fibre. A slightly clearer spot is the
onlv indication of the nucleus in both instances, d and e
elongate at both ends into fibres, one of which (the upper end
of d) terminates in a knob with a defined outline. At the spot
where this knob unites with the body of the cell, a shading,
indicating a cavity, may be clearly perceived, the pigment
being more closely deposited in the neighbourhood of the cell-
wall than in the centre; and lastly, /is a cell which elongates
into fibres on three sides. AVhen a small piece of the
skin of the tadpole is torn in water, separate portions of
these pigment-fibres, or prolongations of the colls filled with
pigment, may be observed to float about isolated. Instances
sometimes occur in which one of these pigment-fibres passes
uninterruptedly from the body of one cell to that of another j
for example, fig. 9, a. We may imagine this to be effected
by the prolongations of two cells meeting at one point.
As the pigment does not move from one cell to another, we
cannot accurately determine whether the partition-walls be-
come absorbed at such a point or not. Such, however,
may be supposed to be the case, otherwise an interruption of
the pigment corresponding to the double thickness of the cell-
wall must be seen at the spot where the prolongations arc in

80 NAILS.

contact. The fibres issuing from the cells often become very
minute in the last part of their course, from which we learn
that the delicacy of fibres does not preclude their being
hollow.

3. Nails.— In order to investigate the structure of the nail
we should make use of that of a child immediately after birth,
or, what is better, that of a mature, unborn, human foetus ;
such an one, when divided into delicate longitudinal sec-
tions, will be found to consist of laminae deposited one upon
another, surface to surface. This laminated arrangement, how-
ever, becomes more and more indistinct upon the under surface
of the nail which lies upon the skin, the nearer we approach to
that portion contained in the fold of skin at the root, and
the posterior half of the part which is embedded in that fold
exhibits no laminated structure whatever, but consists of small
polyhedral cells, many of which present perfectly distinct cell-
nuclei. When a small portion is cut or torn off from the
surface of such a nail, the form of the margins, which present
smooth angular projections, leads at once to the supposition
that the laminae of the nail are not structureless, but pro-
duced by the junction of little scales resembling those of
epithelium. When treated with acetic or concentrated sul-
phuric acid, the scales separate more readily, and in some rare
instances an indistinct nucleus may be recognized in them.
No such scales can be seen in the root of the nail after the
adherent lamella of epidermis has been scraped off,. but polyhe-
dral cells, which are much smaller than the scales, are found
in that situation. Now it is a well known fact that the nail in-
creases from its root, and is constantly pushed forwards. The
polyhedral cells of the root must thus, therefore, become
transformed into those scales by flattening and extension of
their superficies, a process which the independent vitality of the
cells renders easily conceivable. The cells of the nail already
formed increase in size from the same cause, and the growth
of the nail by no means depends upon a mere apposition at its
root, although it is probable that the formation of new cells
takes place in that situation onlv where the nail is in con-
nexion with the organized skin. The nail would certainly be
pushed forwards by the extension of the superficies of those cells,

HOOFS. 81

and by their flattening in reference to its thickness, but the
more the cells become flattened, the thinner must the anterior
part of the nail become. This probably is compensated for,
by a formation of epithelium-scales upon the under surface of
the nail, and especially at its posterior part. If, for example,
an epithelium-scale become attached to the most posterior part
of its under surface, it will be advanced somewhat forwards by
the flattening of the cells above, and the formation of new
cells at the end of the nail. At that part, however, a new
scale is next formed, and laid upon the former one, and as the
advance forwards goes on, a third and fourth are formed, and
so on, so that, by this means, a thickening of the nail must take
place proportionate to its advance from behind forwards. I
consider, therefore, that this thickening of the nail, in conse-
quence of growth from the under surface, and the thinning
consequent upon the flattening of the cells, compensate each
other, and that the almost uniform thickness of the nail is
produced by this means. The superficial laminae of that part
of the nail which lies external to the fold of skin at all events
do not continue to grow. I marked several nails with two
points, by boring them with a needle and colouring the spot
with nitrate of silver ; the marks were made at the root of the
nail, some in the longitudinal, others in the transverse direc-
tion. In the course of two or three months they had advanced
to the point of the nail, but their distance from each other had
not altered in the least.

4. Hoofs. The horny tissue of hoofs, in the foetus at least,
consists entirely of the most beautiful vegetable-like cells. If
a thin transverse lamella be cut off from the hoof of a large
foetal pig, the preparation will present the exact appearance of
vegetable cellular tissue. The following facts prove that the
cells are not flat : in the first place, when the side walls do
not stand quite perpendicular, they may be traced down below
the level of the section, and the depth to which they go may
be estimated ; and secondly, longitudinal sections of the horny
tissue of hoofs present a similar appearance to those made in
the transverse direction. They are, therefore, polyhedral cells,
and some of them, at least, contain a distinct nucleus.

When the tissue is quite fresh, it is not possible to distin-

6

82 FEATHERS.

guisli the particular wall of every cell. But when the foetus
has lain for a time in strong spirit, the horny substance of the
hoof mav be easily separated from the foot, in consequence of
the connexion between the cells having become looser. The
undermost layers of cells, however, remain attached to the
foot. The interior of the layer of horny substance so sepa-
rated, consists of a crumbling mass, somewhat resembling a
boiled yelk. The particles cannot, however, be separated quite
so readily from one another as those of the yelk are. With
the aid of the microscope, this mass is found to be composed
of. irregularly angular bodies, resembling the yelk- substance
when boiled. These bodies are the isolated cells, whose pecu-
liar walls are distinctly perceptible, and some few of them
have a nucleus, which lies upon the inner surface of their
wall. A continuous firm layer of flat epithelium-scales, the
immediate continuation of the outer lamellse of the epidermis,
consisting of flat cells, surrounds these polyhedral cells as an
external covering to the entire hoof. This lamella exists in
the foetal pig at a very early age, the layer of polyhedral
cells being at that time very slight ; in a more advanced stage
of development, however, the latter forms the chief mass of the
horny substance of the hoof. In the recent condition these
cells must also have somewhat firm contents, otherwise, with
so delicate a cell- membrane, the substance could not be so
firm. But its elasticity was such as to prevent my crushing
one of the cells with the compressorium, my object being to ob-
serve whether the cell-contents would flow out, or be torn like
a firm substance. As the cell-contents form a large portion of
this horny substance, whilst the nails consist for the most part
of flat cells without any discernible contents, almost entirely
therefore of cell-walls, a chemical distinction may be presumed
to exist between the two structures.

5. Feathers. The feather is composed of the quill, the
shaft, and the vane, or beard. The elementary structure of
these parts is, however, what most interests us at present ; and
in order to investigate it, at least in order to become acquainted
with the relation which the different elementary formations in
the feather bear to cells, we must take one in which a part of
the shaft is in progress of formation. The feathers at that

FEATHERS. 83

time are surrounded by a dense capsule, which is composed,
throughout its entire thickness, of gigantic tabular epithelium.
The feather is so placed in this capsule, that the shaft and vane
arc folded together to form a hollow cylinder, which is occu-
pied by the so-called organized matrix of the feather (see an
article on this subject by Fr. Cuvicr, in Froriep's * Xotizen/
No. 317). According to Cuvier, a membrane lines the inner
surface of the vane, and gives off septa, which penetrate be-
tween its separate barbs. This membrane, hoAvever, as well as
the septa, is composed of epithelium.

The shaft of the feather consists of a loose medullary sub-
stance (the pith), surrounded by a firm cortex. On making
thin transverse or longitudinal sections of the pith, it is seen
to be composed of beautiful polyhedral cells, which perfectly
resemble the parenchymatous cellular tissue of plants, — as the
substance of cork for example. (See plate II, fig. 10.) The
cell-cavities which have moderately thick, dark partition-walls,
are at first filled with a transparent fluid, but subsequently
become dry, and in that state contain air. Notwithstanding,
however, that this pith so precisely resembles vegetable tissue
in its general appearance, it may be questioned whether these
cells be actually cells in that sense of the word in which we
receive it here, viz. elementary cells of organic structure, and
whether they correspond to vegetable cells. It therefore be-
comes necessary to investigate whether each cell has its pecu-
liar wall, and whether the course of development of each in-
dividual cell be the same as in plants. There is no structure,
however, in which it is easier to follow the process of develop-
ment than in the one before us, chiefly because the cells,
even from the first, have no connexion with the organized

J o

so-called matrix, but remain attached to the fully-developed
cells of the shaft, when the matrix, which terminates ex-
ternallv with a smooth surface, is taken away. The following
description is taken from the large wing-feather of a raven :
it applies however equally well to the feathers of the young
chicken.

The pith, when in progress of formation, is soft and friable.
When a small portion of it is examined, after the component
particles have been separated asunder, it is found to consist
of cells, in various stages of development. Those which

84 FEATHERS.

are most completely developed resemble the cells which we
have seen in the mature feather (see plate II, fig. 11, a)
in every other respect, but that they lie less firmly connected
together, so that they may readily be isolated, and the peculiar
wall of each cell be distinctly seen. The walls are of suffi-
cient thickness to prevent their losing their angular shape,
even when in the isolated state. There are intercellular spaces
between some of them, and such also occur between the fully-
developed cells of the perfect feather. The cell-membrane is
dark and smooth, and the cell-contents consist of a transparent
fluid. A very distinct nucleus is also seen lying upon the wall
of each cell. It is oval, and contains one or two nucleoli,
which are large in proportion to its size (see the figure).
There is no nucleus to be seen in the fully - developed cells,
and it is only in very rare instances that its remains can be
detected ; it must therefore undergo absorption at some sub-
sequent period. The process may, indeed, be followed ;. for
example, the cells which form the stages of transition between
those delineated in fig. 11 a, and fig. 10, are more closely
connected together, the nucleus in them becomes smaller,
and its outline more irregular, the nucleolus meanwhile re-
mains ; at length both disappear. The degree of development
attained by the cells is generally proportionate to their dis-
tance from the matrix ; and as that is situated on the inner
side of the feather, at the part where the shaft exhibits a
furrow, those cells which lie immediately under the cortex, at
the back of the shaft, are the most perfectly developed. Now
the cells when traced from that part inwards towards the ma-
trix, are found to become gradually smaller ; the cell-mem-
branes lose their dark outlines, and present a granulous aspect.
The nucleus of the larger granulous cells has still the same
form as in those previously described with a smooth cell-mem-
brane; its size, however, diminishes with that of the cell.
The cells in this granulous condition resemble most of the
elementary cells of other tissues. Plate II, fig. 11, b, c, repre-
sents the stages of transition. Advancing still nearer towards
the matrix, the cells are no longer recognizable ; all that we
can see being numerous nuclei, which lie close together in a
minutely- granulous substance (plate II, fig. 12).

The process of formation of the cells of the pith is, there •

FEATHERS. 85

fore, as follows : a minutely- granulous mass is present in the
first instance, in which numerous cell-nuclei lie, some of them
exhibiting a nucleolus. Around them the cells are formed,
being at first not much larger than the nuclei, and having a
granulous aspect. The cells gradually expand ; the nucleus
also grows, and soon reaches its full maturity. It remains
eccentrical, lying upon the cell-wall. The cell-membrane re-
tains its granulous aspect for a time; gradually losing it,
however, as the expansion of the cells advances ; at the same
time the contents of the cell-membrane become darker, but the
cell-walls are not at all diminished in thickness. The walls
of the cells, in the next place, become more firmly united
together, so that they cannot be separated from one another
so readily, and at the same time the nucleus gradually dis-
appears. The contents of the cells at last dry up, and they
become filled with air. The development of these cells ac-
cords, therefore, entirely with the vegetable cells, the nucleus
being their true cytoblast ; it is present before the cell, and,
as is generally the case in the cells of plants, afterwards be-
comes absorbed. The cell expands, growing by intussuscep-
tion, and the membrane of the fully- developed cell might,
without much danger of error, be assumed to be more than
ten times heavier than that of the youngest one. The phy-
sical, and probably also the chemical, condition of the cell-
membrane undergoes a change. The cytoblastema, in which
the cell-nuclei are in the first place formed, consists of gra-
nules, analogous to the mucus-granules, in which, according
to Schleiden (Miiller's Archiv, 1838, plate III, fig. 2), the
cytoblasts of vegetable cells originate. According to Schleiden,
those mucus-granules are deposited from a solution of gum
within a parent-cell. The cells of feathers are not formed in
parent-cells, but in the neighbourhood of the organized matrix.
There can be no doubt, however, but that the matrix only
exudes a fluid, which afterwards becomes transformed into a
granulous substance. I have not investigated the mode in
which the nuclei originate in the cytoblastema, whether
by a junction of smaller globules, whether the nucleoli first
exist, and so forth. The growth of the nucleus proceeds for
a time with that of the cell ; for the latter is formed around
the nucleus before it has reached its full size. The cytoblas-

86 FEATHERS.

tema of the cells of the pith of the feather is supplied by the
nearest contiguous substance provided with vessels, that is, by
the so-called matrix. In the young feathers of the hen, how-
ever, I found a layer of very small, extremely pale, round cells
without nuclei, — a sort of imperfect epithelium, — between
the matrix and the granulous cytoblastema, so that not even
so much as an immediate contact exists between the latter and
the organized substance.

The cortical substance of the shaft of the feather is a
fibrous structure. Here the Cell-theory seems, at first sight,
to fail ; but we are soon taught otherwise, when we examine
the generation of the fibres as exhibited in the incompletely
formed portion of the cortical substance of a feather, which
is in progress of formation within the capsule. The cortex
is then seen to consist of large flat epithelium-cells, each
having a beautiful nucleus, with one or sometimes two
nucleoli. Some of these epithelial tables are long flat stripes,
others are of an irregular rhomboidal form. They- are very
firmly connected together. Each cell generates several fibres,
and the transitions may be readily observed at different
parts of the same preparation. Plate II, fig. 13, represents
them. The cells at first are flat tables, having a smooth
margin, a slightly granulous aspect, and containing a very dis-
tinct nucleus (fig. 13, a). Upon their margins and sur-
faces indistinct fibres gradually become visible, which project
out insulated from the edges, but are connected together upon
the surface by the substance of the tables (fig. 13, b). At
this stage the fibres are pale, and the nucleus of the cell still
quite visible. The fibres afterwards become more sharply and
darkly defined; the insulated portions projecting from the
edges are larger, the part of the table connecting them together
becomes more indistinct, and the nucleus begins to wane
although it is still distinctly perceptible, and the nucleolus
especially so (fig. 13, c). At length all traces of the original
cell and the nucleus disappear, and we see only dark, stiff,
thin fibres, which are closely connected together but' may
still be recognized as being insulated for a space, the length of
the original table (fig. 13, d). These fibres, therefore, also
originate from cells, and that not so much bv an elongation
ol the cells, as by their division into several fibres. As the

CRYSTALLINE LENS. S7

fibres which lie close together in the first instance, do not,
as it seems, continue connected with one another, a portion
of the original table must be absorbed, and the following
mav therefore be conceived to be the mode in which the
fibres originate. After the two laminae of the table are in
part or entirely blended together, an absorption takes place
at certain parts, in such a manner, that the portions not
absorbed lie in longitudinal lines, and thus remain as fibres.
The reality of an absorption is, moreover, distinctly shown by
the disappearance of the cell-nucleus. We have no evidence
as to whether the fibres are hollow or not ; it is sufficient for
our purpose to know that they originate by a transformation
of cells.

The quill of the feather has a similar structure to that of
the cortical substance of the shaft.

The vane is composed of separate barbs, and each barb is
again a miniature feather. The following description is taken
from the undeveloped wing-feather of a sparrow. Each bail)
contains a secondary shaft, on the side of which is placed a
secondary vane. The secondary shaft has the same structure
as the principal one, and consists of a cellular medullary sub-
stance (pith), and a firm cortex. The secondary vane is com-
posed of a great many triangles, which lie with their surfaces
close together, having very narrow bases by which they are
fixed upon the secondary shaft. Each triangle is formed of
flat epithelium-cells arranged with their angles overlapping
each other, each having its nucleus. The separate epithelium -
cells are broadest below, diminish more and more towards the
point, and extend proportionately in length. The nuclei lie
in a row, near about the middle line of the triangle. The
last ceil, at the apex of the triangle, is contracted into a long
fibre. The last cell but one, and all the others in succession,
become elongated, at the point at which the next following
cell is attached to them, into pointed processes, which vary in
length, and are extended on both sides of the cells in the plane
of the triangle.

G. The Crystalline lens. The mode in which the lens is
nourished has always been an enigma. Having no vessels, it
has either been regarded as a secretion of its capsule, or its

S8 CRYSTALLINE LENS.

mode of life lias been considered as generally resembling that
of vegetables. We shall find the latter to be the correct view,
and the singularity of the mode of its nutrition disappears
altogether, when we become acquainted with the fact, that the
growth of the organized tissues resembles that of vegetables.
The general statement, that the lens has the vitality of a vege-
table, does not, however, express much, unless the relation of
its elementary structure to the cells of plants be proved. The
lens is known to be composed of concentric layers, made up of
characteristic fibres, which, not to go into details, may be said
to pursue a general course from the anterior to the posterior
surface.

In order to become acquainted with the relation which these
fibres bear to the elementary cells of organic tissues, wre must
trace their development in the feet us. When the lens of a
chick is examined after eight days' incubation of the egg, no
fibres are to be found ; but it is composed of round, extremely
pale, and transparent smooth cells. Some contain the charac-
teristic cell-nucleus, in others it cannot be detected ; and there
are also many nuclei without surrounding cells. Some larger
cells may be observed in the chick at a more advanced period,
which contain in their interior one or two smaller ones (see
pi. I, fig. 10, d, from a foetal pig), and from the manner in which
these cells become flattened against the wall of the parent-cell,
as well as from the presence of the nucleus in other cells, we
may conclude, that these pale globules are actually cells, al-
though a cell-membrane be not distinctly recognizable. Wer-
neck, who first observed them, likewise calls them cells.

The following conditions of the crystalline lens may be ob-
served in Mammalia. In a foetal pig, three and a half inches in
length, the greater part of the fibres of the lens is already
formed ; a portion, however, is still incomplete ; and there are
many round cells awaiting their transformation. The perfected
fibres form a sphere in the centre of the lens ; but there is no
laminated structure as yet perceptible in it. The fibres may
readily be separated from each other, and proceed in an arched
form from the anterior towrards the posterior side of the lens.
This sphere, composed of the perfected fibres, becomes sur-
rounded, in the circumference of the lens, with a thick and
broad zone of fibres, which are as yet imperfectly developed.

CRYSTALLINE LENS. 8H

They have much the same course as the others, that is, they
form arches from the anterior towards the posterior surface.
They do not, however, reach the axis either in front or behind,
but the fibrous zone is thickest in the middle, gradually dimi-
nishes towards the anterior and posterior surfaces of the lens,
and terminates altogether without the fibres meeting anywhere
in front, or reaching the axis. No laminated structure can
be perceived in the zone ; but the fibres may he readily insu-
lated throughout its entire breadth. When the ends of these
fibres are examined, they are found to be either simply rounded
off, or to terminate in a small round dilatation, or to pass over
into larger globules (cells) ; or, on the contrary, it may be more
correctly expressed by saying, that the larger globules or cells
become elongated to these fibres (see pi. I, fig. 12). The
transition from cells to fibres may either be very gradual or
somewhat sudden ; but even in the latter case, the contour of
the cell passes immediately over into that of the fibre, so that
the latter is not merely affixed to the globule, but is a true
continuation of it. Now, these cells which become elongated
into fibres, perfectly accord with other neighbouring cells which
are as yet quite round ; and these again accord with the cells
forming the greater portion of the lens in the embryo chick.
They are round, extremely pale, smooth, transparent cells of
very various size (see pi. I, fig. 10). Some have a very beau-
tiful, sharply-defined, oval nucleus, which, in most instances, is
not flattened, and which lies upon their wall, and encloses one
or two nucleoli. Some cells are scarcely larger than the nu-
cleus which they contain, fig. 10, b, for example. Some of
these enclose young cells (fig. 10, d), and as they may be ob-
served to flatten against the wall of the parent -cell, there would
seem to be no question about the existence of a special cell-
membrane for the latter, and thus the true cellular nature of
these globules appears indubitable. The presence of the nu-
cleus, and the fact of the outlines of the cells being too sharply
defined for mere shadows, would, however, have been sufficient
to render their cellular character probable. The very distinct
nucleoli contained in the nuclei, which are not flattened, lie
upon the inner surface of the wall and not in the centre, as
represented in fig. 11.

Since, then, the round cells, as we have seen in the chick.

90 CRYSTALLINE LENS.

form the primary structure of the crystalline lens, and no
fibres can be detected in the early stage, and since the more
fully- developed lens of the foetal pig exhibited many fibres and
fewer round cells, and at the same time cells which became
elongated into fibres, we cannot but regard the fibres gene-
rally as elongated cells. It is true that a cell-membrane
cannot be distinguished on the fibres, nor can it be distinctly
recognized on the round cells. If, however, the arguments
above cited rendered its existence in the round cells certain,
they must avail equally in the case of the fibres. Nuclei
are also frequently found upon the fibres of the foetal pig
Some of the fibres are flat. I have, also, several times ob-
served an arrangement of the nuclei in rows ; but I do not
know what signification to attach to the fact. A blending of
several cells to form a fibre may also possibly occur j but I
have no observations decisive of the point. In fishes also
in a young pike for instance, the elongation of the cells into
fibres may often be very distinctly seen.

Brewster found that many fibres of the crystalline lens espe-
cially m fishes, exhibit the remarkable peculiarity of having
their margins serrated. PI. I, fig. 13, represents such a fibre
taken from the innermost lamina of the lens of a pike The
fibres are flat, and their sharp margins furnished with W
teeth, which are so disposed, that two neighbouring fibres lock
into each other by them. We have here an instance of perfect
analogy to a form of vegetable cells, which is delineated in fig
14 : it is an epidermal cell of a species of grass. It is very
much elongated, quite flat, and furnished on the sides with
teeth precisely similar to those of the fibres of the lens which
m like manner, fit in between the denticulations of the con-
tiguous cells. The fibre-cells of the crystalline lens which are
delineated, have somewhat longer teeth in comparison with the
breadth of the cell - they represent, however, some of the most
strong y denticulated fibres. On pursuing the examination
from the external towards the internal laminas, the same lens
will be found to present all possible stages of transition in this
serration, from the smooth or only minutely -notched cells,
to such as are strongly denticulated like those in the figure.
Has striking accordance of so remarkable a form of animal
structure with a similar modification of vegetable cells, is a

INDEPENDENT CELLS, ETC. 91

brilliant confirmation of the correctness of the view, that the
fibres of the crystalline lens arc really cells, however much
they may deviate from the fundamental type of the cellular
form.

There is no longer, therefore, any more difficulty in explain-
ing the process of nutrition in the lens, than there is that of
plants. The cells grow by their own independent force, and
blood-vessels are unnecessary, as the nutrient fluid can be con-
ducted from one cell into another. A morbid change of the
cell-vitality, rendering the cell- contents opaque, is also possible.

The structures included in this class, notwithstanding the
strong general resemblance which they bear to each other, have
furnished us with far more varied modifications of the cellular
form than the previous class exhibited ; indeed, these so-called
unorganized tissues have already prefigured the type of all the
changes by which the organized tissues are developed from sim-
ple cells. Here, also, the fundamental form of the cells is that
of a sphere, which, in consequence of their close contact,
passes over, from mechanical causes, into a polyhedral figure.
Two different modifications of this fundamental form occur,
which cannot be explained mechanically ; they are the flatten-
ing of the cells on two opposite sides to form tables, and their
elongation in two directions into cylinders or fibres. We have
already seen an instance of flattening of the cells in the blood-
corpuscles of the previous class. It is not only more strongly
marked here in the tabular epithelium, where the cell-cavity is
quite obliterated, but a modification even of this form is pre-
sented to us in the elongation of these tables on two sides into
flat stripes, as seen in the epithelium of the internal coat of veins
for example, and still more distinctly in the cortical substance
of the shaft of the raven's feather. The epithelium of many of
the mucous membranes, that of the intestine for instance, which
Henle describes as consisting of little palisade-like cylinders
placed close to one another, furnishes us with a rudimentary
form of the elongation of cells into cylinders and fibres.
Sometimes these little cylinders become acuminated at their
lower extremity, or they may diminish throughout their entire
length from above downwards, and thus become small cones.

92 INDEPENDENT CELLS UNITED

This prolongation of cells into long cylinders (called fibres)
is much more remarkable in the crystalline lens. The fibres
or cylindrical cells of the lens, however, themselves undergo
very important modifications, inasmuch as they often become
flattened on two sides into bands, and then the margins of
these bands become denticulated. This serration is probably
produced by a more forcible expansion, and therefore bulging-
out of the walls of these bands at different points, which follow
each other at pretty regular distances, whilst the intervening
points, situated close to them, remain stationary. All the dif-
ferent stages of this serration, may be observed in the lens of
the fish, if the fibres are examined from the exterior towards
the centre of the structure. Now, in this flat and serrated
condition, the cells of the crystalline lens perfectly resemble
those of the epidermis of some grasses, and this accordance
with indubitable vegetable cells is a proof that, despite the
modifications which they undergo, they do not lose their
cellular character. If the explanation I have given of the
mode in which the serration is produced be correct, it will not
materially differ in principle from the elongation of the cells
into cylinders and fibres. For, in the latter case, a more
forcible expansion of the cells is likewise presumed to take
place in certain situations : the sole difference being, that in
the latter case it takes place only at one or two opposite points
of a cell, whereas with the serration it occurs at many sepa-
rate ones. At this stage of our inquiry, we are reminded of
the form of many pigment-cells, in which this expansion of the
cell, at certain spots, takes place on several sides, and in a far
higher degree, causing the cell to assume an irregular stellated
form. The prolongations of these cells, however, retain their
character as hollow processes, even when almost as minute as
the fibres of cellular (areolar) tissue.

The distinction between cell-membrane and cell- contents is
nowhere more distinctly defined than in the fully-developed
cells of this class. In the perfected cells of the pith of
feathers, for example, it is as marked as we ever find it to
be in plants. When traced backwards to their earliest stages
of development, their true cellular formation scarcely admits
of a doubt, although the cell-membrane, for reasons given at
page 36, cannot be so clearly distinguished. The elementary

INTO CONTINUOUS TISSUES. 93

cells of the tissues of the following classes, in most instances,
do not advance beyond this early stage in the development of
the feather-cells, but the changes necessary to the formation
of the subsequent tissues occur at this period j their cellular
nature is, however, quite as certain as is that of the young
feather-cells, although it be not possible to recognize their cell-
wall so clearly as in their perfectly-developed condition.

The matter contained in the cells is either a transparent
fluid, as in the cells of the pith of feathers previously to their
becoming dry, or in the crystalline lens, when it contains
albumen j or, a minutely- granulous mass, as in many epithelium-
cells, or pigment-granules ; or, it is altogether absent, and the
cell-walls, in consequence of their flattening, are in immediate
contact. The air in the cells of the pith of mature feathers
simply penetrates from without, during the process of their
desiccation. With the exception of some of the cells of the lens,
all the cells of this class are invariably furnished with a nucleus
of the characteristic form. It is not, however, a persistent
structure, as in the previous class, but in very many instances
becomes absorbed when the cells have reached maturity ; sueh
is the case in the pith of the feather, the superior laminae of
the epidermis, the nails, crystalline lens, &c. &c.

As a general rule the cells remain independent during all
these changes, that is to say, each cell retains its especial wall,
and its own peculiar closed cavity. More or less complete
Mendings of the cell-walls, and even of their cavities also,
occur, however, as exceptions even in this class. The epithelial
scales of the nail are so intimately connected together, that it
is rarely possible to trace the contour of one of them in its
entire circumference ; and the same appears to be the case
with the epithelium in the vessels of the adult. The coalescence,
however, does not appear to be perfect, for, by the employment
of concentrated acids, the scales of the nail may be separated
somewhat more readily from each other. A union of the
cavities of several cells seems to occur in the pigment-cells.
A prolongation of a cell filled with pigment may be seen to
pass uninterruptedly to the cavity of another cell (plate II,
fig. 9, a). In such an instance, probably, the prolongations of
two cell-cavities join at a certain point, the cell-walls unite
together there, and the partition-wall becomes absorbed, and

94 INDEPENDENT CELLS UNITED

thus an uninterrupted passage from one cell-cavity into the
other is produced. I am not certain as to whether a similar
process does not take place in some fibres of the crystalline
lens.

The transformations which the cells undergo are not, how-
ever, restricted to those already mentioned. A completely
opposite process occurs in the cortical substance of the shaft
of feathers, viz. a division of the cells into fibres. By this
process, out of a single cell several fibres are generated, which,
in the first instance, are united together by the rest of the
substance of the cell, but at a later period of development may
be insulated to a considerable extent. An elongation of the
cells into these fibres takes place, indeed, at the same time,
but the major portion of each fibre is formed by the division
of the bodies of the cells.

With respect to the formation of the cells of this class, we
find it to be a constant rule, that their size increases in pro-
portion with their age, a fact which Henle has already pointed
out with regard to the epithelium. We have seen in the dif-
ferent tissues, that the nucleus is first present, that the cell is
then formed around it, the nucleus, therefore, being the true
cytoblast, and that it holds the same position in these cells
that it does in those of plants, being fixed eccentrically upon the
internal surface of the wall. Cell and nucleus advance in
growth for a time, the former, however, much more vigorously
than the latter. The nucleus is generally absorbed after the
formation of the cell is completed. The generation and growth
of the cells and all the phenomena connected with the nucleus
resemble those of the vegetable cells, and we may unhesi-
tatingly draw a parallel between them. In no class is the
quantity of the cytoblastema smaller than in this. In the
immature state the walls of the cells lie close together, with
at the most, but a minimum of intercellular, substance be-
tween them at points where three cells are in contact, and it
is only between those nuclei, around which no cells have as yet
formed, that a somewhat larger quantity of cytoblastema is
present.

The class of cells now treated of, and the teeth which will
be examined in the following class, have been comprised under
the term unorganized tissues, and their growth represented as

INTO CONTINUOUS TISSUES. 95

dependent upon a secretion of the so-called matrix. If by
this it is meant that the substance of horn is secreted bv the
matrix and hardened in the air, the view is manifestly an
erroneous one ; what Ave call horny substance being either
merely the cell-walls, when, for example, the cells are flat, and
there are no cell-contents, or the cell-walls and cell-contents
together, when the cells are polyhedral, as in hoofs. All these
cells are independent structures, which grow organically. Bnt
if, by the above description, it is meant that the organized
matrix only furnishes (or secretes) the cytoblastema, no im-
portant objection can be raised. The cells of the horny tissue
require a nutritive fluid for their growth. This is supplied to
them by the blood, as it is in all tissues. As, however, the
blood-vessels themselves do not pass between the cells of the
hornv tissue, the nutritive fluid must be furnished bv the
nearest substance in which blood-vessels exist, and in this
sense the nearest organized substance may be called, matrix.
But whether this cvtoblastema which exudes from the matrix
have a specific character, and on that account horn-cells are
formed in it — or whether their formation take place in it for
the same reason that the muscle-cells, those of areolar tissue,
and so on, originate in other parts of the body, that is to say,
whether it is determined by the plan of the entire organism,
— is a question which does not as yet admit of a decision. It
is, however, a characteristic of all the cells of this class (with
the exception of the crystalline lens, which I have not examined
in reference to the point), that the new cells are not generated
between those already formed, but onlv in the cvtoblastema
nearest to the organized substance, if not, indeed, always in
immediate contact with it. The teeth were necessarily sepa-
rated from this class, because, as we shall see hereafter they
present quite a different relation of the cells. The new cells
of cartilage, so Ions: as it does not contain any vessels, are not
only formed upon the surface of the tissue, but also between
the most recently-formed cells.

The chorda dorsalis forms the transition from this class to
the following one. The cell-walls remain separate in the
highest stage of their development, and it is only in their
rudimentary forms, in the osseous fishes for example, that they

96

COALESCENCE OF THE CELL-WALLS ONLY.

coalesce and exhibit fibres between the cell-cavities. It does
not appear to possess any vessels. The formation of new cells
goes on at the extremities, for instance, at the point of the
tail of the tadpole \ it is not, however, limited to the surface,
but appears to take place between the most recently-formed
cells, for cytoblasts may be observed in the intercellular sub-
stance between the cells which have reached maturity. In
this respect the chorda dorsalis resembles cartilage, but differs
again from it, in that, as Miiller discovered, it undergoes no
change in boiling water, and also, in that, the nuclei are flat,
while those of cartilage- cells are round or elliptical.

If the chorda dorsalis be reckoned in this class, it affords,
as we have seen, an example of the generation of cells within
cells. A different signification might, however, be ascribed
to these young cells within the true cells of the chorda dorsalis,
for they do not seem to be formed like their parent-cells,
from cytoblasts. A generation of cells within cells takes
place also in the lens. In all the other tissues of this class,
with few exceptions, the formation of new cells takes place
only on the outside of those already existing.

CLASS III.

Tissues, in which the cell-walls have coalesced with each other,
or with the intercellular substance.

This class comprises the firmest structures of the animal
body, namely, cartilage, bone, and the ivory and osseous sub-
stance of the teeth. The following is the type of these tissues
in their mature state : they present either a multitude of small
roundish cavities in a firm transparent substance, or cavities,
from which canaliculi issue out in a stellate form : or asrain,
merely canaliculi dispersed through the tissue with tolerable
regularity. The cavities do not communicate immediately
with each other ; the canaliculi, however, often unite together.
A special cell -membrane cannot be distinguished in any of
them in the mature condition, but in an earlier stage the
cavities may be proved to be cells, that is, hollow spaces en-

THE TEETH. 97

closed by a peculiar membrane, and the canaliculi are then also
seen to be hollow prolongations of cells. The intermediate
substance between the cavities is produced in one of the fol-
lowing wavs : either the walls of the cells become thickened,
and then coalesce to form an homogeneous substance, or, which
is much the more frequent mode, the intercellular substance
is developed in greater quantity, and a coalescence takes place
between it and the unthickened or only slightly thickened
cell-walls. I cannot positively assert that a blending of the un-
thickened cell-walls with the intercellular substance takes place
universally : I cannot do so, for instance, with respect to the
cartilages of the higher animals, and so far the mere coalescence
of the cell-walls is not a certain characteristic of this class
of tissues. Should it be found not to prevail universally we
must look for a distinctive character in the abundant develop-
ment of a firm intercellular substance — a peculiarity which
is presented by no other class.

1. Cartilage and Bone. As these tissues have been already
treated of (pp. 15-33), the reader is referred to that part of the
work.

2. The Teeth. The teeth were formerly classed with the
bones, but have of late been treated of as non-vascular struc-
tures under the head of horny tissues. Since Miescher's
discovery, however^ that the vessels of bone also traverse
only the medullary canaliculi, since Miiller observed that
the teeth, like the bones, afford gelatine by boiling, and
Retzius discovered osseous corpuscles in the ivory, it seems
more correct to class the teeth with the bones again, and
the more so, as we now know that the presence or absence
of vessels proves no essential difference in the growth. The
coalescence of the cell-walls which appears to take place in the
ivory of the teeth forms an additional reason for our classing
them with bone. The teeth, as is well known, consist of ivory,
osseous substance, and enamel.

98

ENAMEL OF THE TEETH.

a. The enamel.

The enamel consists, according to Purkinje, of square, or,
according to Retzius, of hexagonal closely-aggregated prisms,
which stand nearly perpendicular upon the surface of the ivory,
and pass outwards in a slightly curved direction. It is at first
soft, and if some of it be scratched off in that state, we obtain,
what Muller has described as needle-shaped bodies pointed at both
extremities. According to Purkinje, Raschkow, and Retzius,
some organic substance remains after the young enamel has
been treated with hydrochloric acid, whilst Berzelius asserts
that the enamel of mature teeth does not contain two per
cent, of organic matter. For further details I refer the reader
to the excellent works of Purkinje, Raschkow and Frank el,
and those of Retzius, J. Muller, and v. Linderer

If an immature tooth of a child or mammal (the pig, for
instance) be removed from its capsule and placed in dilute
hydrochloric acid, the organic substance of the enamel which
remains after the solution of the earthy matter, may be sej^a-
rated from the ivory entire. It has exactly the form and size
of the enamel previous to the action of the acid. It is very
soft, and breaks readily in the direction of the fibres of the
enamel. Examined with a high magnifying power and subdued
light, it is found to be composed, like the enamel itself, of
closely-aggregated prisms, which may be insulated from one
another, so that each one forms an independent structure.
(See pi. Ill, fig. 3.) This organic substance, therefore, cannot
be, as Raschkow and Retzius considered, a mere deposit from
the moisture with which the enamel-fibres are at first sur-
rounded, and thus a sort of cast of the enamel-fibres, but
either the fibres must result from an ossification of these
prisms, or the prisms must be hollow, and the inorganic
substance deposited within them. When the enamel of the
pig's tooth is examined with a subdued light, the contour of
tjhese organic prisms is found to be so dark in comparison with
their interior, that it can scarcely be regarded as the mere
shaded outline of a solid prism, but suggests the idea of a
cavity surrounded by a thin membrane. This distinction is,

ENAMEL OF THE TEETH. 99

however, much less striking in human teeth, so that the que -

tion as to which of the two views is correct must remain un-
decided.

What, tlieu, is the process of formation of these enamel-
prisms ? According to Purkinje and Raschkow, the crown of the
growing tooth is surrounded externally by a peculiar membrane,
the enamel-membrane, the inner surface of which is composed
of short hexagonal fibres, which stand perpendicularly upon the
membrane, and are directed towards the enamel, so that each
fibre of the enamel-membrane corresponds to an enamel-fibre.
On examining a portion of this membrane, particularly that
part which lies nearest to the root of the tooth, we readily
recognize in it the characteristic nuclei, some of them being
furnished with nucleoli. They lie in a minutely granulous
substance. This granulous aspect, however, is seen to be pro-
duced, in many situations, by granulated cells which contain the
nuclei. Each nucleus is surrounded by a circular areola of
small granules, and seems to lie in a minutely granulated
globule, which we know to be the rudimentary form of
most elementary cells. Some of these cells are prolonged
into very delicate fibres; they appear to be young cells of
areolar tissue; most of them, however, are round. The
fibres or prisms of the membrane, which have a direction
from its inner surface towards the enamel-fibres, have
assumed an hexagonal form, which Raschkow attributes to
their close contact. They very closely resemble the columnar
epithelium upon mucous membranes, only that they arc
prismatic in their entire length, that is, so far as they project
out from the membrane to which they are attached. I am
inclined therefore, to regard them as merely elongated cells.
In the recent state they also contain a very distinct nucleus,
which encloses its nucleolus. (See pi. Ill, fig. 4.) In the upper
part of the enamel-membrane they lie quite close together ;
but in the portion nearest to the root of the tooth, they
diminish in number and stand insulated, so that at this
part the structure of the membrane beneath them may
also be recognized, and I suppose the round cells before
mentioned to be the earlier condition of these prismatic
cells. What, then, is the relation which these prismatic cells
of the enamel-membrane bear to the prisms of the enamel?

100 ENAMEL OF THE TEETH.

Purkinje and Raschkow regarded each fibre of the enamel-
membrane as an excretory organ, a little gland which secreted
the enamel-fibre corresponding to it. With our altered views of
the growth of unorganized l tissues, however, this explanation,
previously so plausible, loses much of its probability. Various
other explanations might be offered in place of it, but I have
not made sufficient observations to enable me to decide upon
the correct one. Firstly, one might suppose the organic basis
of the enamel-prisms to be cells, which are formed, and continue
to grow independently upon the dental substance, having no
other connexion with the prisms of the enamel-membrane than
that the latter furnishes their cytoblastema. This explanation,
however, would compel us to regard the remarkable accordance
which exists between the prisms of the enamel-membrane and
those of the enamel as an accidental circumstance. But we
should be obliged to adopt such a view, if it could be proved
that another peculiar substance intervened between the enamel-
membrane and the enamel, and I have several times observed
such an one on the molar teeth of swine. It is very soft and
full of vesicles, having the appearance of a slag. I think
Purkinje mentions it also, but I cannot find the precise passage
at this moment. It lay between the enamel-membrane and
the tooth, but I am not certain whether it was also present
at those points where the formation of the enamel had already
commenced, and whether, therefore, it actually interrupted the
continuity of the enamel-membrane with the formed enamel.
We might suppose, as a second explanation, that the enamel-
prisms are uninterrupted continuations of the prisms of the
enamel-membrane, which become filled towards one end with
calcareous salts. This is a very improbable explanation, and
the connexion between the two structures is of too loose a
nature to warrant its adoption. A third, and as I am at pre-
sent disposed to think the most probable, explanation is, that
the prismatic cells of the enamel-membrane separate from it,
and coalesce with the enamel already formed, while at the same
time their cavities either become filled with calcareous salts,
or they become ossified throughout their entire thickness, their
cavity being previously filled with an organic substance. This
explanation makes the formation of the enamel accord with

1 [The author appears to use this word- as synonymous for " non-vascular." — Trans.]

IVORY OF THE TEETH. 101

the growth of the other unorganized tissues treated of in the
previous class. If we suppose, for example, that the little
cylinders (columnar epithelium) of the mucous membranes
(which, according to Henle, are constantly being thrown off)
could become ossified at the moment when they separated from
the surface of the mucous membrane, we should obtain a cover-
ing to the membrane, consisting of little calcareous cylinders,
each of which, however, would still have its organic basis like
the enamel-fibres. Beneath this covering would be other
cylinders not as yet ossified, which, when they in like manner
became calcified would add to its thickness, whilst new cvlinders
grew forth from the mucous membrane. The quantity of the
organic basis is extremely small in the teeth of adults which
have been exposed for a considerable time to the action of the
saliva, a circumstance which I suppose to be referrible to its
undergoing a chemical solution in that fluid.

b. The ivory.

This is known to consist of a structureless 1 substance,
traversed by a great many minute canals. These canals
(tubes) have for the most part a radiate course from the
cavity of the tooth towards its external surface, and, accord-
ing to Retzius, often give off branches as they proceed.
Their peripheral terminations are extremely minute ; they are
thicker towards the dental cavity, and, when the pulp is removed,
open freely into it. Midler observed that the tubes projected
beyond the intermediate substance from the fractured surface
of thinly-ground lamina?, and of lamella? which had been
macerated in hvdrochloric acid, and were surrounded therefore

«/ J

by a special membrane ; Retzius also remarked the same
upon a transverse section. Purkinje and Midler noticed that
when teeth are placed in ink, the fluid penetrates into the
tubes ; they must therefore be hollow. If any of them contain
calcareous matter, it must be only the most minute ones.
According to Retzius, many teeth present corpuscles which
resemble those of bone, and like them send forth minute
radiating canaliculi.

1 See note to page .'>!•.

102 IVORY OF THE TEETH.

What relation then does the ivory bear to the cells ? I
must at once avow that I cannot give a positive reply to
this question, and that T only communicate the following im-
perfect investigation for the sake of presenting a connected
view of my subject. The formation of the dental substance
is described by Purkinje and Raschkow as follows : " Primordio
substantia dentalis e fibris multifariam curvatis convexis late-
ribus sese contingentibus ibique inter se concrescentibus com-

posita apparet In ipso apice istas fibras pequaliter quam-

cunqne regionem versus se diffundunt, attamen parietes laterales
versus directio Ion gitudinalis prasvalet, dum fibras sinuosis flexibus
a^qualique modo se invicem contingentes ibique ubi concavas
apparent lacunas inter se relinquentes, ab apice coronali radicem
versus ubicunque procedunt. Non nisi extremi earum fines
tunc molles sunt ceteras autem partes brevissimo tempore in-

durescunt Substantias dentalis formationis secundum

crassitudinem processus pari modo ac primo ejus ortu cogi-

tandus est. Postquam fibrarum dentalium stratum

depositum est, idem processus continuo ab externa regione
internam versus progreditur, germinis dentalis parenchymate
materiam suppeditante. .... Convexas fibrarum dentalium
flexuras, quae juxta latitudinis dimensionem crescunt, dum ab
externa regione internam versus procedunt, sibi invicem appo-
sitas continuos canaliculos effingunt, qui ad substantias dentalis
peripheriam exorsi multis parvis anfractibus ad pulpam dentalem
cavumque ipsius tendunt, ibique aperti finiuntur, novis ibi,
quamdiu substantias dentalis formatio durat, fibris dentalibus
aggregandis inservientes." (Raschkow, Meletemata circa Mam-
malium dentium evohdionem. Vratislav. 1835, p. 6.)

I must admit that I do not clearly understand some of this
description, but if I rightly comprehend it, the dental substance
originates from fibres which are formed in strata around the
pulp (the latter supplying the material for the purpose) ;
that these fibres then coalesce, leaving, however, spaces be-
tween them which are the dental tubes. Since, according
to Muller, the tubes are furnished with special walls, we can
no longer regard them as mere spaces between the fibres.
His observation, however, does not affect the explanation of
the formation of the firm substance.

IVORY OF THE TEETH. 103

If a tootli be removed from its capsule, and macerated
for some days in Bliffhtly diluted hvdroelUoric acid, the dental
substance, which on the first withdrawal of the calcareous salts
possessed a cartilaginous consistence, becomes so very soft that
it can only be removed from the acid in very small portions
with the forceps. This pappy mass is found on examination
to consist of fibres, which may here and there be insulated.
(See pi. Ill, fig. 5.) These fibres arc too thick to be the
walls of the tubes ; thev form the entire substance. Nor
can they well be an artificial product, the result of the acid
penetrating into the tubes, and dissolving, in the first instance,
the substance in immediate contact with them, so that the
intercellular substance remained undissolved in the form of a
fibre ; they are too regular and smooth for that. It appears
rather that the dental substance is composed of these fibres,
which have become blended together, that they are therefore
identical with those fibres, by the coalescence of which, accor-
ding to Purkinje and Raschkow, the dental cartilage is formed,
and that this coalescence is not so complete, but that it may
be artificially dissolved. The fibres have the same course as
the tubes in human teeth, but I could no longer perceive the
tubes between them in this preparation ; I could, however,
recognize the fibres everywhere, save in the most external layers
which lav immediatelv under the enamel, in which situation
the mass was more completely broken down by the acid, and
traversed by more minute fibres of a different kind, having
the most confused and varied directions, and which I suppose
to have been the remains of the dental tubes.

We must therefore regard the dental substance as composed
of fibres blended together, between which run tubes provided
with special walls. The fibres and tubes are nearly per-
pendicular to the dental cavity in human teeth. "What con-
nexion now is there between the fibres, or the tubes, and cells ?
I should incline to the old opinion, that the dental substance
is the ossified pulp. According to Purkinje and Paschkow,
the pulp in the first instance consists of globules, of nearly
uniform appearance, but has neither vessels nor nerves. At a
subsequent period vessels appear in it, and afterwards nerves.
Upon the surface of the pulp, the globules arc more regularly
arranged, and more extended in the longitudinal direction,

104 IVORY OF THE TEETH.

and are directed towards the outside perpendicularly, or at a
slightly acute angle.. These elongated globules are clearly
cylindrical cells. In recent teeth, the characteristic nucleus
with its nucleoli may be distinctly seen in them, and
they very closely resemble the prisms of the enamel-membrane.
(PI. Ill, fig. 4.) The interior of the pulp also consists of
round nucleated cells, bety,reen which the vessels and nerves
pass. When the pulp of a young tooth is detached from its
cavity, and the dental substance is examined (without further
preparation, or after the earthy matter has been withdrawn),
a stratum of the cylindrical cells of the pulp will be found
to remain attached to its internal surface, at least to the
lower part of it, where the newly -formed dental substance
is vet thin and soft. These cells are of about the same
size, and have the same course as the solid fibres of the
dental substance ; and since, on the one hand, they clearly
belong to the pulp, which follows from their accordance with
the cylindrical cells that remain attached to the rest of its
surface, and as, on the other hand, they are still more firmly
connected with the dental substance than with the pulp, and
remain affixed to the former, I suppose a transition to take
place at that part, and the cylindrical cells of the pulp to be
merely the earlier stage of the dental fibres, i. e. that the cells
become filled with organic substance, solid and ossified. In
some instances, these little cylinders are not found upon
the dental substance, but a quantity of cell-nuclei seen
in their place ; these are very pale, and so intimately united
with the dental substance, that they readily escape notice; when,
however, attention is once attracted to them, it is impossible to
mistake them, and they lie side by side with extremely
small interspaces. The facility with which the two struc-
tures may be separated, has been adduced as an argument
against the opinion that the dental substance is the ossified pulp,
and I fully acknowledge the weight of the objection. But the
following circumstances deprive it of at least some of its import-
ance. Firstly, some portion of the pulp actually remains
attached to the dental substance ; again, in ribs which are half
ossified, the cartilage may easily be separated from the ossified
portion; and lastly, the separation must be effected with
more facility in the tooth, in consequence of the greater

IVORY OF THE TEETH. 105

difference in consistence between the dental substance and the
pulp. There are therefore, at least, reasons enough to war-
rant our entering more particularly into the details of this
opinion. The pulp accords with all the other tissues of
the foetus, therefore with cartilage, in being composed of
cells : the difference between its consistence and that of the
cartilage of mammalia, depends on this, that the quantity of
cytoblastema (to which the latter owes its hardness) is very small,
for the cylindrical cells of the pulp lie quite close together,
at least such is the case on its surface. In this respect, the
pulp bears a closer analogy to certain cartilages of animals
lower in the scale, in which there is also only a small quantity
of cytoblastema present, and the consistence of the cartilage is
principally occasioned by thickening of the cell-walls. As I
have not actually observed the transition, I do not know
whether the filling up of the cavities also takes place by thick-
ening of the cell- walls, in this supposed conversion of the cells
of the pulp into the dental fibres. If such be really the case,
the cavities of the cells are in general so completely obliterated
by it, that no cartilage-corpuscles remain. From the observa-
tions of Ketzius, however, it might be supposed that some of
the cells retain their cavities, and even become transformed
into stellated cells ; for he saw true osseous corpuscles in the
dental substance. When the uppermost stratum of the pulp
consisting: of cylindrical cells has become converted into dental
substance by ossification, the round cells lying immediately
next beneath it in the parenchyma of the pulp, must first com-
mence their transformation into cylindrical cells, the vessels of
the stratum must become obliterated, and then this stratum
ossified, and so on.

"What, then, are the dental tubes ? Retzius compares them
to the calcigerous canaliculi of bone which issue from the
osseous corpuscles, and I was myself at first of that opinion ;
for I regarded them as prolongations of cells, the bodies of
which lay in the pulp. For, when the pulp is drawn out
from the cavity of a pig's tooth, and its margins examined,
it will be seen that each of the cylindrical cells of the surface

w

of the pulp becomes elongated into a short minute fibre towards
the dental substance, and that these fibres are about as nume-
rous as the tubes projecting upon the surface of the pulp. 1

106 OSSEOUS SUBSTANCE OF TEETH.

thought formerly, that thev became elongated to form the dental
tubes, and that the intertubular substance was merely intercellu-
lar substance between these prolongations. But I was compelled
to relinquish that idea in consequence of there being no such
appearance in the human tooth, and because the explanation
led to difficulties with respect to the teeth of the pike, in
which, according to Retzius, an immediate transition of the
dental into the osseous substance takes place. If one of the
largest teeth in the lower jaw of the pike be sawn off, deprived
of its earthy matter by means of hydrochloric acid, and
then divided into thin longitudinal sections, the dental sub-
stance will be seen to form a hollow cone, which is filled with
osseous substance. The dental substance is transparent, and con-
sists of fibres which have a direction from the point towards the
base of the cone. Canals traverse the osseous substance,
resembling the Haversian medullary canals of ordinary bone,
only they are not so regular. The dental tubes then are con-
nected with these canals of the proper osseous substance, and
may be distinctly seen issuing from them in a funnel-shaped
form. The canaliculi soon ramify in the dental substance,
and, as they run across the thickness of the dental cone, inter-
lace with the dental fibres. According to this view, the
dental tubes would correspond to the medullary or Haversian
canaliculi of bone, and not to the calcigerous canaliculi pro-
ceeding from the osseous corpuscles. It appears impossible,
however, to be assured of the right explanation of all the struc-
tural relations of the dental substance, until its development is
examined in teeth differing widely from each other in con-
struction.

c. Osseous substance of the teeth.

This requires no particular explanation, as it entirely ac-
cords with the ordinary osseous substance.

Having examined in detail the tissues comprehended in this
class, and compared them one with another, we have yet to
consider the entire class in relation to those which have been
previously discussed, and to observe how much our knowledge
of the transformations which the cells are capable of under-
going, has been advanced bv the study of it.

COMPARATIVE RETROSPECT. 107

It is easy to see -which elements of the tissues of this and
the preceding class correspond. There the whole tissue
consisted of cells, closely crowded together, and the in-
tercellular substance was almost nil. Here we find the like
arrangement only in the lowest stage of development of the
most simple cartilages. In such as are more highly developed,
those of all the mammalia for example, the cells lie surrounded
by a larger quantity of intercellular substance, which fun us
the proper cartilaginous substance ; but the cell-walls contri-
bute onlv verv slightly, or not at all, to its formation. The
proper firm substance of these higher cartilages, therefore, has
its analogy in the former class, only in the minimum of cyto-
blastema by which the cells are connected, while, on the other
hand, it corresponds with that which, in the first class, was
the fluid, wherein the isolated cells were formed. The carti-
lage-cells in this class, however, correspond precisely to the
epithelium-cells, the feather-cells, &c. &c, in the preceding
one, and the blood-corpuscles, mucus-corpuscles, &c. in the
first class.

We have not found any new changes in the form of the
cells in this class. Most of them were angular, somewhat
approaching the circular form ; and stellated cells, so far at
least as we may be permitted to regard the osseous corpuscles
as such, were also frequently met with. (See pp. 29, 30.)
Some cells, which were remarkably elongated, were observed
near to the surface of several cartilages, in which situation
they are known as greatly elongated cartilage- corpuscles ;
still, however, this appearance is never presented by the cells
of this class in so remarkable a degree as it is by those
of the crystalline lens in the previous one. The fibro-
cartilages, on the other hand, form the immediate transition
from this to the following class, for in them a bundle of fibres
seems to be formed out of each cartilage-corpuscle, a process
which we shall consider more minutely when treating of cel-
lular (areolar) tissue in the next class.

AVc have observed the formation of cells around the pre-
viously-existing nucleus, and their progressive growth, goin Li-
on in this class in a similar manner to that exhibited in the
preceding, and the true cartilage-cells were also seen to form
around a cvtoblast which lav external to the cells already

108 COMPARATIVE RETROSPECT.

developed. On the other hand, a formation of cells also takes
place within the true cartilage-cells, but it is probable that
they have a different signification from those within which they
are generated. A deviation from the previous class seemed to
occur with respect to the spot at which the young cells are
formed, in relation to the entire tissue. In the former class,
so far as we could perceive, they were formed at that part
only where the tissue was in immediate contact with the
organized substance. The formation of the new cells in
cartilage, it is true, did not take place throughout the entire
thickness of the tissue, but (so long at least as the cartilage
itself is not furnished with vessels) only near the sur-
face, and therefore, at the spot where it was in contact with
the organized substance ; still, however, it not only took
place at that point of contact, but went on also between
the cells most recently formed, as if cartilage had a greater
capacity of imbibition, so that the cytoblastema penetrating
from the blood-vessels into the parenchyma arrived at the
deeper seated portions of the tissue more speedily; and, there-
fore, retained its fresh plastic force even in that situation;
or, as if the cartilage itself possessed a higher vitality, and,
therefore, the cytoblastema retained its productive power for
a longer period, although penetrating quite as slowly as in
the previous class.

Although the modifications in the form of the cells of this
class vary but slightly from those of the preceding one, yet we
see two striking changes in the cells and their cytoblastema,
namely, the coalescence of the cell-walls and ossification. The
thickening and transformation of the cell-walls were very dis-
tinct in the last class, for example, in feathers. Here a still
more strongly- marked thickening of the cell-walls takes place
in several cartilage- cells. The external contours of the walls,
however, gradually disappear in such instances, and a coales-
cence takes place to such an extent as to leave merely the
cell-cavities perceptible, lying in an homogeneous substance.
The blending of the cell-walls takes place either between the
walls of neighbouring cells, in instances where they are in
immediate contact, or, with the intercellular substance, when
the cells are surrounded by it. Further investigations are re-
quired in order to decide the question as to whether this

COMPARATIVE RETROSPECT. 100

blending be so complete that it cannot in any way be dissolved,
the simple fact being, that the cell-walls arc no longer dis-
cernible with the microscope. I shall not bring forward the
splitting of the dental fibres as examples here, nor indeed
make any reference to the teeth in this retrospect, their ex-
planation being as yet too problematical. It has, however,
been already mentioned as a doubtful point, whether a coales-
cence of the walls actually takes place in all cartilage-cells,
for instance, in those of the higher animals.

Ossification appears to occur especially, perhaps exclusivclv,
in those cartilages which have a greater quantity of intercel-
lular substance. It consists probably in a chemical union
between the calcareous salts and the firm portion of the car-
tilage substance. In the first commencement of the process
the cartilage frequently acquires a granulous appearance, which
subsequently disappears, the entire substance meanwhile be-
coming gradually dark. At the same time the cartilage-cells
undergo a transformation into the osseous corpuscles, a process
which must probably be explained as analogous to the for-
mation of the stellated pigment-cells. There is reason to
suppose that the osseous corpuscles and the canaliculi which
issue from them, also become filled with earthy salts by the
calcifying process.

The class of cells now under consideration has yet another
point of especial interest for us, since it is the first in which
organized structures, that is, structures provided with vessels,
occur. The accordance between the elementarv cells of

m

unorganized animal tissues and vegetable cells might be con-
ceded, without granting a connexion between the organized
tissues (which are especially characteristic of animals) and
the structure of vegetables. A distinction had alwavs been
drawn between the growth of the organized and that of the
unorganized structures ; and much had alreadv been said
in a general way about a vegetative growth of the non-vascular
structures, the crystalline lens for instance, though the analogy
which existed between their elementary particles was not
proved. Cartilage, then, is the first structure which teaches us
that a tissue, which, at a later period at least, contains vessels,
is composed of cells, perfectly according, in their development,
with those of plants ; and, therefore, that a similar formative

HO FIBRE-CELLS, ETC.

principle is the basis both of the organized and unorganized
tissues. We shall have further evidence of this presented to
us in the following classes, which comprise the rest of the
tissues, — those, indeed, which are most perfectly organized,
and the most important to the animal organism. In them
we shall also find that the formation of cells is the general
principle of development, and that their elementary particles
are derived from cells, although at the first glance one would
scarcely imagine that any connexion could exist between them
and cells.

CLASS IV.

Fibre-cells, or tissues, which originate from cells that become

elongated into bundles of fibres.

Mere fibres are all that can be detected as the elementary
components of the tissues of this class when they are examined
in the mature animal. But when we investigate the mode in
which they are generated, we see that the fibres are formed
only as prolongations of cells, which, in most instances, are
elongated in two opposite directions, sometimes terminating
at once in a fasciculus of fibres, at other times in a single fibre,
which afterwards splits into several finer ones. This con-
stitutes the characteristic feature of the class. We are already
acquainted with the type of the prolongation of cells into fibres
in the pigment -ramifications, osseous corpuscles, &c. The
cells next to be considered differ from them in the following
particulars : the fibres originating from any one cell generally
lie together in a fasciculus, and in these prolongations of the
cells, it is principally the wall which is most strongly developed,
whilst, in the former instances, the cells though extended into
fibres, were chiefly rendered conspicuous by their cavities.
This class comprises the Cellular (areolar), Fibrous, and Elastic
tissues.

1. Cellular [areolar) Tissue. This tissue is known to be
composed of extremely minute, tough, smooth fibres, having
a pale outline, and usually a serpentine course ; they
may be seen in their natural state in the mesentery with-

AREOLAR TISSUE. HI

out any dissection. Most areolar tissue may be distended bv
forcing air into it, and then innumerable cellular spaces arc
seen communicating with each other in it j it is not known
whether these are produced artificially, or whether they existed
previously. Areolar tissue also frequently contains fat- vesicles,
which, according to Gurlt, arc surrounded by a thin and
transparent, but not fibrous, pellicle, often have an hexagonal
form, and in that respect resemble vegetable tissue. (Gurlt's
Physiologie dcr Haussaugethiere, p. 19.) In order to become
acquainted with the relation which these constituent parts of
areolar tissue bear to the elementary cells, we must refer to
the formation of the tissue in the foetus.

If we examine some areolar tissue from the neck, or from
the bottom of the orbit of a foetal pig measuring three inches
and a half in length, we shall find it to be a gelatinous substance,
somewhat more consistent than the vitreous humour of the eve
and, in its earliest state, quite as transparent ; as development
proceeds, however, it becomes more of a whitish colour, and
loses its gelatinous quality. When examined with the micro-
scope, small corpuscles of various kinds are seen in greater
or less numbers ; they are not, however, sufficiently numerous
in a foetus of the size specified to form the entire gelatinous
substance, but must necessarily be situated in a transparent,
structureless,1 primordial substance of a gelatinous nature, which
we will for the present call cytoblastema. The whiter this
substance appears to the unaided eye, the greater is the
number of corpuscles contained in it ; their quantity, there-
fore, is continually increasing during development, while
that of the cytoblastema constantly diminishes. As in con-
sequence of its transparency, the cytoblastema cannot be
seen, but is only inferred to exist from the circumstance
that the corpuscles, which are visible under the microscope,
could not, at the period when they are but few, form the
entire jelly, and that when moved, it is plainly seen that
they are held together by some invisible medium, so it is no
longer possible to convince ourselves of its existence, when the
corpuscles are very numerous. It is probable, however, that it
remains between the fibres of the areolar tissue throughout life.

1 Vide note at page 39.

112 AREOLAR TISSUE.

This cytoblastema is present in the greatest quantity, and
therefore most distinctly demonstrable in the jelly which lies
between the chorion and amnion in the foetus of the pig at a
somewhat more advanced period, and where it may be rendered
very clearly perceptible on the margin of the preparation by
colouring it with iodine. It is quite as evident in the cellular
tissue of the young tadpole. An indistinct fibrous appearance
is sometimes given to it by drawing it asunder ; but a fibrous
structure must not be inferred from that fact simply, since all
tenacious matter assumes that appearance under similar cir-
cumstances. Since the number of the corpuscles in the cyto-
blastema continually increases as development proceeds, it
•would appear that the cytoblastema must be regarded as the
primary formation, so that wTe may suppose some of it to be
first present, and then the corpuscles originate in it ; at the
same time, however, new cytoblastema is formed, in which new
corpuscles are in like manner generated, whilst the formation
of those in the previously-existing cytoblastema proceeds
simultaneously.

Three kinds of these corpuscles may be distinguished in the
mammalian embryo ; one, which is developed at an earlier
period than the rest, and is found in all the areolar tissue
throughout the foetus, and two others, which are formed
subsequently, and, as it would seem, do not occur in the areolar
tissue of some parts. We shall, therefore, designate the first
(which is the only essential kind) proper corpuscles of areolar
tissue, or — in accordance with the signification which will
shortly be determined for them — fibre- cells of areolar tissue ;
the second kind are fat-cells ; the third form round cells of
areolar tissue, the precise signification of which I have not yet
been able to make out.

a. Proper corpuscles of areolar tissue, or fibre-cells of areolar
tissue. The areolar tissue is not found in the same stage of
development in every part of the same foetus. When some of
the tissue that has reached about its middle stage of develop-
ment is removed from the neck of a pig's foetus, measuring
from four to seven inches in length, and examined with the
microscope, a quantity of corpuscles of various forms are ob-
served in it. The majority of them, however, appear as they

AREOLAR TISSUE. l;

are represented in pi. Ill, fig. 6, a, being spindle-shaped or

longish corpuscles, which arc thickest in the middle and gra-
dually elongated at both extremities into minute fibres. They
may therefore be described as consisting of a thicker portion,
or body, and fibres, which proceed from it.

The body is either round or slightly compressed upon the
sides. The surface is covered with very minute granuh .
Within the thickest portion of it lies another small corpuscle
of a circular or generally oval form, and which again eneloses
one or two small dark points, and accords entirely with the
common cell-nucleus. It is therefore probable, that the entire
corpuscle is a cell containing a nucleus. The nuclei have not
a similar size in all the cells ; there is a much more striking
variation, however, in the relative size of the cells and the
nuclei. In the largest cells, such as a, fig. G, the body is
almost as thick again as the nucleus, and it may be observed
that the nucleus does not lie in the centre, but upon the wall
of the cell. In most instances, however, the cells are rela-
tively smaller, scarcely larger indeed than the nucleus ;
insomuch, that the fibres often appear to proceed immediately
from the nucleus, as at b in the figure : the cell in
that instance encompasses the nucleus quite closely. Cells
frequently become separated during the process of preparation
for the microscope, and float about singly in the water, with a
portion of the fibres issuing from them. By causing them to
roll, when so detached, it maybe satisfactorily seen that many
of them are somewhat flattened laterally, and that the nucleus
is attached to the inside of the cell-wall. The larger cells,
under such circumstances, appear as though the granulous
aspect were produced by the external Avail only, therefore by
the cell-membrane, the interior being filled with a clear fluid.

The cells pass by a gradual process of acumination into the
fibres, it being quite impossible to discern any defined boun-
dary between them. The fibres are pale, minutely granulous
like the cells, and frequently give off branches. Their course
is usually straight. It is difficult to find out how they terminate ;
but they are generally lost in a bundle of extremelj minute
fibres.

The above-described corpuscles, then, are the fibre-cells of
areolar tissue in the middle stage of their development, a con-

S

, 14 AREOLAR TISSUE.

ditioii in which they immediately attract attention in the
investigation of that tissue in the foetus. We shall in the next
place consider the earlier, and then the subsequent stages of
their development. In addition to the corpuscles before men-
tioned, others may be seen in very young areolar tissue, which
are not elongated into fibres, but are more or less round. They
are granuloasand contain a nucleus with nucleoli, and as they
present all the stages of transition up to those cells which are
prolonged into fibres, we must regard them as being the un-
developed fibre-cells. Various forms of them are delineated
in pi. Ill, fig. 6. I will not assert that all round cells in
foetal areolar tissue are young fibre-cells; for we shall
presently become acquainted with some which are not. It
is only after the commencement of the process of acumina-
tion that the young fibre-cells can be distinguished from these;
in the earliest state, when they are as yet quite round, almost
all cells are alike. It is difficult to determine positively
whether or not these cells are formed around a previously
existing nucleus ; probably, however, such is the case, as there
are no cells to be seen without nuclei, although there are many
nuclei observed without investing cells.

The following, then, are the results of our investigation into
the progress of development of areolar tissue, in so far as
we have as yet pursued it. In the first place, small round
cells are formed (probably around a previously existing
nucleus), in the structureless jelly-like cytoblastema of the
tissue. The cells, furnished with the characteristic nucleus,
become acuminated in two opposite directions, and these acumi-
nations elongate into fibres, that sometimes give off branches,
and at length split into fasciculi of extremely minute fibres,
which, in the early stage, cannot be distinctly perceived to be
insulated. As development proceeds, the splitting of the two prin-
cipal fibres, issuing from the body of the cell into a bundle of
more minute fibres, continually advances nearer towards the cell,
so that, at a later period, a fasciculus of fibres issues immediately
from the body. of the cell (see pi. Ill, fig. 7.) At a subsequent
period, this process of splitting reaches as far as the nucleus,
and at length goes quite through the body of the cell,
and the nucleus then lies merely upon a fasciculus of
fibres. At the same time the fibres in the progress of de-

AREOLAR TISSUE. 115

velopment are rendered smooth, become distinctly and indivi-
dually discernible, and assume their waving course; in short,
they acquire the appearance of the ordinary fibres of areolar
tissue. (See the figure.) As the process of splitting advances
from both sides towards the nucleus, the fibres in its neigh-
bourhood are those which arc longest united together, and
that part of the cell is the last to undergo division. The
nucleus remains for a time lying upon the fasciculus of
fibres ; and when it is at last absorbed, we have a bundle of
fibres in the place of the original cell. The figure repre-
sents a nucleated cell, which is elongated at the upper end
into the characteristic fibres of areolar tissue, each one being
individually perceptible ; the upper part of the body of this
cell has also begun to split into fibres. With regard to
the elongation downwards, it is not possible to distinguish
whether there are separate fibres yet formed, and collected into
a cord, or whether it is still merely a simple prolongation of
the cell.

It now becomes a question how the elongation of the
cells into fibres, and their division, and at a later period
the splitting of the body of the cell also into more minute
fibres, can be conceived to take place. "We have already
observed a prolongation of the cells into fibres in several in-
stances, and have traced it minutely in the stellated pigment-
cells. The onlv difference between them and the fibre-cells of
areolar tissue is, that in the latter, the elongation generally
takes place in two opposite directions only, a circumstance
which also frequently occurs with pigment-cells j whilst, on the
other hand, the cells of areolar tissue also frequently become
elongated into fibres on several sides ; see, for example, pi. Ill,
fig. 8. There is often a striking resemblance in form between
some of the cells of areolar tissue and those of pigment; com-
pare, for instance, pi. Ill, fig. 6 a, with pi. II, fig. Se. Analogy
would lead us to regard those fibres as hollow; but since the
cell-contents are not so characteristic in them as they are in
the pigment-cells, a cavity might really exist, but not fall
under observation, in consequence of the minuteness of the
fibre ; the appearance of the fibres, therefore, proves nothing,
either in favour of or against their hollowness. Since, however,
we are already acquainted with many extremely minute hollow

HC AREOLAR TISSUE.

prolongations of cells, and as the transformation of trie cells
into fibres in the areolar tissue, takes place by a gradual
acumi nation, it seems to me, for the present, more probable
that they are hollow rather than solid. If we imagine the
formation of the fibres from a cell to take place by the
cell-wall growing more vigorously at two opposite limited
spots than it does at any other part, we can then conceive
that the division of these main fibres into branches, and their
prolongation into fibrils, may be effected by the same process.
The question as to the hollo wness or solidity of these fibrils, is
still less capable of being settled by observation than that
with respect to the larger fibres. Analogy is in favour of
their being hollow, and the minuteness of an object forms
no limit to nature's operations.

The splitting into fibres, which, as we have seen, pursues a
retrograde course from the branches towards the main fibres,
and thence towards the body of the cell, might be illus-
trated in the following manner : — suppose that part of a
glove which corresponds to the hand to be the body of a cell,
and the fingers to be a fasciculus of fibrils. If the membrane
situate in the angle between two fingers grow in the
direction of the hand, the glove will at length be split into
five portions. But a difficulty arises with respect to the
fibre-cells of areolar tissue, which is, that the division into
fibres advances from two opposite sides towards the body of the
cell, and, therefore, the fibres of one side must ultimately cor-
respond with those of the other. This, however, admits of no
further explanation than the healing of the corresponding pri-
mitive fibres in the reproduction of nerves does. Meanwhile
the above arc only attempts to convey a clear idea of the
results of my investigations, modes of representation which are
susceptible of various modifications, provided they be not made
to contradict the observations ; the latter may be briefly summed
up as follows : — cells, furnished with the characteristic nucleus,
are present in the first instance, which become elongated on
two opposite sides, more rarely on several sides, into fibres, and
these are prolonged into more minute fibres. At a later period
the principal -fibres, and then also the bodies of the cells are
split into fibres, so that a small fasciculus of fibrils, with a
nucleus fixed upon it, remains in the place of the original single

AREOLAR TISSUE. 117

cell. Last of all, the nucleus also disappears, and fibrils alone
remain. All these transformations proceed in a homogeneous
cytoblastemaj which probably also continues to exist between
the fibres of areolar tissue in the adult.

b. Adipose cells. In the later periods of foetal existence,
adipose cells occur in many situations in addition to the fibre-
cells before described. They are usually first seen in small
groups between the fibre-cells. They are round cells of very
various sizes, which are generally completely filled by a single
fat-globule. The cell-membrane which closely encompasses
the contents, is most minutely granulous, or, according to
Gurlt, homogeneous. It is in most instances very thin, being
about half the thickness of a blood-corpuscle, but sometimes it
is much thicker, and in the subcutaneous areolar tissue of the
thigh of a rickety child, at the age of twelve months (probably
in connexion with the disease), was almost as thick as the
breadth of a human blood-corpuscle. In the early stage, this
cell-membrane encloses a very distinct nucleus of a round or
oval form, which is sometimes flattened. When the former is
thin, the nucleus presents itself externally as a little promi-
nence upon the round fat -globule, which is closely encom-
passed by the cell-membrane ; but when thick, the nucleus lies
embedded in it. It contains one or two nucleoli. It is not
uncommon for adipose cells to contain a number of small
globules instead of one fat-globule, in such instances, one of
them is generally remarkable for its larger size. The adipose
cells are best seen in the fat found in the cranial cavity of a

m

young carp, before it has attained the length of six inches.
(See pi. III. fig. 10.) They there lie in so soft a substance,
that they may be insulated without any difficulty, and float
singly in the water in which they are examined. Some are so
larsre as to be visible even with the unaided eve. When
examined under the microscope with a magnifying power
of 450, the cell-membrane is readily recognized, it is very
thin, and closely encompasses the contents. It rises into
a little prominence on one side, within lies a proportion-
ately large, and very beautiful cell-nucleus, which is oval,
but not flattened, and contains one or two very distinct
nucleoli. Some of these fat-cells have two such nuclei, which

118 AREOLAR TISSUE.

have precisely similar relations to the cell, and both elevate
the cell-membrane into a prominence at the points where
they are attached. "When one of these cells is pressed
under the compressorium, the cell-membrane is at first
remarkably expanded, and then tears to a very limited
extent, allowing the fat to flow out. W7hen the pressure is
discontinued, it contracts again strongly. It has a minutely
granulous aspect, is soft and very elastic, but not fibrous.

In close apposition, the cells become flattened against one
another into polyhedral shapes, and, as Gurlt remarks, they
then resemble vegetable cells in their appearance. We, how-
ever, may go further, and regard them as corresponding in
signification also. In them the fat forms the cell- contents, as
the pigment does in its cells, and the ethereal oil, &c. in those
of plants. In its physiological signification of nutritive deposit
it has more analogy with starch than with any other substance.
I know not whether the nucleus is the part first formed in
these cells, or not. Nuclei without any investing cells are
found in the cranial cavity of the carp, lying with the adipose
cells in the surrounding cytoblastema ; these, however, may
be nuclei of fibre-cells of areolar tissue. Sooner or later the
nuclei become absorbed. They were still quite distinct in the
adipose cells of the subcutaneous areolar tissue in the thigh
of the before-mentioned rickety child twelve months old,
whilst I could not detect any in the neck of a foetus at the
seventh month. The absorption of the nucleus proceeds in one
of two ways ; either its external contour becomes gradually
indistinct, some granulous substance merely being left in its
place, which substance also disappears at a later period, or
small fat-globules are formed both within the nucleus itself,
and in its immediate proximity, which go on increasing in
size, whilst the nucleus gradually disappears. The cell-mem-
brane probably remains, even in the mature condition of the
tissue, and Gurlt has made the very interesting observation,
that in emaciated persons, the ordinary adipose cells are filled
with serum.

c, The third kind of cells which occur, in the areolar tissue
of the foetus are round, for the most part extremely pale and
transparent (pi. Ill, fig. 9.) They vary very much in size,

AREOLAR TISSUE. 119

most of tliem being much larger than the fibre-cells, and some
as large as the largest adipose cells. They can very rarely
be seen without the aid of the most favorable Light, but when,

under such circumstances, the observer has once detected one
of them, and become familiar with the degree of its trans-
parency, they may be recognized in great numbers. They
have a distinct nucleus attached to the internal surface of their
wall, containing one or two nucleoli. The nucleus always
attracts attention first ; the cell surrounding it is either quite
transparent, and void of granules, or has granulous contents,
and this granulous deposit is first formed in the neighbourhood
of the nucleus, the remaining portion of the contents being
still transparent. (See the figure.) Gradually, the entire con-
tents appear to become granulous. These cells are distinguished
from the fibre-cells of areolar tissue by the circumstance of
their becoming much larger than the latter, and their not
being elongated into fibres, and from the adipose cells, in that
they do not contain fat. I have found them in areolar tissue
taken from the bottom of the orbit, and from the neck of a
foetal pig, but do not know whether they occur in the areolar
tissue of all parts of the body ; nor can I determine their sig-
nification. They might be regarded as cellular spaces which
had been produced by the distension of the areolar tissue with
air. In such case, thev must communicate with one another
in the course of their further development. But this appears
to me to be somewhat improbable j and those spaces may be
merely artificial productions. I should rather regard the cells in
question as a modification of the adipose cells. For since, ac-
cording to Gurlt, the ordinary adipose cells in the adult may
contain mere watery fluid, one may also conceive the cells
destined to the formation of fat becoming completely developed,
without that formation actually taking place within them.
There are, indeed, adipose cells which contain fat even in the
earliest stage of their development, but that is no reason
whv the formation should not take place at a much later period
in other cells. The granulous deposit in many of them might
be regarded as the transitional step to the formation of fat.

The cellular tissue of the foetus differs in its chemical con-
stitution from that of the adult, since we cannot obtain any

120 AREOLAR TISSUE.

gelatine from it by boiling, none at least which has the pro-
perty of gelatinizing. The integument was removed from a
pig's foetus measuring four inches in length, cut up into pieces,
and steeped in distilled water for a clay. It was then boiled
for twenty-four hours. The last process caused it to crumble
into small particles that clouded the fluid, in which also some
large lamellae of epidermis floated. When examined with the
microscope the epidermis exhibited the same structure as it
did previous to being boiled ; the nuclei in the separate cells
were also distinct. A quantity of fibre-cells floated in the fluid
in the same state as when they, in their recent condition, com-
posed the entire cutis, that is to say, longish corpuscles extended
at both extremities into somewhat long fibres. The cell-nucleus
could still be distinctly recognized in some of them. Thus
the process of boiling, which had not produced any effect upon
the fibre-cells or the fibres issuing from them, had dissolved
the connecting cytoblastema, by which they had been held
together in the recent state. The fluid was then filtered.
Acetic acid caused a precipitate which was not dispersed by an
excess of acid. A solution of alum produced a much more
copious precipitate, which, in like manner, was not redissolved
by an excess of alum, or at least not completely. Tincture
of gall-nuts caused a thick clouding, spirits of wine only a
slight one Hydrochloric acid clouded the fluid, and an
excess of acid did not render it clear again. These reactions
accord with what Guterbock has called pyine, save that the
clouding produced in the latter by hydrochloric acid, was
redissolved by an excess of the acid. A portion of the filtered
fluid was evaporated almost to dryness, but even after twenty-
four hours, there was no trace of the formation of a jelly
observable. In order to separate the component particles of
this, in all probability, still very heterogeneous fluid, in some
degree from one another, some pure alcohol was added to that
portion of it which had been so long evaporated, whereby a
very copious precipitate was produced. This was separated by
filtration and washing, first with pure alcohol, and afterwards
with spirits of wine of 80 per cent, strength, then dried, and
again dissolved in boiling water. Acetic acid and alum caused
precipitates in this solution, which were not again dissolved

AREOLAR TISSUE. 121

by the addition of either of those substances in excess. With
respect to hydrochloric acid, the result was the same as before
described.

It cannot appear at all surprising that the areolar tissue of
the foetus differs from that of the adult, when it is known that
many cell-membranes undergo a change in their chemical
constitution at different stages of their development, and that
the growth of the cells is not a mere mechanical expansion.

Previous to quitting the subject of areolar tissue, we must
consider some other processes, by means of which a new for-
mation of it takes place in the adult. If (as I have already
laid down as an axiom in my first essays, Froriep's Notizen,
1838, Nos. 91, 103, and 112) the formation of cells be really the
principle of development of all organic structures, it must apply
no less to pathological than to physiological processes; and
that it really does so, is proved by the investigations of Henle
with reference to the new products resulting from inflammation,
namely, exudation, suppuration, and granulation; the results of
his observations are communicated in Hufeland's Journal,
vol. lxxxvi, No. 5.

Vogel pronounced the pus-corpuscles to be epithelium, in
consequence of their resemblance to epithelial cells, and there
was much of probability in the statement, so long as it ap-
peared that every diversity in the physiological signification
of an elementary structure was based upon a recognizable
diversity of formation. But this conclusion lost its importance,
when I brought forward the formation of cells as the common
principle of development of elementary structures, which were
perfectly distinct in a physiological sense, and at the same time
showed the most opposite tissues to be developed from cells,
which, in the first instance, perfectly resemble each other, and
present no distinction either in appearance or in the signi-
fication of their individual parts. H£nle, however, proved a
positive difference between the epithelial cells and pus-cor-
puscles, for he found that the nuclei of the youngest epithelial
cells were not broken down by the action of acetic acid like
those of the pus-corpuscles. The latter must, therefore, be
regarded as peculiar cells, which arc developed in the scrum
of pus in the same manner that all other cells originate in
their cvtoblastema ; the only difference being that in this

122

AREOLAR TISSUE.

case the cytoblastema is fluid. Beneath the pus, in healing
wounds, lie the granulations, composed of a firm cytoblastema,
in which lie a quantity of cells. Henle thus describes the
microscopical structure of granulations : " The most superficial
part presents cells, which resemble the pus-granules, except that
their nuclei are not broken down by acetic acid? In the
deeper strata, the nuclei are very distinct, and the envelopes
are polygonal, in consequence of mutual pressure. Wood has
already drawn attention to their resemblance to epithelial
cells. Deeper still the envelopes of the cells are found passing
through all the gradual transitions of the fibres of areolar
tissue, just as in the immature areolar tissue of the embryo.
The first rudiments of these fibres are the longish nucleated
corpuscles, which Giiterbock observed, and compared to the
cylindrical epithelium. Hence it follows, that the formation
of new cells proceeds upon the surface of the granulations,
and that the transformation of the latter into cellular tissue
(cicatrix-material, narbensubstanz) proceeds successively from
the bottom of the wound towards the surface/' As no gelatine
can be obtained from the granulations by boiling, Giiterbock
thought that those fibres in the granulations and exudations
which resemble the areolar tissue ought not to be regarded
as the actual fibres of that tissue, but as merely those of fibrin.
But, as we have seen above, the entire areolar tissue of the foetus
also does not afford any gelatinizing gelatine ; and since Henle
observed a similar course of development in these fibres to
that which I had pointed out in the areolar tissue of the
foetus, we must regard them as the young fibres of that
tissue (although they may differ from the mature tissue in
their chemical qualities), and the granulations as nothing more
than a primitive formation of areolar tissue.

A formation of areolar tissue similar to that in the foetus
takes place also in exudations resulting from inflammation.
R. Froriep (Klin. Kupfertafeln, lite Lief. Weimar, 1837,
Th. lxi) had already observed that irregular granules, some of
which seemed to be extended on one or both sides into thin
fibres, existed in the exudation of pericarditis, in addition to
the fibres resembling areolar tissue. " These elongated gra-
nules of fibrin/' he continues, " seem to be the commencements
of the formation of the new mass of tissue, that is, the rudi-

FIBROUS TISSUE. 123

ments of the newly-forming cylindrical fibres of the areolar
tissue composing the proper false membranes, or substance of
the cicatrix." Thus Froriep had already observed the gene-
ration of fibres, resembling those of areolar tissue, by the elon-
gation of corpuscles ; what he here calls fibrine globules, arc,
no doubt, the nucleated fibre cells becoming elongated into
fibres. Henle examined the exudation by which wounds
that heal by the first intention are closed, and found, that,
in this also, cells are formed which undergo transformation
into fibres of areolar tissue by an elongation of their envelope,
just as in the foetus. He thence concludes, that the formation
of exudations and granulations are essentially similar processes.
The exudation-globules (exsudatkugeln) discovered by Valentin,
and described also by Gluge, which, according to the former,
occur in every form of exudation, are, he says, allied to pus-
corpuscles ; and Henle also found that their nuclei are like-
wise broken down by the action of acetic acid.

Suppuration, therefore, differs from exudation and granula-
tion only in this circumstance, that a more fluid cytoblastema
is formed, in which fewer perfect cells are developed. It
represents an intermediate stage between the formation of the
firm tissues and the true function of secretion ; between which
two processes again no essential difference exists.

2. Fibrous Tissue. The fibres of tendons and those of
areolar tissue, differing but little from each other, and it
being impossible to define precisely the respective limits of
the two structures in the perfectly developed condition, we
accordingly find that they agree in their mode of origin.
Cells, resembling the fibre-cells of areolar tissue, are found
in the tendons of the foetus at a very early age. They
are arranged with their long axis corresponding to that
of the tendon, and are prolonged in two opposite directions
into fibres, which again subdivide into more minute ones.
(See plate III, fig. 11.) These cells split into fibres pre-
cisely in the same way as those of areolar tissue ; they have a
nucleus similar 'to theirs in shape, which remains for a period,
but is at last absorbed, leaving nothing but the fasciculus of
fibres persistent. All these processes, however, take place
much earlier in fibrous tissue than in the areolar, so that,

124 ELASTIC TISSUE.

unless the tissue is investigated in a very young foetus, we can
only detect cell-nuclei intermixed with fibres, or nuclei, in
whose immediate proximity a small fasciculus of fibres arises
on both sides. At an early stage of development the tendons
have a gray appearance, not having assumed the white colour
of the adult tissue. This fact is probably connected with
a chemical difference existing between the young and perfectly
developed fibrous tissue, as in areolar tissue. The quantity of
the cytoblastema in which these cells are formed, and by which
the fibres and tendons, when perfected, are probably connected
together, must be extremely small, and cannot in any way be
demonstrated by observation. Its existence can only be inferred
by analogy with areolar tissue : it will be remembered that it
was proved to be present in the foetal condition of that tissue.
The quantity of cytoblastema, in comparison to the fibres pre-
sent, seems to me to be the principal distinction between areolar
and fibrous tissue in the adult. The fibrous tissue contains
a great many more fibres within a given space than the areolar
does, and they are not more minute than those of the latter
tissue. There is just as great a difference, however, between
fibres of areolar tissue tali en from different parts of the body,
as there is between the ordinary fibres of tendons and the most
common form of areolar tissue, so that a very gradual transi-
tion takes place.

3. Elastic Tissue. The distinction between elastic and
fibrous tissue is exhibited at a very early period. But my
investigations into the history of the development of this tissue
are very incomplete, and extended only so far as to render
it probable that it presented no exception to the principle of
development from cells. I made use of the aorta of a foetal
pig and the ligamentum nuchse of a foetal sheep for the purpose.
The tissue taken from these two parts was very, different in its
general character. In a pig's embryo, of six inches in length,
the aorta had already acquired its yellowish colour and perfect
elasticity. The external coat could be easily drawn off in
long pieces, almost, indeed, as a distinct tube. Having drawn
off a small portion of the middle coat (which, in order to avoid
any suspicion of epithelium being mixed wTith it, was so care-
fully done, that the internal surface of the vessel remained

ELASTIC TISSUE. 125

uninjured), and torn it asunder a little, it was examined with
the microscope ; tlic first appearance presented was that of i
great quantity of isolated cells, floating about in the sur-
rounding fluid, each of which had its peculiar nucleus. (See
plate III, fig. 12.) This easy separation of the cells is
never seen in the same degree in the areolar and fibroid
tissues, as they are there connected together by the cytoblas-
tema, and by the tough fibres issuing from the cells. These
cells of the coat of the aorta vary very much in shape. See
the figure.) Some are round, but most of them oblong, some
terminate with a blunt extremity, others are acuminated on

a/ '

two or more sides, others again are prolonged into small pro-
cesses/ which again subdivide, but never extend to any great
length. Many of them are somewhat compressed laterally.
They all have a granulous aspect, but that appearance, so far
as one can judge by roiling the cells about, seems to be refer-
rible to the cell-membrane, and the contents appear to be
transparent. The nucleus, enclosing one or two nucleoli,
is attached to the interior of their walls. It is sometimes
round, at others more or less elongated. These cells have
become disengaged from the small portion of the coat of
the artery before described. When the preparation itself is
examined, many more cells are observed in it, and in addition
to them, distinct elastic tissue, consisting of a network of
minute, elastic, rough (?) (rauher) fibres, such as are found
nearest to the internal coat of the aorta in the adult. (See
Eulenburg, de Tela elastica, fig. 9.) It dees not, however,
present any fibres so thick as those which are found in the
external layers of the same part. A blighted nucleus may
be recognized here and there in the network. What relation,
then, do these cells bear to this still delicate, but so far as
regards its characteristic features, perfectly-formed elastic
tissue ? Analogy would lead us to suppose them to be
the primitive formation ; I sometimes also thought, that in
rare instances I could observe an immediate transition ; that
I could see, for instance, one of these cells, furnished with a
nucleus, pass continuously on one side into a small portion of
reticular tissue, resembling in appearance the undoubted clastic
tissue, whilst on its other side it retained its perfect cellular
figure. But this occurred so rarely, that I am not enabled to

126 ELASTIC TISSUE.

state that a transition of these cells into elastic tissue was
proved by observation. If, however, such be really the process
of formation, as, from analogy, we are entitled to suppose,
the bodies of these cells must then take a much more important
share in the formation of the fibres than those of areolar tissue
do, and the formation of the elastic fibres of the aorta holds a
middle position between the generation of the horny fibres in
the cortical substance of feathers (see p. 86, and pi. II, fig. 13)
and the production of fibres in areolar and fibrous tissues.
The reticular appearance of elastic tissue loses its singularity,
when it is conceived to be generated in the same manner
as those horny fibres in the feather, that is, partly by
an elongation of the cells, and partly by a splitting of
their bodies. The splitting of the elastic fibres is not to
be regarded as an isolated phenomenon, since such division
undoubtedly occurs in transitional stages in the development
of all forms of areolar and fibrous tissue in the foetus.
In this respect the elastic tissue seems to remain at a lower
stage of development. Purkinje and Rauschel observed a
darkish point in the centre of a transverse section of the
elastic fibres of the aorta, and a dotted line in the course of
the fibres, and thence inferred the existence of a rudimentary
canal in their interior. This supposition, which I must
confess formerly struck me as being a very bold one, has
much more weight now, inasmuch as it is not improbable
that all fibres which are formed by the prolongations of
cells (even those of areolar tissue) are hollow, at least, that
they are not composed throughout of one uniformly solid
mass. If, as an observation of Valentin's seems to indicate,
still more minute fibres may be rendered visible by the aid
of caustic potash in those of ordinary elastic tissue, I
should be inclined to regard them as analogous to the primi-
tive muscular fibres, whose signification, as we shall subse-
quently see, differs entirely, in a morphological view, from the
primitive fibres of areolar tissue.

Whilst the elastic tissue of the aorta taken from a very
young foetal pig exhibited in the manner before described the
main characteristics of the tissue, namely, its yellowish colour
and elasticity, the ligamentum nucha? of a sheep's foetus, at a
much later period of gestation, was but very slightly developed.

FIBRE-CELLS, ETC. 127

It had a gray and translucent appearance, exhibited no elas-
ticity, and when examined with the microscope, presented no
trace of its future structure. A gray cord, indistinctly marked
with longitudinal fibres, was seen, in which a great many
cell-nuclei might be recognized. I did not prosecute any fur-
ther researches, as the presence of the nuclei was sufficient
proof that there was nothing essentially different in the type
of its formation.

On casting a retrospective glance over the class of fibre-
cells which we have just been considering, we find that it
forms a very natural and somewhat strictly defined group
amongst the tissues. The tissues comprised in it are generated
from nucleated cells, which are transformed into fasciculi of
fibres by elongation, in the first place, and by the splitting of
the bodies of the cells themselves into separate fibres at a sub-
sequent period. The fundamental phenomenon previously
described at page 39 is distinctly presented in the formation
of these cells ; a structureless, gelatiniform mass, the cytoblas-
tema, is first present, and lies outside the cells already formed.
The cells are developed in this, the nucleus being, in all pro-
bability, the earliest formation. The growth of the cells
proceeds, and they become transformed into fibres in the
manner described. The quantity of the cytoblastema con-
tinually diminishes in proportion to the cells or fibres which
are forming, but probably part of it remains persistent be-
tween the fibres throughout the whole of life j in the mature
condition, however, it exists in greater quantity in areolar than
in fibrous or elastic tissue.

The mode of generation teaches us which parts of these
tissues correspond to the constituents of those hitherto treated
of. The elementary cells of areolar tissue, before undergoing
change, correspond morphologically with the cartilage and
epithelium-cells, the mucus-corpuscles, &c. ; and as a fasciculus
of fibres is generated from each cell of areolar tissue, a whole
fasciculus of fibres of areolar tissue accordingly corresponds to
what was an individual cartilage- or epithelium-cell, in the pre-
vious classes. The structureless cytoblastema between the
fibres of areolar tissue corresponds, however, to the firm inter-
cellular substance, forming the principal mass of most cartilages,

128 FIBRE-CELLS, ETC.

or to the minimum of cytoblastema between the epithelium -
cells : or, lastly, to the fluid, in which the cells of the first
class are formed. In this way one can also readily understand
how fibro-cartilage forms a gradual transition between true
cartilage and fibrous tissue ; it only requires that the cartilage-
cells pass through the same transformations as the elementary
cells of areolar tissue.

As the present class was based upon the alteration in form
which the cells comprised in it undergo, it necessarily could not
present many modifications in the shape of the cells, and accord-
ingly exhibits throughout merely an elongation of nucleated
cells into fasciculi of fibres, and a subsequent splitting of the
bodies into fibres. We have already seen the types of these
changes in the second class, where the pigment-cells and those of
the crystalline lens, &c, became elongated by a more vigorous
growth of the cell-membrane at different spots ; and the class
now before us merely affords us an instance of the same pro-
cess in a higher degree, since here, one side of one of these
more highly developed fibre-cells gives origin to a great num-
ber, or even a whole fasciculus, of fibres. The cells of the
cortex of the feather also furnished us with an example in the
same class of the division of the body of the cells into fibres.
Inasmuch as the prolongations of the pigment-cells remain
hollow, however minutely they may ramify, one may suppose the
same to be the case Avith regard to the fibres of the tissues
now under consideration. The decision of this point would,
as we shall subsequently see, be of great importance for
the theory of nutrition; but it is quite impossible to deter-
mine it by observation, in consequence of the cells of this class
not possessing any characteristic contents like those of pigment.
An observation by Purkinje and Raiischel was quoted, however,
in favour of these cells being hollow. If the hollowness of the
fibres of areolar tissue, &c, could be proved, there would
then be a division of a single cell into many cells at each
transformation of a fibre-cell, and thus the fibrous tissues would
not lose their cellular character.

The fibre- cells undergo chemical changes during their
growth and gradual transformation into the fibres of areolar
tissue, since that tissue, when boiled, even long after the forma-
tion of fibres has commenced, yields no gelatinizing gelatine.

ELASTIC TISSUE. 129

The formation of the fibres of areolar tissue from cells, having
been typified already in the second class, it follows that orga-
nization, or the presence of blood-vessels, does not establish
any essential difference in the growth of the elementary par-
ticles ; for this class belongs to the perfectly organized tissues,
and areolar tissue is highly vascular. The unorganized tissues
were formerly said to grow by apposition, and the organized
by intussusception. We have already discussed this distinction
at page 95. It is so far correct, that the young cells of unorga-
nized tissues are not formed throughout the entire thickness of
the tissue, but only in the neighbourhood of that surface, on
which they are in contact with vascular substance, and where
they therefore obtain the freshest cytoblastema. But if this
distinction between the surface and parenchyma of the tissue
be not present, in consequence of the blood-vessels being dis-
tributed throughout its whole thickness, the young cells are
then also generated in every part of the tissue ; and such is
the case with areolar tissue. The primary distinction, there-
fore, merely consists in the absence or presence of vessels, the
difference in the place of formation of the new cells being but
a secondary distinction. The elementary particles grow in both
instances and by the same powers. We shall see hereafter how
far the presence of vessels facilitates certain processes which
occur during growth. The essential phenomena of growth,
and, therefore, also the fundamental powers called into activity
by it, are similar in both. But why a formation of vessels
should take place in areolar tissue and not in epithelium, is a
question for future discussion.

class v.

Tissues, generated from cells, the tvalls and cavities of which

coalesce together.

The following is the type of formation in this class : inde-
pendent cells, by which we mean such as have a special wall and
cavity, are present in the first instance ; these we shall call
primary cells. They are either round or cylindrical, or of a
stellate figure. When round or cylindrical, the primary
cells are applied together in rows, the contiguous portions of

130 MUSCLE.

the cell-walls then become blended, in such manner that
merely simple septa remain, dividing each succeeding cell-cavity
from its neighbour. These septa, however, become absorbed,
so that the cavities of the different cells communicate. Instead
of a number of primary cells, we then have one single long
one, which we shall call a secondary cell. The cavity of such
a one, therefore, consists of the united cavities of the original
cells, and its cell-membrane of all their blended cell-mem-
branes, except that the parts with which they were in contact
are absorbed. The growth of the secondary cell proceeds
like that of any simple independent cell. This appears to
be the process of formation in muscle and nerve, so far, at
least, as the observations, which will presently be communi-
cated, extend. When the primary cells have a stellate figure,
their bodies are not applied in rows, as in nerve and muscle,
but are generated in larger interspaces filled with cytoblas-
tenia or with cells of another kind. Their prolongations,
however, come in contact, the walls coalesce at the points
of junction, and the blended septa then become absorbed, so
that the cell-cavities, which were at first separated, now com-
municate. In this manner, when several prolongations of
one cell come into contact with those of another, or of several
others, we obcain, in the place of isolated, hollow, stellate cells,
a network of canals, which are, in the first instance, somewhat
thicker at the parts corresponding to the bodies of the cells,
but become of pretty equal dimensions, in consequence of
more vigorous expansion of the communicating prolongations.
This appears to be the mode in which the capillary vessels are
formed. The following detailed statement of observations
upon the relation which muscles, nerves, and capillary vessels
bear to elementary cells, will show how far the description
just given, as the probable mode of formation, is to be regarded
as proved by these, as yet, very incomplete researches, and will
also indicate what deficiencies have yet to be supplied.

1. Muscle. To ascertain the relation which this tissue
bears to the elementary cells, we must have recourse to the
history of its development. I was, unfortunately, prevented
from investigating the earliest formation of muscular fibre, in
consequence of not being able to obtain any very young

MUSCLE. 131

embryos j but the deficiency in my researches may be sup-
plied from the description given by Valentin (Entwicklimgs-
Geschichte, p. 268), from which the following passage is
extracted : " Long before separate muscular fibres can be dis-
cerned, the globules of the primitive mass are seen, arranged
in parallel lines, particularly when they arc lightly pressed be-
tween two pieces of glass. The granules then appear to be
drawn somewhat nearer together, to become completely
coalesced, in some situations, while at others the blending
takes place only on the one or the other side, and to be com-
bined into one transparent mass. In this way filaments arc
formed, which, in some situations, have an appearance like
strings of pearls, at others, on the contrary, are less sharply
indented ; they often also continue slightly puckered on one
side, whilst the margin of the other has already become more
straight. At a subsequent period, all trace of granules or
division in the filament vanishes, and its outline becomes sym-
metrically transparent and cylindrical. The muscular fibre
usually undergoes no other change until the sixth month, ex-
cept that its substance becomes somewhat darker and its
cohesion closer. The first traces of transverse striae are ex-
hibited in the sixth month. These fibres are the primitive
fasciculi of muscle and not the primitive fibrils, which latter
are formed by a splitting of the fasciculus into smaller fibres.
From the period at which the muscular filaments become
transparent and uniform, masses of globules, of a more or less
spherical form and somewhat larger than the blood-corpuscles,
begin to accumulate between them. They diminish again
afterwards, and, blending with the gelatiniform mass which
connects them, become converted into the connecting areolar
tissue."

The youngest embryos in which I have investigated the
generation of muscle were those of the pig, measuring three
and a half inches in length. If a portion of one of the super-
ficial dorsal muscles be removed from an embryo pig of that
size, and examined under the microscope upon a black ground,
a transparent gelatiniform mass is observed, in which parallel
fibres (primitive fasciculi of muscle) run in close contact,
having- a whiter appearance than the surrounding gelatinous
substance. As development proceeds, the transparent sub-

132 MUSCLE.

stance diminishes in quantity, the muscular fibres lie closer
together, and have a more intensely white appearance upon
the black ground. When some of this transparent substance,
taken from a foetus of the size before mentioned (and in order
to exclude as completely as possible the embryonal cellular
tissue which surrounds the entire muscle, a portion should be
cut out from the centre of the muscle), is examined with a
magnifying power of 450, it exhibits various kinds of granules
differing in size, and lying in a finely granulous mass. On
examining these granules more accurately, they are found to
vary, both in size and appearance, being round or oval, more
or less opaque or transparent. A great many of them may be
recognized as cell-nuclei by their form. In many instances,
even when they are still connected together, the granulous
substance around them is more or less distinctly seen to have
a defined globular figure, within which the nucleus lies. This
is, however, observed most distinctly when any of the granules
become separated from the transparent substance, and float
about in the fluid upon the object-glass. A quantity of globules
are then seen floating about isolated, each one containing the
characteristic cell-nucleus, which is placed eccentrical, varies
much as to its size, and is often furnished with nucleoli. (See
pi. Ill, fig. 13.) We are already familiar with this as the
rudimentary form of most cells. The finely granulous por-
tion of the transparent mass is formed, in part, of the bodies of
the cells, which, when in close contact, are difficult to distin-
guish, and in part, of the cytoblastema in which the cells have
been generated. Some of these cells which float about are
becoming elongated into fibres, which are manifestly those of
areolar tissue. Such instances, however, are rare, and these
cells seem to be something quite peculiar. They might be re-
garded as the primitive cells of new muscular fibres ; but from
the manner in which Valentin expresses himself, one should
infer that they are formed at a later period, for he says,
" masses of globules begin to accumulate between the mus-
cular fasciculi from the period at which they become trans-
parent j* it is clear that he here refers to the nuclei of these
cells. This must, therefore, remain an undecided point.

We next examine the muscular fibres (primitive fasciculi)
in the dorsal muscles of the same foetus. Thev do not all

MUSCLE. 133

resemble one another in general character j some are more
irregular, more granulous, whilst others are relatively smooth.
The smoother ones represent cylinders, which are generally
more or less flattened (sec pi. IV, fig. 3), in which they are
delineated from the brachial muscles of a foetal pig seven inches
in length, a representing the flat surface, b the marginal. The
cylinder a presents a dark margin, and an internal clear por-
tion, a distinction which is yet more manifest in c} where the
dark margin is broader and sharply defined on its inner edge,
so that it has quite the appearance of a hollow cylinder.
I must, however, remark, that but very few fibres present this
appearance sufficiently distinct to satisfy the mind of the ob-
server. But in many instances it was so manifest, that no
other explanation seemed left than to suppose the fibre a hollow
tube. In the clear portion of the cylinder, which corresponds
to the cavity, (in addition to some small granules,) larger oval
corpuscles are seen, which are often very much extended in
the longitudinal direction. Their form at once shows them
to be nuclei, and they frequently contain one or two nucleoli.
The distance at which they lie from one another is more or
less regular in different instances. They do not lie in the
axis of the fibre, but eccentrically, upon and within the thick-
ness of the wall, as is seen when the fibre rests upon its
margin. (See the fibre b.) That delineation exhibits a regu-
larity in their position, since a nucleus lies upon the one side
of the wall, the second on the opposite, and the third again
upon the first side, and so on ; such, however, does not appear
to be the case in every instance. The nuclei are flat, for
when viewed edgeways they have the appearance of mere
stripes. The thickness of the wall of the cylinder seems
to vary, as is shown by a comparison of a with c. The
latter, c, the wall of which is the thicker, already presents an
appearance of transverse striae. The nuclei, however, arc also
still visible in it, as well as small isolated globules which are
contained in its cavity. Muscular fibre docs not present any
appearance of a cavity after the period of development before
mentioned has passed, but the nuclei remain visible for a long
time, lying in the thickness of the fibre, and often project
upon the outside in the form of small prominences.

The other form of muscular fibre is delineated in pi. IV,

134 MUSCLE.

fig. 1, from the dorsal muscles of a fetal pig of three inches
and a half in length. They are in general somewhat thicker
than those last described, more irregular, not so smooth,
but more granulated. The existence of a special wall to the
fibre and of a cavity in its interior, may be quite as distinctly,
or even more clearly, recognised in many of these. (See the
fibre c in fig. 1.) The wall is not so smooth as in the other
form of muscular fibre. The contents are always very granulous.
Distinct cell-nuclei, and not unfrequently nucleoli also, even
in the natural state, may often be perceived in them. Com-
monly, however, only the circular or oval outlines of the
nuclei are distinctly perceptible, in consequence of the other
granules which are contained in the cavity of the fibre lying
above them, and the general granulous nature of the fibre
renders an accurate discernment of the nucleus particularly
difficult. But if a drop of acetic acid be added, the fibre
becomes perfectly transparent, and swells ; the nuclei, on the
contrary, remain dark, shrivel up slightly, and may then be
distinguished with perfect accuracy. This is exemplified by
fig. 2, which represents the fibre c of fig. 1 after having been
treated with acetic acid. The indubitable cell-nuclei, partially
furnished with nucleoli, are there seen, with isolated small
dark granules between them. The nuclei have indeed under-
gone a slight change from the acetic acid, but they do not all
present a regular aspect even in the recent state. The
majority of them are flat. In the recent state, some appear to
be placed on their edges, presenting an appearance as though
the cavity of the fibre were divided into compartments by
small thick transverse striae. The nuclei lie much nearer
together in this than in the form of muscular fibre previously
described, so that the distance of the central points of two
nuclei from one another, is generally equal to, or even less
than, the thickness of the fibre.

This second form of muscular fibre appears to be an earlier
condition of the first. The younger the embryo the more
abundant is this form of fibre, and it gradually becomes less so
as development proceeds. The steps of this transition may
readily be conceived. The fibre becomes extended in its
entire length, is thereby rendered thinner, the cell-nuclei
are removed farther from one another, and in some instances

MUSCLE. 135

are also elongated in the direction of the fibre. Some
of the nuclei, those for instance which appear to be placed on
their edges, may possibly become absorbed at the same time,
for they never present that position at a later period. The de-
velopment of the whole cylinder proceeds simultaneously, Its
granulous aspect disappearing, and the small granules of the
cavity also diminishing in quantity. All the stages of transi-
tion from the second form into that first described may be
observed. The extension does not appear to take place quite
regularly, but may be stronger at particular parts, so that,
for a considerable extent, a fibre may be somewhat narrow,
and present no nucleus, and then again an intumescence oc-
curs in which a nucleus lies.

It now, however, becomes a question how the form of mus-
cular fibre last described is generated, and what its elementary
form may be. It presented a cylinder, which is most probably
hollow, and may be presumed to be closed at both ends, since
the muscular fibres terminate abruptly at the tendons, with a
well-defined and bluntly-rounded extremity. Cell-nuclei lie
within this cylinder at very small distances from one another.
Is the cylinder an elongated cell, in which nuclei are formed
as the rudiments of new cells, which, however, are not deve-
loped ; or are the nuclei the remains of cells, which, by coales-
cence with one another and absorption of the septa, form the
entire fibre or cylinder ? Or, in other words, is the fibre
generated by a coalescence of cells ?

I have not observed the stages of transition in which
original cells arranged themselves in a linear series to form a
fibre, the recent embryos at my command not being sufficiently
young for the purpose. I have, indeed, met with an appear-
ance in the form of muscular fibre last described, which might
be regarded as an indication that those fibres are composed of
small portions joined together. Their margins were incur-
vated at different spots, and a line, indicative of a division, ran
transversely across the entire thickness of the fibre. I have
endeavoured to delineate this appearance in pi. IV, fig. 1, b,
but I have not succeeded in representing its true character,
and it was not, in itself, conclusive. There are some other argu-
ments in favour of the fibre of muscle being composed of sepa-
rate particles. Many of the muscles of fishes or tadpoles, for

136 MUSCLE.

instance, when simply torn, separate into microscopic particles,
-which have an almost similar length. The same takes place,
according to C. H. Schultz, during the digestion of muscle in
the stomach, and, according to Purkinje, in muscle which is
exposed to the action of an artificial digestive fluid. The
observations of Valentin, already mentioned, admit, however,
of no other explanation than that previously given ; and the
history of the period of formation deficient in my researches
(from the cause before stated) may be completed from his.
According to him, " globules of the primitive mass, arranged
longitudinally, in a linear series, are present previous to the
muscular fibres. The granules, then, seem to draw somewhat
nearer together, and to coalesce, at some parts completely, at
others, on the contrary, only on the one or other side. In
this manner threads are formed, which present at some spots
the appearance of strings of pearls, whilst at others they
are less sharply indented ; they are also often seen to be still
wrinkled on one side, while on the other their margin is
already nearly a straight line. The expression " granules of
the primitive mass" (Urmasse), or other similar terms, have
been hitherto used to denote either the elementarv cells
themselves or their nuclei, indiscriminately ; in consequence
of the distinction between them, and their relation to each
other being unknown. In the passage quoted, Valentin can-
not have meant the nuclei, for, as we have seen, they do not
coalesce. What he calls globules of the primitive mass must,
therefore, be the elementary cells furnished with their nuclei,
and in their earliest stage of development ; that is, before they
have undergone any transformation. The following arguments
may likewise be adduced in favour of the correctness of the
explanation which assumes these " globules of the primitive
mass" to be cells. In the first place, the structure formed by
their coalescence, namely, the primitive fasciculus of muscle,
is hollow ; and, secondly, in the early stage of development of
the fasciculi, the cell-nuclei lie just so closely together, as
they would if each nucleus had pertained to a previously round
cell. If these nuclei were subsequent formations, generated
in the primitive fasciculus of muscle, as in a cell, they ought
to be more numerous in old than in young muscles.

It, therefore, seems scarcely to admit of a doubt, that

MUSCLE. 137

each primitive muscular fasciculus is a secondary cell, formed
by the coalescence of primary round cells, each furnished with
a nucleus, and which were arranged together in a row. After
the coalescence of the contiguous portions of the cell-walls has
taken place, an absorption of the septa remaining between the
cavities of the two neighbouring primary cells must commence,
since no such septa can be perceived within the secondary cell
at a later period. If the little transverse striie, by which the
cavity of the fibres is sometimes divided, be actually nuclei
placed transversely upon their edges, they are probably such
as lay upon that part of the wall of the cells which was ab-
sorbed. It seems that the coalescence of the cells, however,
is not sufficiently complete to prevent a separation taking
place more readily at the points of junction than elsewhere,
and on this the phenomena of the artificial division of muscle
before mentioned probably depend.1

When I made my first communication upon the formation
of the primitive fasciculi of muscles by the coalescence of cells
(Froriep's Notizen, No. 103), the only corresponding instances
known to exist among vegetable cells were those of the spiral
and lactiferous vessels. The interest attached to the subject
has very much increased since Meyen's discovery* of a much
more striking analogy in the cells of the liber or inner bark
— (bastzellen). (Wiegmann's Archiv, 1838, p. 297.) He found
that th?se long-extended cells, when boiled in hydrochloric
acid, fell into small particles of nearly equal length ; and
investigation into the development of the cells of the liber in
buds showed, that in the early period a corresponding quantity
of distinct, somewhat longitudinally extended, prismatic, pa-
renchymal cells are present, which are placed with their
extremities accurately arranged one upon another, that they
unite together at those parts, and that their septa arc after-
wards absorbed.

The secondary muscle-cell passes subsequently through all
the changes incident to a simple cell. Its wall is at first thin,

1 It might be important to examine whether the zigzag plications of muscles,
during contraction, have not perhaps some connexion with the length to which the
portion of a muscular fibre generated from one single cell has become expanded, so
that probably the angle of each flexion coincides with the point of junction of two
cells.

138 MUSCLE.

and it contains many small granules in its cavity in addition
to the nucleus. A transformation of the cell-contents then
takes place, the granules gradually disappearing ; the wall of
the cell at the same time becoming thicker at the expense of
the cavity, so that eventually the latter completely disappears,
and the entire secondary cell is converted into a solid cord.
The cell-nuclei at first remain whilst this thickening of the
cell-wall is going on, and become enclosed by it, rather than
pushed into the cavity of the cell. They are at length entirely
absorbed. Is, then, the thickening of the wall of the secondary
muscle-cell a thickening of the cell-membrane itself, as ap-
peared to be the case in cartilage? or is it a secondary deposit
upon its inner surface, so that the cell-membrane is chemi-
cally and microscopically distinct from the substance, by
means of which the secondary cell becomes converted into a
solid cord ? The latter is the more usual case in vegetables.
The position of the cell-nuclei affords important evidence for
the solution of the above question ; for as those bodies, gene-
rally at least, lie firmly attached to the inner surface of the
cell-membrane, they would be pushed towards the interior by
a thickening of the cell-membrane itself, whilst a secondary
deposit upon its inner surface, must enclose and fix them
there, unless they should become separated altogether from
the cell-wall. Now, in muscle, they actually remain lying
in the circumference of the fasciculus, as represented by
pi. IV, fig. 3, b. This fact, then, renders it probable that the
thickening of the wall of the secondary muscle-cells is due only
to a secondary deposit. Such a supposition must, however, have
been adopted, independent of the argument just raised, since
the muscular fasciculi are, as it seems, enclosed by a struc-
tureless membrane. The fasciculi have been long described as
invested by a sheath, but that investment has been considered
to be composed of cellular tissue, and to correspond in the
primitive fasciculi to the cellular tissue, by which the larger
fasciculi are separated from one another. This membrane
seems, however, to have quite a different signification, and
to be the cell-membrane of the secondary muscle-cell. It
is structureless, very transparent, and appears as a very narrow
and sharply-defined border around each primitive fasciculus. I
well know how readily such an appearance is produced by a

MUSCLE. 139

mere optieal deception, and that one can never be positive
with respect to it unless it be observed that the margin in
question does not accurately follow every bend of the fasci-
culus. It is, therefore, difficult to be convinced of this in
mammalia ; but in all those larva? of insects which present
the broad transverse stria? of the fasciculi, discovered by
Miiller, the membrane, when the continuity of the proper
muscular substance of a primitive fasciculus has been broken
at a certain point, may be distinctly observed passing over
uninterruptedly from the one portion to the other. PI. IV,
fig. 4, represents such a fasciculus ; the membrane encompasses
it so loosely (this larva had been preserved in spirits of wine)
that a portion of the muscular substance could even change
its position within the cavity. The membrane, where entirely
isolated from the other parts of the preparation, shows itself to
be quite structureless, and, indeed, the sharply-defined ex-
ternal contour renders it very improbable that it should be
composed of areolar tissue. I, therefore, consider it extremely
probable that it represents the cell-membrane of the secondary
muscle-cell. It thus not only serves to isolate the fasciculi,
but forms an essential constituent part of them. PI. IV,
fig. 5, exhibits this structureless membrane upon a muscular
fasciculus of the pike ; this preparation, however, was not
quite convincing, inasmuch as the inferior edge of the fasciculus
was covered by muscles lying above it. By means of this mem-
brane, the muscular fasciculus remains, throughout its entire
existence, a cell with a closed membrane and a cell cavitv,
the latter being filled with a firm substance, the peculiar
muscular substance. It, therefore, clearly follows from the
above that nervous fibres cannot pass between the primi-
tive fibres (fibrils) of muscle ; and that the latter cannot
separate from their fasciculi, so as to pursue a more extended
and independent course, as is common with fibres of areolar
tissue ; since, in either case, the cell-membrane must be
ruptured.

The true muscular substance, which is thus, in the first
place, formed as a secondary deposition upon the inner surface
of the secondary muscle-cell, and continues to be so deposited
until the entire cavity is filled, is composed in its mature con-
dition, of very minute longitudinal fibres, the so-called primi-

MO MUSCLE.

tivc fibres (fibrils) of muscle. These longitudinal fibres do not
appear to represent the original condition of the secondary
deposit, but the latter is structureless at first, and its trans-
formation into fibres takes place subsequently. The change
seems, however, to commence at a very early period, and
indeed before the cavity is completely filled. The transverse
Btriae of the muscular fasciculi, which, according to my mode of
explanation, are produced by the peculiar form of the primitive
fibres, likewise make their appearance before the complete
filling up of the cell-cavity, as pi. IV, fig. 3, c, exhibits.

According to the observations of Meyen on the formation
of the cells of the liber, after the coalescence of the cells and
absorption of the septa, a secondary deposit also takes place
upon the common cell-membrane in the same way that we
have observed to take place in muscle ; but I know of nothing
amongst vegetables analogous to a secondary deposit consisting
of longitudinal fibres. On the contrary, according to Valentin,
such deposits appear to take place in plants universally in
spiral lines. The beaded appearance which the primitive mus-
cular fibres here and there present, might perhaps be regarded
as the result of this tendency to a spiral formation, the intu-
mescences (beads) being so placed, as to produce the transverse
strise, and the latter may perhaps be spiral and not circular.
This is, however, a mere conjecture, and requires further re-
search.

The involuntary muscles, such as do not present the trans-
verse striae, appear to originate in a manner similar to that
just described. They differ, however, from the voluntary or
striated muscles, in their fibres being generally shorter than
those of the latter; probably, therefore, fewer primary cells
arrange themselves together to form a secondary cell, and
their fibres are commonlv thinner and flat. T found in a
human uterus, which contained a mature foetus, some long
muscular fibres of the breadth of the common primitive fasci-
culi of voluntary muscles, which were so flat as scarcely to
amount to 0-0010 to 0-0015 of a line in thickness. The
involuntary muscles, likewise, have cell-nuclei, proving that
the fibres composing them do not correspond to the primitive
fibres (fibrils), but to the primitive fasciculi of the voluntary
muscles. An opposite view of the matter might be taken

NERVES. Ml

from the circumstance of their frequently exhibiting no trace
of longitudinal striae, and that probably the greater portion of
them do not contain other more minute primitive fibres, or
at least only such as are imperfectly developed. In this re-
spect they are not so highly developed as the voluntary
muscles. Perhaps the peculiar secondary deposit upon the
cell- membrane of the secondary cell is all that is essential to
the contraction of muscle ; and it may not be important that
that substance should consist of minute longitudinal fibres.

In order briefly to recapitulate our researches into the
generation of muscle, the process may be thus stated. Hound
cells, furnished with a flat nucleus, are first present, the
primary cells of muscle. These arrange themselves close
together in a linear series ; the cells thus arranged in rows,
coalesce with one another at their points of contact ; the septa,
by which the different cell-cavities are separated, then become
absorbed, and thus a hollow cylinder, closed at its extremities,
the secondary cell of muscle, is formed, within which the
nuclei of the original cells, from which the secondary cell has
been formed, are contained, generally lying near together on
its wall. This secondary cell, then, passes through all the stages
of a simple one. It expands throughout its entire length,
whereby the nuclei are farther removed from one another,
and sometimes even become elongated in the same direction.
A deposit of a peculiar substance, the proper muscular sub-
stance, takes place at the same time upon the inner surface
of the cylinder, by which the cavity is at first narrowed, and
at length completely filled. The cell-nuclei lie external to this
substance, between it and the cell-membrane of the secondary
cell.

The transverse striae in the voluntary muscles become more
manifest, and the deposited substance is more distinctly seen
to be composed of longitudinal fibres, as the foetus advances
in age. The nuclei are gradually absorbed. The cell-mem-
brane of the secondary muscle-cell remains persistent through-
out life, so that each primitive muscular fasciculus is always to
be regarded as a cell.

2. Nerves. The nervous system presents two forms of
elementary structure : 1st, fibres, nervous fibres in the ex-

142 NERVOUS FIBRES.

tended sense of the term, including the fibres of the brain and
spinal cord : 2d, globules, ganglion-globules, in addition to the
ganglia occurring in the brain and spinal cord. Our task is
to point out the relation which these two forms of elementary
structure bear to the elementary cells.

Ne7*vous Fibres.

Of these, there are two different forms : «, the common
white nervous fibres ; b, the gray, so-called organic fibres.

a. White nervous fibres. They have the appearance of
fibres, which, when examined microscopically, exhibit very
dark margins, and these margins are produced by a substance
apparently identical with that which gives them their white
colour when examined with the unaided eye. Since the cause
of this colour does not appear to be situated in the whole
fibre generally, but to be confined to its external portion, this
latter may be termed the white substance of the nervous
fibres. The margin of a fibre generally presents a double
outline on both sides, so that it has the appearance of a
hollow tube, and the distance between the two outlines,
then, denotes the thickness of the white substance. According
to the researches of Remak, the white substance of every
nervous fibre may be removed by pressure, and an extremely
pellucid, pale band, which was previously surrounded by
the white substance, then remains, corresponding to that
which, previous to the manipulation, seemed to be the contents
of the tube. (See R. Remak, Obss. Anat. et Microsc. de Syst.
Nerv. Struc, Berol. 1838.)

Two opinions with respect to the nervous fibres may be
deduced from the above observations ; either this pale band is
the proper nervous fibre, and the white substance only a
sheath (cortex) around it (this is the view taken by Remak),
or the nervous fibre is actually a hollow fibre, the wall of
which is formed by the white substance, the contents of which,
however, are not fluid, but composed of a tolerably firm sub-
stance, namely, the above-mentioned band.

The history of the development of the nervous fibres must

:

NERVOUS FIBRES. 1 13

explain the relation which they bear to the cells. Remak1
describes the early condition of the nerves in the following
manner : " The substance of the cerebro-spinal nerves of the
rabbit, in the third week of embryonal existence, consists of
corpuscles, some of which are irregularly spherical, others
slightly elongated, having a very delicate filament adhering
to them ; they arc mostly transparent, and arranged in rows
without, however, presenting any distinctly perceptible fibrous
structure." And 1. c. page 153, he says, " A structureless and
general globular mass is the original form, from which the
primitive fibres of the cerebro-spinal nerves are developed.
These primitive fibres are at first varicose, and contain no
medulla; most of them pass into the cylindrical form, through
the intermediate stage of transitional fibres."

I have investigated the development of nerve in the fcetal
pig. The nerves of the foetus have not the shining white
colour, presented by those of the adult animal, but are gray
and transparent, and the younger the embryo the more strik-
ing are these appearances. We are, therefore, quite prepared
to find that microscopic investigation showrs the white sub-
stance of the fibres to be less perfectly or not at all developed.
If a nerve, taken from a foetal pig of about six inches in length,
be spread out, in the usual mode of preparation by tearing it
under water, some fibres are seen which very much resemble
those of the adult animal, and which are furnished witli
outlines almost as dark. The greater part of the substance,
however, does not form connected fibres, but consists of separate
round globules, or more or less long, irregular little cylinders,
arranged with their long axes in the direction of the course of
the nerves, having outlines, however, quite as dark as those of
the nervous fibres. These appear to be what Remak refers
to in the description previously quoted. In addition to them,
however, a substance of quite another appearance is seen,
which has not the dark outline, docs not appear pellucid but
granulated, and in which the celh nuclei arc distinctly recog-
nisable.

When the other constituent parts predominate, the nuclei

1 Miiller's Arcliiv, 183G, p. 148. Respecting the microscopic structure of the
brain and spinal cord of the foetus, see Valentin, Entwickelungsgeschichte, p. 183.

144 NERVOUS FIBRES.

may very probably be overlooked, or possibly be regarded
as extraneous substances. But they are in fact the primitive
structure of nerve, for the younger the foetus the greater is
their relative quantity, and in a pig's foetus of three inches in
length, I found them the sole constituent of nerve, none of
the fibres furnished with the dark margins, nor any of the
cylinders or globules being visible at that period of deve-
lopment. The development of nerve, however, does not appear
to proceed uniformly in all individuals ; for the dark globules
and cylinders were already present in some other pigs' em-
bryos, which were scarcely any larger. PI. IV, fig. 6, repre-
sents a portion of the ischiatic, and fig. 7, of the brachial
nerve of such a foetus. We observe a palish, and very
minutely- granulated cord, which, in consequence of certain
longitudinal shadings, such as the delineation exhibits, pre-
sents the appearance of a coarse fibrous structure. Round or
for the most part oval corpuscles, which are immediately recog-
nised as cell-nuclei, and which sometimes also contain one or
two nucleoli, are generally seen in the course of these shaded
parts, throughout the entire thickness of the cord. Sometimes
a fibre separates from such a cord, and stands out isolated, as
at a in both the figures, and the nuclei are then seen to lie in
the course of the fibres. A single fibre presents several nuclei
in its course, as was also observed in secondary muscle-cells
(see fig. 8, b), but I have never remarked it in the cells of the
fourth class, the fibre-cells. Although the (nervous) fibres
cannot at this early period be distinctly perceived to be hollow,
the wall not being distinguishable microscopically from the
contents, yet we shall see that the progress of development
renders it highly probable that they are so. If then these
(nervous) fibres are so far analogous to the early condition of
secondary muscle-cells, that they are hollow, and in various
parts of their course contain nuclei, whose form shows them
to be ordinary cell-nuclei, it is probable that they are gene-
rated in a similar manner to muscle ; that is, that they are
formed by the coalescence of primary cells, to which the nuclei,
just noticed as present upon the fibres, have pertained; so
that thus the nervous fibres would be secondary cells, cor-
responding to the secondary muscle-cells, or primitive muscular
fasciculi. The actual observation of the primary cells of nerve

NERVOUS FIBRES. 1 15

in their independent state, is very difficult, from the circum-
stance of our being unable at that period to distinguish between
them and the surrounding tissues ; for a whole organ is then
composed entirely of independent cells, which have not as yel
undergone any transformation. It is true I saw an independent
cell, furnished with a nucleus, which seemed to have separated
from the nervous cord, in one of the preparations alluded to,
fig. 6 b; but I cannot positively assert that it had actually
separated from that particular part, nor that it was a primary
nerve-cell, for the cells in that preparation had not as yet un-
dergone any change. In this instance, therefore, we must
content ourselves, for the present at least, with the analogy to
muscle.

These fibres, or secondary nerve-cells, differ very much in
their appearance from the subsequent nervous fibres, which are
furnished with distinct but not dark outlines : thev have a
pale, granulated aspect. By progressive development, however,
they become converted into the white fibres, and pi. IV,
fig. 8, d, represents the transition. The part of the figure
to the right hand exhibits the fibre yet in the early condition,
pale, granulated, and furnished with a cell-nucleus ; in the
portion to the left, it has completely assumed its subsequent
form : it has a dark outline, is not granulated, and the one
portion passes immediately into the other. The identity be-
tween these pale fibres and the subsequent white nervous fibres
is thus established.

In what then does this transformation of the pale granu-
lated fibres into the white fibres consist ? Clearly in the
development of the white substance ; we may, however, imagine
three different modes in which this development may take
place. It may take place, lstly. By the white substance form-
ing as a sheath (cortex), around each fibre, and in this manner
enclosing it. By this mode of explanation the fibre would be
identical with the pale band discovered by Remak, which
would therefore be the cell-membrane itself. 2dly. The white
substance might be regarded as a transformation and thicken-
ing of the cell-membrane of those fibres, or secondary nerve-
cells. According to this view, the white substance would be
the cell-membrane, and Bemak's band the firm contents of the
secondary cell. 3dlv. The white substance may be formed as

10

146 NERVOUS FIBRES.

a secondary deposit upon the inner surface of the cell-mem-
brane^ being chemically distinct from the latter, and the
remainder of the cell-cavity may then, and not until then, be-
come filled up by Remains band.

It -will be seen that the above question is analogous to
that raised when we were treating of muscle, viz., whether
the proper muscular substance be a thickening of the original
cell-membrane itself, or a secondary deposit upon it. The
reply is not, in either instance, essential to the proof of the
origination of nerves or muscle from cells, but it is of so much
the more importance for the explanation of the structure of a
perfectly- developed nerve. If any conclusion may be drawn
from the few observations which I have made on this point,
the latter view appears to me the most probable, viz., that
the white substance is a secondary deposit upon the inner
surface of the cell-membrane. The white substance of each
nerve is surrounded externally with a structureless and peculiar
membrane, which appears to be minutely granulated. This
membrane presents itself as a narrow, clear border, which
is readily distinguished from the dark contours of the white
substance. This membrane seems hitherto to have been in-
cluded with the neurilema or with the cellular tissue, which
surrounds the nervous fibre, and although its external outline
is generally very sharply defined in the nerves of the frog, it
would be difficult, on examination of the entire nerve of a
mammal, to arrive at any comdction of its distinct and sepa-
rate existence, did not opportunities of observing it in an
isolated state present themselves. PL IV, fig. 9 a, represents
such a preparation, taken from the cranial portion of the
nervus vagus of a calf. The continuity of the white substance
has here been broken by the process of preparation; but
where it still exists, the double contours, (and thus the thick-
ness of the white matter), may be clearly distinguished. But
the nerve still exists at the part where the white substance is
separated, its sharply-defined external margins may be seen,
although their contours are but pale, and it may be observed
that this pale outline does not pass into the external dark
one of the white substance, but is continued on the outside of
it as a narrow border, parallel to the two outlines of the
white substance. The white substance of nerve is, therefore,

NERVOUS FIBRES. I 17

surroundecl externally with a thin, pale membrane, which has
a sharply-defined external margin. If the membrane be very
thin, it cannot be recognised as the pale border round the
nervous fibre; it is still, however, distinctly visible at situations
where the white substance is destroyed. (See fig. 9 b.) The
mere fact of the membrane possessing a defined external
border, is evidence against its being composed of areolar tissue ;
and even the portion which docs not contain any white substance,
presents no appearance of a fibrous structure j it simply ap-
pears to be somewhat minutely granulated. If this be correct,
the membrane can have no other signification than that of
cell-membrane of the nervous fibre, or secondary nerve-cell.
The white substance is then a secondary deposit upon its
inner surface. The position of the cell-nuclei is also favorable
to this view. Most of the cell-nuclei, presented by the nervous
fibres in their earliest and as yet pale condition, disappear
during the formation of the white substance, a circumstance
which is common to most other cells. Some, however, appear
to remain for a longer period ; occasionally, although rarely, a
cell-nucleus is here and there seen upon the side of a nerve,
(the white substance of which is completely developed), lying
in the pale border, which surrounds the white substance.
Fig. 9, c and d, exhibits them from the nervus vagus of a
calf. At c the white substance, corresponding to the nucleus,
even forms a slight projection into the cavity of the fibre.
This nucleus seems therefore actually to belong to the fibre,
and to lie upon the inner surface of the cell-membrane, while
the white substance is so deposited, that the nucleus remains
situated external to it. The band discovered by Remak would
then be the proper cell-contents. Meanwhile I beg that the
above may be regarded simply as an attempt at an explana-
tion, the accuracy of which must be decided by further
researches, for much more extensive investigations and a sepa-
rate and distinct consideration arc absolutely necessary for
accurate decision of so important a subject.

According to the foregoing explanation, therefore, each
nervous fibre is, throughout its entire course, a secondary cell,
developed by the coalescence of primary nucleated cells.
With respect to these cells, we remark, lstly. An external, pale,
thin cell-membrane, having a granulated but not a fibrous

148 NERVOUS FIBRES.

aspect, the inner surface of which constantly exhibits cell-
nuclei in the very early period of the development of nerve ;
but in the somewhat more advanced stage, when the white
substance is developed, they are only occasionally found. 2dly.
That the white, fat-like substance to which the peculiar appear-
ance and distinct outline of the nerves are chiefly referable, is
deposited upon the inner surface of this cell-membrane. When
this deposit is thick, its double contour (to which the nerve is
indebted for its tubular appearance), may be recognised ; this,
however, escapes observation when only a thin stratum of
white substance is present. Morphologically considered, it
therefore corresponds to the peculiar substance of muscle, for
that is likewise developed as a secondary deposit upon the mem-
brane of the secondary muscle-cell. 3dly. That the rest of the
cell-cavity appears to be filled up by a firm substance, namely,
the band discovered by Ptemak. There seems to be no struc-
ture analogous to this band in perfectly-developed muscles, for
there, the secondary deposit, that is, the formation of the pro-
per muscular substance, proceeds until the cavity of the
secondary cell is completely filled.

We have thus traced the development of nerve to its per-
fect state, without those irregular globules and little cylinders
with the dark outlines, (which were mentioned at page 143,
as occurring at a middle stage of the development of nerve
in addition to the pale fibres and the matured nervous fibres),
having proved to be a transitional step in the process. I
am inclined to regard them as an artificial product, caused
by pressure and the action of water upon the as yet very
delicate nerve. If, for example, water penetrate through the
cell-membrane by imbibition, the oil-like white substance re-
tracts into separate rounded bodies, and the facility with which
this takes place is proportionate to its slight degree of con-
sistence. This is often seen even in fully-developed nerves ;
an entire nerve frequently separates from this cause into sepa-
rate globules or little cylinders, which have sharply -defined
outlines, so that merely the cell-membrane proceeds uninter-
ruptedly, in the form of a pale stripe, from the external wall of
one of the dark portions to that of the other. Valentin has
given a delineation of such a state of the nervous fibre, (Acta
Acad. Leopold. Nat. Curios, vol. xviii, pi. Ill, fig. 7). As the

NERVOUS FIBRES. 149

white substance is less consistent in the foetus, it separates the
more readily, and the artificial generation of such globules is
very easy of observation in foetal nerves.

The growth of nerves neither proceeds from the circum-
ference towards the central organs, nor vice versd, but their
primary cells are included amongst those from which every
organ is formed, and which, so far at least as their appearance
is concerned, present no marks by which they can be distin-
guished from other cells. They arc first characterized as
nerves, when they become arranged in rows and coalesce to
form a secondary cell. After that coalescence each nervous
fibre forms a separate cell, which pursues an uninterrupted
course from the organ, in which its peripheral extremity is
situated, to the central organ of the nervous system. The
white substance of nerves does not appear to be formed at
so early a period in their peripheral extremities, as it is in
their trunks. The Medizinischen Zeitung for August 1837,
contains a description which I gave of some nerves from the
tail of frog's larvae, which presented an appearance quite dif-
ferent from ordinary nerves, inasmuch as they had a pale con-
tour and no perceptible cavity. They were nerves in an early
stage, previous to the development of the white substance.
They represent the only form of nervous matter which we find
in the tail of very young larvae. Some isolated nerves, having
the ordinary appearance of the dark contours, gradually make
their appearance, and afterwards increase in quantity j they
were first observed in the neighbourhood of the muscular fas-
ciculus which traverses the middle of the tail. The development
of the white substance appears therefore to advance from the
trunks towards the circumference. These white fibres become
more minute and paler towards the periphery. Sometimes such
a fibre seems to terminate suddenly with even an incomplete
acuniination. But, on a more accurate observation, some ex-
tremely delicate, very thin filaments are generally seen going
off from it. The pale immature fibres in the tail of the
frog's larvae also subdivide. A question now arises arc those
more minute fibres (which at least present an appearance of
subdivision) already prepared within an ordinary white primi-
tive nervous fibre, or are they actual subdivisions? Since each
nervous fibre is a secondary cell, and retains its character as

150 NERVOUS FIBRES.

a simple cell, and since the simple cell-membrane continues to
exist distinct from its secondary deposits, and from the cell-
contents, it is quite conceivable that fibres may be generated
in the secondary deposits or in the cell- contents, as they are in
muscle, although we have as yet no evidence of the fact ; but
these fibres could no more issue out free from the white
nervous fibre, than the primitive fibres of muscle could from
secondary muscle-cell, because, in order to do so, they must
necessarily rupture the cell-membrane of the secondary cell.
These subdivisions, therefore, so far as the structure from
whence they issue corresponds to an ordinary nervous fibre,
and is not merely a fasciculus of very minute secondary nerve-
cells, cannot be a mere appearance, nor anything but actual
divisions, a simple secondary nerve-cell becoming elongated into
several minute fibres, in a manner analogous to that which
we have witnessed in the fibre-cells, (see page 115.) The
nerves in the tail of the tadpole may therefore be described
as terminating by the nervous fibres, that is, the secondary
cells becoming split in different directions after the manner
of fibre-cells or stellate cells. In the memoir before alluded
to, I have noticed some swellings upon the pale nervous fibres
in the tail of the tadpole. They have a double signification ;
some which are marked off from the rest of the fibre by a
sharply- defined outline are the nuclei of the cells, from which
the fibres have been generated ; the majority, however, which
pass into the fibre without a well-defined contour, as gene-
rally occurs at situations where the fibres divide and diverge
towards different sides, are the bodies of the original cells,
which (especially when they become elongated at different
parts into fibres) remain somewhat thicker than the prolonga-
tions themselves; the pigment -cells, pi. II, fig. 9 a, exhibit
this appearance.

b. Gray or organic nervous fibres. The gray cords, which,
according to the researches of Retzius and J. Miiller, are derived
from the sympathetic nervous system, and mingled with the
cerebrospinal nerves in which they sometimes pursue a long
isolated course, owe their gray appearance, according to the in-
vestigations of Remak, " to the peculiar structure of the primi-
tive fibres, which arise in the ganglia. They are not tubular,

NERVOUS FIBRES. 151

that is, surrounded with a sheath, but naked, being transparent,
almost gelatinous, and much more minute than most of the
primitive tubes. They almost always exhibit longitudinal lines
upon their surface, and readily separate into very minute fibres.
In their course they are very frequently furnished with oval
nodules, and covered with certain small oval or round, more
rarely irregular, corpuscles, which exhibit one or more nuclei,
and in size almost equal the nuclei of the ganglion-globules."
(Observationes anat. ct microsc. de system, nervos. structura.
Berol., 1838, p. 5.)1

These corpuscles may at once be recognised, both in Remak's
delineations, and when examined in the natural state, to be cell-
nuclei, which are round or oval, and frequently furnished with
one or two nucleoli. Thev are attached to the most minute fibres,
and as they are thicker than the fibres, they often appear to be
situated only on their outside. Observation, however, does
not warrant the conclusion that such is actually the fact. In
the secondary muscle-cells (in which the nuclei decidedly lie
within the cell) it frequently appears, and especially in the
later periods of development, previous to the disappearance of
the nuclei, as if the nuclei lay externally to the cell, inasmuch
as they become pushed towards the outside. But no doubt
the cell-membrane is at the same time elevated upon them, as
we saw to be so distinctly the case in the fat-cells. (PI. Ill,
fig. 10.) Now, these most minute organic fibres, furnished
with nuclei, precisely resemble the earlier condition of the
white nervous fibres, as they were represented in pi. IV, fig.
8, a b. Both have the same pale, minutely-granulated ap-
pearance, and both present cell-nuclei in their course. The
only difference is, that the organic fibres are much more
minute and the nuclei smaller. Each single nucleated or-
ganic fibre (I do not mean an entire fasciculus of them) cor-
responds to a white primitive fibre, and is probably, like it, a
secondary cell, which has been generated by a coalescence of
primary cells, whose nuclei are the nodules described by

1 Remak's discovery of the peculiar structure of the organic nervous fibres ex-
plains an observation previously communicated by me upon some extremely minute,
pale, nervous fibres, which did not appear tubular, and wen' nodulated at different

spots, and which I discovered in the mesentery of frogs. No doubt they were or-
ganic fibres.

1/52 GANGLION-GLOBULES.

Remak as existing upon these fibres. The similarity between
the organic fibres and that which I have described as the
earlier condition of the white nervous fibres, might be adduced
as an objection to my description of the formation of nerves,
and it might be said, that that form seemed to be the earlier
form of the white nervous fibre, because the organic nerves
were developed earlier than the white, and, therefore, organic
fibres were the only ones present in the first instance. Ob-
servation of the actual transition, as represented in pi. IV,
fig. 8, c d, would, however, refute this argument. Each pale,
nucleated fibre becomes a white nervous fibre, as an immediate
consequence of the formation of the white substance, which
is probably a secondary deposit upon the internal surface
of the hollow fibre. The formation of this white substance,
which, according to analogy, must occur in every one of the
minutest fibres, either does not take place at all in the organic
fibres, or does so at a much later period, and their peculiarity
therefore consists in their remaining stationary at an earlier
stage of development, and either never attaining to the higher
development of ordinary nerves, or only at a much later period,
(a point which might be decided by comparing their numbers
in old and young individuals.) One can conceive that the
function of the organic nerves, whether it be actually a che-
mico- vital one, or consist merely in the production of in-
voluntary motion, requires less-developed nerves, in the same
way that the involuntary muscles do not attain the same de-
gree of development as the voluntary.

2. Ganglion-globules.

These occur in the gray substance of the brain and spinal
cord and in the ganglia, having generally the appearance of
comparatively large granulous globules, enclosing a round vesi-
cle, placed eccentrically, and which again exhibits in its
interior one or two small dark points. According to Eemak,
two of these vesicles sometimes occur in one globule. Valentin
(Nov. act. Acad. Leopold, xviii, p. 196), calls attention to
the similarity between their composition and that of the egg,
he compares the vesicle of the ganglion-globules to the germi-
nal vesicle, their parenchyma to the yelk-substance, and ascribes

GANGLION-GLOBULES. J

a protecting investment of fibres resembling areolar tissue to
both structures. This is certainly a very striking comparison,
but the external investment must not in either instance be re-
garded as a something unessential, as a structure composed of
other elementary parts, for the ganglion-globules, like the yelk,
are true cells, and their external covering is an essential com-
ponent part of them ; it is the cell-membrane. The vitelline
membrane of the bird's egg, while contained in the ovary,
is perfectly structureless, not composed of more minute ele-
mentary parts ; the same is the case with the investment of the
ganglion-globules. They are both of them true simple cells.
The parenchyma of the ganglion-globules forms the cell-con-
tents, and the vesicle in their interior is the cell-nucleus ; the
small corpuscles which it contains are the nucleoli. The vesicle
of the ganglion-globules lies, as in other cells, eccentrically
upon the internal surface of the cell-membrane. This cell-
membrane may be most distinctly observed in the ganglion-
globules of the sympathetic nerves of the frog, previous to their
junction with the sacral plexus. (See pi. IV, fig. 10, a.) Tt there
appears comparatively dark, and sharply defined, both externally
and internally, so that its thickness may be readily measured.
Valentin has already remarked, that the capsule of the gan-
glion-globules is thicker in the lower animals. In the situation
before mentioned in the frog, it seems as though a ganglion-
globule were sometimes formed within another cell. (See fig.
10, b.) The ordinary contents of these ganglion- globules ^is a
minutely-granulous, yellowish substance. On one occasion,
however, I saw a ganglion-globule from the head of an ox (I
do not precisely know from what part it was taken), in which
the granulous appearance was confined to the surface, the inte-
rior being clear, — a fact which was rendered distinctly percepti-
ble by causing the globule to roll about, It is nothing remarkable
that two nuclei should sometimes occur in one ganglion-glo-
bule ; we have observed this already in several cells, in those
of cartilage for instance. In those instances, however, only
one of them was the true cell-nucleus, the cytoblast of the car-
tilage-cell, the other being a subsequent formation within the
cell.

10i CAPILLARY VESSELS.

3. Capillary vessels.

Plate II, fig. 9, represents two stellate pigment-cells, which
have coalesced at a. In that instance two cells had been gene-
rated at some distance from one another, their bodies may still
be distinguished as two spots somewhat thicker than the rest
of the structure. These cells became elongated on different
sides into hollow processes, which, like the cavities of the bodies
of the cells, are filled with pigment. Two processes of the two
cells came into contact at a, and then coalesced, the separation
at the point of union appears to have been absorbed also at the
same time, so that the cavities of the two cells communicate
immediately with one another ; at all events there is no appa-
rent interruption to the pigment, which forms the contents of
the cells and their prolongations, (See page 78.) Now, if we
imagine several such stellate cells to^be developed on a large
surface at similar distances from one another, and the several
prolongations issuing from each separate cell to coalesce with
those issuing from the other cells, in the manner represented
in the figure at a, the result will be a network of canals ex-
tending over the entire surface, and all communicating with
each other. The size of the meshes of the network is deter-
mined by the distance of the cells from each other, and by the
number of the prolongations issuing from each cell. Such,
then, appears to be the process by which the capillary vessels
are formed.

The observations, on which this mode of formation of the
capillary vessels is based, were made partly on the tails of very
young tadpoles, and partly on the germinal membrane of the
heir's eg^. They are as follows :

1. The capillary vessels, in the tail both of the fully-deve-
loped and young tadpoles, are seen to be surrounded by a
thin, but distinctly perceptible membrane, which does not ex-
hibit any fibrous arrangement. (See pi. IV, fig. 11.) The
variety in the thickness of this membrane in different in-
stances sufficiently explains why we cannot distinguish it in all
capillary vessels, just as we cannot detect the cell-membrane
even in the blood-corpuscles, although there can be no doubt
of its existence. Where the capillary vessels exhibit a fibrous

CAPILLARY VESSELS. 155

structure, they have arrived at a more complicated stage of
their formation, and I regard sucli fibres as distinct from their
cell-membrane.

2. Very distinct cell-nuclei occur at different spots upon
the walls of the capillaries, both of the young and fully-deve-
loped tadpole. They appear to lie cither in the thickness of
the wall, or on the internal surface of the vessels, on which
they often form a projection. (See fig. 11.) They admit of a
double explanation. They are either the nuclei of the primary
cells of the capillaries, or nuclei of epithelial cells, which in-
vest the capillary vessels. It is true that epithelial cells occur
in vessels which have a great resemblance to capillary vessels,
if they are not actually such, as may be very distinctly seen
in the vessels of the membrana capsulo-papillaris in a foetal
pig of from four to six inches long, where some of them pro-
ject, in the form of half-spheres, into the cavity of the vessel;
but there were no epithelial cells perceptible surrounding the
nuclei in the capillaries of the tadpole's tail. On the contrary,
these nuclei frequently seemed to lie free upon the internal
wall of the vessel, and must have been much more abundant
had they been nuclei of epithelial cells. That these are the
nuclei of the primary cells of the capillaries is, therefore, most
probable, although this exclusive argument by no means decides
the question.

3. In the tail of very young tadpoles, the capillary network
presents, besides the ordinary cylindrical canals which have an
equal diameter, and in which the blood flows in a regular cur-
rent, other vessels of an irregular form. Unfortunately I
neglected to make a drawing of them ; they accord, however,
in all essential particulars with the capillaries of the germinal
membrane of the hen's egg represented in pi. IV, fig. 12,
except that the meshes of the vascular network arc much
larger in the tail of the tadpole. They are not regularly
cylindrical. They are generally widest in situations where
branches are given off, sometimes wider even than the ordi-
nary capillary vessels. (See a, b in figure 12.) The branches
diminish very rapidly as they leave those broad parts, and
widen again as they approach another dilated portion They
present every degree of narrowing from vessels in which it
could scarcely be remarked, to those which are reduced so

156 CAPILLARY VESSELS.

much as to be scarcely thicker than a fibre of areolar tissue
(as in c). Brauches are also sometimes given off from these
wider parts, which likewise diminish very rapidly to the same
degree of minuteness, without reaching another dilated part
(as at d e), and which are, therefore, blind ones. According
to the above view of the development of the capillaries, these
appearances may be explained in the following manner : the
wider portions, a, b, &c, are the bodies of the primary cells.
Hollow processes, as at d, are sent out from the bodies of the
cells as the result of a more vigorous growth in different situ-
ations, precisely as is the case in all stellate cells. These
prolongations meet with similar ones from other cells, and thus
produce the form c. But being hollow, they are capable of
expansion during their growth, and thus the canal c becomes
converted into /, and at length into g, which is as wide as an
ordinary capillary vessel. A more accurate analysis of the
observations, however, is necessary to enable us to judge of
the correctness of this explanation. It might be doubted, in
the first place, whether these were really capillaries. The blood
flows uninterruptedly through the ordinary capillaries, but there
are no blood-corpuscles in these canals, at least in the more
minute ones ; they are, therefore, more difficult to discover,
and readily give rise to a doubt whether they are canals.
But their direct continuity with the ordinary capillaries may be
clearly demonstrated, and blood-corpuscles actually enter the
wider ones. If they be true capillary vessels, they may either
be ordinary ones in a state of contraction, or they must repre-
sent a certain stage of their development. But if it be difficult
to conceive that a capillary vessel can have the power to con-
tract itself almost to the minuteness of a filament of areolar
tissue, such an assumption cannot be supported at all in
respect to the blind branches, which do not join any other
vessel, as at d, This form might, indeed, be admitted to be a
certain stage of development, although not of the kind de-
scribed above ; but branches might be sent off from the
capillaries already existing, which again might give off others.
The objection, that such an explanation does not account for
the varying width of these capillaries, might be met by as-
suming that circumstance to depend upon the surrounding
substance. It is, therefore, necessary to see the primary cells

CAPILLARY VESSELS. 157

previous to their union with the actual capillaries. Now it
is certain that a great many stellate cells arc found in the
tail of the tadpole. They lie beneath the epithelium and pig-
ment-cells on the same plane with the capillary vessels; arc
smaller than the pigmcnt-cclls, and contain a colourless or
palish yellow substance ; they send off processes on different
sides, which vary in number very much in different instances,
but are generally short, and for the most part do not join
with processes from other cells. Their shape has no sort of
connexion with that of the pigment-cells which lie above them,
for when, as is the case in many larvae, the latter only send
off prolongations on two sides, these cells exhibit several pro-
cesses on different sides. They cannot, therefore, be young
pigment-cells. Such branches of the capillaries, as those at d,
sometimes appear to be connected with one of those stellate
cells, and the others might, therefore, be regarded as young
cells of capillary vessels which had not as yet begun to
anastomose. These anastomoses, however, are not sufficiently
evident to enable me positively to assert their existence. The
great number of these stellate cells, and their presence at all ages
of the tadpole, are also circumstances unfavorable to the suppo-
sition that they are primary cells of capillaries. They might,
indeed, be conceived to indicate a lower stage of development,
as not having yet undergone any change, and that eventually
capillary vessels may be developed from some, whilst others
continue their existence without such a transformation, and
fill the place of cells of areolar tissue. That, however, would
be somewhat too hypothetical, and I shall, therefore, not ad-
duce these cells as proof of the existence of primary cells of
capillary vessels. The uncertainty which attaches to the ob-
servations on this point in the tail of the tadpole appears,
however, to be removed when we examine the incubated
hen's egg.

4. When the germinal membrane of an hen's egg which
has been subjected to thirty-six hours' incubation (at which
period the formation of red blood has commenced, and is dis-
tinctly perceptible), is placed under the microseope, and the
area pellucida examined with a magnifying power of 450, the
capillary vessels arc readily distinguished in it by their yel-

158 CAPILLARY VESSELS.

lowish-red colour. Notwithstanding repeated endeavours, I
cannot succeed at this season of the year when the hens are
moulting, in subjecting eggs to incubation for so long a period,
I can, therefore, only give a representation of these vessels
from a recollection of what I observed in the early part of
this year. (See pi. IV, fig. 12.) In some situations the capil-
laries are perfect, and connected with the larger vessels ; at
others they have the appearance represented in the figure, and
illustrated previously by observations on the tail of the tadpole.
In addition to these capillaries, which form a network of
canals of irregular caliber and give off blind branches, some
separate irregular corpuscles are seen, such as h and i, which
do not appear to be connected with the vascular network.
These bodies send off blind processes of various forms in
different directions, and have the appearance, therefore, of
stellate cells. They have a yellowish-red colour, like that of
the bone-capillaries, which circumstance is alone sufficient to
suggest the supposition that they are cells of capillary vessels
in progress of development. This becomes much more pro-
bable, when we observe some of these corpuscles, such as k,
already connected with the true capillaries. We may, there-
fore, with a high degree of probability at least, regard them
as the primary cells of capillary vessels ; and in that case the
description of the formation of these vessels, previously given,
would be the correct one. The following would, therefore, be
the mode in which the formation of the capillaries and of the
blood takes place in the germinal membrane : among the
cells which compose the germinal membrane, some which are
deposited at certain distances from one another, are deve-
loped into the primary cells of capillary vessels by becoming
elongated on different sides so as to form stellate cells.
The processes of the different cells come into contact and
coalesce, the septa are absorbed, and in this manner a network
of canals of very irregular caliber is produced, the prolonga-
tions of the primary cells being much thinner than the bodies
of the cells. These processes of the cells or passages of com-
munication undergo expansion until they and the bodies of
the cells all attain one equal width, until, in fact, a network of
canals of uniform caliber is formed. The fluid portion of the

CAPILLARY VESSELS. i;,<»

blood constitutes the contents of the primary cells, as well as of
the secondary ones — the vessels produced by their coalesces
and the blood-corpuscles are young cells which are developed
in their cavities.

Thus this last class, comprising tissues, which, in their
functions, are the most characteristic of the animal kingdom
exhibits the same principle of development that we have met
with in all the others; namely, that cells originate in the
first place, and that these become transformed into the ele-
mentary parts of the tissues. The elementary cells in this
class, however, undergo more essential changes during their
transformation than those of any previous one. They not
only do not remain, as in the first two classes, independent,
that is provided with a special cavity and particular wall ; not
only does a coalescence of the w'alls of neighbouring cells take
place, as in the third class, but the cavities of the different
cells also unite together in consequence of the absorption of
the coalesced partition-walls of the several cells, so that the
primary cells cease to exist as distinct objects. It is to a cer-
tain extent the opposite process to that which occurred in the
fourth class, where, in addition to the prolongation of the cells,
a splitting of them into several, probably hollow, fibres, a sort
of division of the cells took place. The type of the trans-
formation of the primary cells, as presented by nerve, muscle,
and capillary vessels, is not, however, altogether limited to this
class, but has been already exhibited by previous classes, and
even in plants. Some of the pigment-cells have been cited
before as examples, and the generation of the cells of the liber
observed by Meyen was brought forward as an instance of
perfect analogy in vegetables.

The independent existence of each separate primary cell is,
no doubt, lost as a consequence of this perfect coalescence of
several cells ; not so, however, its character as Cell in general.
On the contrary, several primary cells contribute to form one
secondary cell, having the full signification of one independent
cell. Each secondary cell in muscle and nerve forms a closed
Whole, and the distinction between cell-membrane and cell-
contents or secondary deposit seems to continue throughout life.
In this way the nerves bring every part of the body into con-

160 CAPILLARY VESSELS.

nexion with the central portions of the nervous system by
means of a single uninterrupted cell. The different parts of
the body, however, are connected together by another kind
of uninterrupted secondary cell, namely, the capillaries. The
capillary system, generated from several primary cells, forms
one single secondary cell. The cavity of the secondary cell
communicates with that of the large vessels. Researches are
still required to decide the question whether these latter are
mere dilatations of the capillaries, or whether they are formed
simply by the junction of other elementary parts. In the
latter case the capillary vessels would open into a cavity alto-
gether distinct from their own, just as a vegetable cell opens
into an intercellular space. It sometimes occurs that the cavi-
ties of certain vegetable cells open directly outwards, but such
instances are very rare.

As a primitive muscular fasciculus, a nervous fibre and a
capillary vessel are corresponding formations in this class ; we
may also compare these structures with the elementary parts
of other tissues. The elementary cells of all tissues correspond
with one another, being formed universally according to similar
laws. A blood-corpuscle, an epithelial cell, a cartilage-cell, an
elementary cell of areolar tissue (therefore, also a fasciculus of
areolar tissue formed from it), correspond to an elementary
cell of muscle, &c. There is no structure analogous to an
entire primitive fasciculus of muscle or a secondary muscle-cell
or a nervous fibre amongst the principal component parts of
the tissues previously discussed, because with them the forma-
tion of secondary cells only occurs as an exception. A mus-
cular fasciculus differs, therefore, from a fasciculus of areolar
tissue, and a primitive fibre of areolar tissue has no analogy
with a primitive muscular fibre.

SECTION III.

REVIEW OF THE PREVIOUS RESEARCHES THE PORMATH l

PROCESS OF CELLS THE CELL THEORY.

The two foregoing sections of this work have been devoted
to a detailed investigation of the formation of the different
tissues from cells, to the mode in which these cells are de-
veloped, and to a comparison of the different cells with one
another. We must now lav aside detail, take a more e\-
tended view of these researches, and grasp the subject in its
more intimate relations. The principal object of our investi-
gation was to prove the accordance of the elementary parts of
animals with the cells of plants. But the expression " plant-
like life " (pnanzen-ahnliches Leben) is so ambiguous that
it is received as almost synonymous with growth without
vessels; and it was, therefore, explained at page 6 that in
order to prove this accordance, the elementary particles of
animals and plants must be shown to be products of the same
formative powers, because the phenomena attending their deve-
lopment are similar ; that all elementary particles of animals
and plants are formed upon a common principle. Having
traced the formation of the separate tissues, we can more
readily comprehend the object to be attained by this compa-
rison of the different elementary particles with one another, a
subject on which we must dwell a little, not only because it is
the fundamental idea of these researches, but because all
physiological deductions depend upon a correct apprehension
of this principle.

When organic nature, animals and plants, is regarded as a
Whole, in contradistinction to the inorganic kingdom, we do
not find that all organisms and all their separate organs are
compact masses, but that they are composed of innumerable
small particles of a definite form. These elementary particles,
however, are subject to the most extraordinary diversity of

11

162 GENERAL RETROSPECT.

figure, especially in animals ; in plants they are, for the most
part or exclusively, cells. This variety in the elementary
parts seemed to hold some relation to their more diversified
physiological function in animals, so that it might be established
as a principle, that every diversity in the physiological signi-
fication of an organ requires a difference in its elementary
particles ; and, on the contrary, the similarity of two elemen-
tary particles seemed to justify the conclusion that they were
physiologically similar. It was natural that among the \ery
different forms presented by the elementary particles, there
should be some more or less alike, and that they might be
divided, according to their similarity of figure, into fibres, which
compose the great mass of the bodies of animals, into cells,
tubes, globules, &c. The division was, of course, only one of
natural history, not expressive of any physiological idea, and
just as a primitive muscular fibre, for example, might seem to
differ from one of areolar tissue, or all fibres from cells, so would
there be in like manner a difference, however gradually
marked between the different kinds of cells. It seemed as if
the organism arranged the molecules in the definite forms
exhibited by its different elementary particles, in the way
required by its physiological function. It might be ex-
pected that there would be a definite mode of development
for each separate kind of elementary structure, and that it
would be similar in those structures which were physiologi-
cally identical, and such a mode of development was, in-
deed, already more or less perfectly known with regard to
muscular fibres, blood- corpuscles, the ovum (see the Supple-
ment), and epithelium-cells. The only process common to
all of them, however, seemed to be the expansion of their
elementary particles after they had once assumed their proper
form. The manner in which their different elementary par-
ticles were first formed appeared to vary very much. In
muscular fibres they were globules, which were placed together
in rows, and coalesced to form a fibre, whose growth proceeded
in the direction of its length. In the blood-corpuscles it was
a globule, around which a vesicle was formed, and continued
to grow ; in the case of the ovum, it was a globule, around
which a vesicle was developed and continued to grow, and
around his again a second vesicle was formed.

GENERAL RETROSPECT. 163

The formative process of the cells of plants was clearly
explained by the researches of Schlciden, and appeared to be
the same in all vegetable cells. So that when plants were
regarded as something special, as quite distinct from the
animal kingdom, one universal principle of development was
observed in all the elementary particles of the vegetable or-
ganism, and physiological deductions might be drawn from it
with regard to the independent vitality of the individual cells
of plants, &c. But when the elementary particles of animals
and plants were considered from a common point, the vege-
table cells seemed to be merely a separate species, co-ordinate
with the different species of animal cells, just as the entire
class of cells was co-ordinate with the fibres, &c, and the
uniform principle of development in vegetable cells might be
explained by the slight physiological difference of their elemen-
tary particles.

The object, then, of the present investigation was to show,
that the mode in which the molecules composing the elemen-
tary particles of organisms are combined does not vary
according to the physiological signification of those particles,
but that they are everywhere arranged according to the same
laws ; so that whether a muscular fibre, a nerve-tube, an ovum,
or a blood-corpuscle is to be formed, a corpuscle of a certain
form, subject only to some modifications, a cell-nucleus, is uni-
versally generated in the first instance; around this corpuscle
a cell is developed, and it is the changes which one or more
of these cells undergo that determine the subsequent forms of
the elementary particles ; in short, that there is one common
principle of development for all the elementary particles of
organisms.

In order to establish this point it was necessary to trace
the progress of development in two given elementaiy parts,
physiologically dissimilar, and to compare them with one
another. If these not only completely agreed in growth,
but in their mode of generation also, the principle was
established that elementary parts, quite distinct in a phy-
siological sense, may be developed according to the same laws.
This was the theme of the first section of this work. The
course of development of the cells of cartilage and of the

164 GENERAL RETROSPECT.

cells of the chorda dorsalis was compared with that of vege-
table cells. Were the cells of plants developed merely as
infinitely minute vesicles which progressively expand, were
the circumstances of their development less characteristic
than those pointed out by Schleiden, a comparison, in the
sense here required, would scarcely have been possible. We
endeavoured to prove in the first section that the complicated
process of development in the cells of plants recurs in those
of cartilage and of the chorda dorsalis. We remarked the
similarity in the formation of the cell-nucleus, and of its
nucleolus in all its modifications, with the nucleus of vegetable
cells, the pre-existence of the cell-nucleus and the development
of the cell around it, the similar situation of the nucleus in
relation to the cell, the growth of the cells, and the thickening
of their wall during growth, the formation of cells within
cells, and the transformation of the cell-contents just as in
the cells of plants. Here, then, was a complete accordance
in every known stage in the progress of development of two
elementary parts which are quite distinct, in a physiological
sense, and it was established that the principle of develop-
ment in two such parts may be the same, and so far as could
be ascertained in the cases here compared, it is really the

same.

But regarding the subject from this point of view we are
compelled to prove the universality of this principle of develop-
ment, and such was the object of the second section. For so
long as we admit that there are elementary parts which originate
according to entirely different laws, and between which and
the cells which have just been compared as to the principle of
their development there is no connexion, we must presume
that there may still be some unknown difference in the laws

a/

of the formation of the parts just compared, even though
they agree in many points. But, on the contrary, the greater
the number of physiologically different elementary parts, which,
so far as can be known, originate in a similar manner, and
the greater the difference of these parts in form and physio-
logical signification, while they agree in the perceptible phe-
nomena of their mode of formation, the more safely may
we assume that all elementary parts have one and the same

GENERAL RETROSPECT. 165

fundamental principle of development. It was, in fact,
shown that the elementary parts of most tissues, when
traced backwards from their state of complete development
to their primary condition are only developments of cells,
which so far as our observations, still incomplete, extend,
seemed to be formed in a similar manner to the cells com-
pared in the first section. As might be expected, according
to this principle the cells, in their earliest stage, were almost
always furnished with the characteristic nuclei, in some the
pre-existence of this nucleus, and the formation of the cell
around it was proved, and it was then that the cells began to
undergo the various modifications, from which the diverse forms
of the elementary parts of animals resulted. Thus the apparent
difference in the mode of development of muscular fibres and
blood-corpuscles, the former originating by the arrangement of
globules in rows, the latter by the formation of a vesicle
around a globule, was reconciled in the fact that muscular
fibres are not elementary parts co-ordinate with blood-cor-
puscles, but that the globules composing muscular fibres at
first correspond to the blood-corpuscles, and are like them,
vesicles or cells, containing the characteristic cell-nucleus,
which, like the nucleus of the blood-corpuscles, is probably
formed before the cell. The elementary parts of all tissues
are formed of cells in an analogous, though very diversified
manner, so that it may be asserted, that there is one universal
principle of development for the elementary parts of organisms,
however different, and that this principle is the for/nation of
cells. This is the chief resn.lt of the foregoing observations.

The same process of development and transformation of
cells within a structureless substance is repeated in the for-
mation of all the organs of an organism, as well as in the
formation of new organisms ; and the fundamental phenomenon
attending the exertion of productive power in organic nature
is accordingly as follows : a structureless substance is [ire-
sent in the first instance, which lies either around or in the infe-
rior of cells already existing ; and cells arc formed In it in ac-
cordance witJi certain laws, which cells become developed ill
various ways into the elementary parts of organisms.

The development of the proposition, that there exists one gene-

166 GENERAL RETROSPECT.

ral principle for the formation of all organic productions, and
that this principle is the formation of cells, as well as the conclu-
sions which may be drawn from this proposition, may be com-
prised under the term cell-theory, using it in its more extended
signification, whilst in a more limited sense, by theory of the
cells we understand whatever may be inferred from this pro-
position with respect to the powers from which these pheno-
mena result.

But though this principle, regarded as the direct result of
these more or less complete observations, may be stated to be
generally correct, it must not be concealed that there are some
exceptions, or at least differences, which as yet remain unex-
plained. Such, for instance, is the splitting into fibres of the
walls of the cells in the interior of the chorda dorsalis of osseous
fishes, which was alluded to at page 14. Several observers
have also drawn attention to the fibrous structure of the firm
substance of some cartilages. In the costal cartilages of old
persons for example, these fibres are very distinct. They do
not, however, seem to be uniformly diffused throughout the carti-
lage, but to be scattered merely here and there. I have not ob-
served them at all in new-born children. It appears as if the
previously structureless cytoblastema in this instance became
split into fibres ; I have not, however, investigated the point
accurately. Our observations also fail to supply us with any
explanation of the formation of the medullary canaliculi in
bones, and an analogy between their mode of origin and that
of capillary vessels, was merely suggested hypothetically. The
formation of bony lamella around these canaliculi, is also an
instance of the cytoblastema assuming a distinct form. But
we will return presently to an explanation of this phenomenon
that is not altogether improbable. In many glands, as for
instance, the kidneys of a young mammalian foetus, the
stratum of cells surrounding the cavity of the duct, is enclosed
by an exceedingly delicate membrane, which appears to be an
elementary structure, and not to be composed of areolar tissue.
The origin of this membrane is not at all clear, although we
may imagine various ways of reconciling it with the formative
process of cells. (These gland-cylinders seem at first to have
no free cavity, but to be quite filled with cells. In the kidneys

GENERAL RETROSPECT. 1G7

of the embryos of pigs, I found many cells in the cylinders,
which were so large as to occupy almost the entire thickness
of the canal. In other cylinders, the cellular layer, which
was subsequently to line their walls, was formed, but the cavity
was filled with very pale transparent cells, which could be
pressed out from the free end of the tube.)

These and similar phenomena may remain for a time un-
explained. Although they merit the greatest attention and re-
quire further investigations, we may be allowed to leave
them for a moment, for history shows that in the laying down
of every general principle, there are almost always anomalies
at first, which are subsequently cleared up.

The elementary particles of organisms, then, no longer lie
side by side unconnectedly, like productions which are merely
capable of classification in natural history, according to simi-
larity of form ; they are united by a common bond, the
similarity of their formative principle, and they may be com-
pared together and physiologically arranged in accordance
with the various modifications under which that principle is
exhibited. In the foregoing part of this work, we have treated
of the tissues in accordance with this physiological arrange-
ment, and have compared the different tissues with one
another, proving thereby, that although different, but similarly
formed, elementary parts may be grouped together in a natural-
history arrangement, yet such a classification does not neces-
sarily admit of a conclusion with regard to their physiological
position, as based upon the laws of development. Thus, for
example, the natural-history division, " cells," would, in a
general sense, become a physiological arrangement also, inas-
much as most of the elementary parts comprised under it have
the same principle of development ; but yet it was necessary to
separate some from this division ; as, for instance, the germi-
nal vesicle, all hollow cell-nuclei, and cells with walls composed
of other elementary parts, although the germinal vesicle is a
cell in the natural-history sense of the term. It docs not
correspond to an epithelium-cell, but to the nucleus of one.
The difference in the two modes of classification was still
more remarkable in respect to fibres. The mode of their
origin is most varied, for, as we saw, a fibre of areolar tissue

168 SURVEY OF CELL-LIFE.

is essentially different from a muscular fibre; while, on the
other hand, a whole primitive muscular fasciculus is identical
in its mode of origin with a nervous fibre, and so on. The
existence of a common principle of development for all the
elementary parts of organic bodies lays the foundation of a
new section of general anatomy, to which the term philoso-
phical might be applied, having for its object — firstly, to
prove the general laws by which the elementary parts of
organisms are developed; and, secondly, to point out the dif-
ferent elementary parts in accordance with the general princi-
ple of development, and to compare them with one another.

SURVEY OF CELL-LIFE.

The foregoing investigation has conducted us to the princi-
ple upon which the elementary parts of organized bodies are
developed, by tracing these elementary parts, from their per-
fected condition, back to the earlier stages of development.
Starting now from the principle of development, Ave will recon-
struct the elementary parts as they appear in the matured
state, so that we may be enabled to take a comprehensive view
of the laws which regulate the formation of the elementary
particles. We have, therefore, to consider — 1, the cytoblas-
tema; 2, the laws by which new cells are generated in the
cytoblastema ; 3, the formative process of the cells themselves ;
4, the very various modes in which cells are developed into the
elementary parts of organisms.

Cytoblastema. — The cytoblastema, or the amorphous sub-
stance in which new cells are to be formed, is found either
contained within cells already existing, or else between them in
the form of intercellular substance. The cvtoblastema, which
lies on the outside of existing cells, is the only form of
which we have to treat at present, as the cell-contents form
matter for subsequent consideration. Its quantity varies ex-
ceedingly, sometimes there is so little that it cannot be recog-
nized with certainty between the fully-developed cells, and can
only be observed between those most recently formed; for
instance, in the second class of tissues ; at other times there is

SURVEY OF CELL-LIFE. 169

so large a quantity present, that the cells contained in it do
not come into contact, as is the case in most cartilages. The
chemical and physical properties of the cytoblastema are not
the same in all parts. In cartilages it is very consistent, and
ranks among the most solid parts of the body; in areolar
tissue it is gelatinous; in blood quite fluid. These physical
distinctions imply also a chemical difference. The cytoblas-
tenia of cartilage becomes converted by boiling into gelatine,
which is not the case with the blood; and the mucus in which
the mucus-cells are formed differs from the cytoblastema of
the cells of blood and cartilage. The cytoblastema, external
to the existing cells, appears to be subject to the same
changes as the cell-contents ; in general it is a homogeneous
substance ; yet it may become minutely granulous as the re-
sult of a chemical transformation, for instance, in areolar
tissue and the cells of the shaft of the feather, &c. As a
general rule, it diminishes in quantity, relatively with the deve-
lopment of the cells, though it seems that in cartilages there
may be even a relative increase of the cytoblastema propor-
tionate to the growth of the tissue. The physiological relation
which the cytoblastema holds to the cells may be twrofold :
first, it must contain the material for the nutrition of the
cells ; secondly, it must contain at least a part of what remains
of this nutritive material after the cells have withdrawn from
it what they required for their growth. In animals, the cyto-
blastema receives the fresh nutritive material from the blood-
vessels; in plants it passes chiefly through the elongated cells
and vascular fasciculi ; there are, however, many plants which
consist of simple cells, so that there must also be a transmis-
sion of nutrient fluid through the simple cells ; blood-vessels and
vascular fasciculi are, however, merely modifications of cells.

Lav)S of the generation of new cells in the cytoblastema. —
In every tissue, composed of a definite kind of cells, new cells
of the same kind arc formed at those parts only where the
fresh nutrient material immediately penetrates the tissue.
On this depends the distinction between organized or vas-
cular, and unorganized or non-vascular tissues. In the former,
the nutritive fluid, the liquor sanguinis, permeates by means
of the vessels the whole tissue, and therefore new cells origi-

170 SURVEY OF CELL-LIFE.

nate throughout its entire thickness. Non-vascular tissues,
on the contrary, such as the epidermis, receive the nutri-
tive fluid onlv from the tissue beneath; and new cells
therefore originate only on their under surface, that is, at the
part where the tissue is in connexion with organized sub-
stance. So also in the earlier period of the growth of carti-
lage, while it is yet without vessels new cartilage-cells are
formed around its surface only, or at least in the neigh-
bourhood of it, because the cartilage is connected with
the organized substance at that part, and the cytoblastema
penetrates from without. We can readily conceive this to be
the case, if we assume that a more concentrated cytoblastema
is requisite for the formation of new cells than for the growth
of those already formed. In the epidermis, for instance, the
cytoblastema below must contain a more concentrated nutri-
tive material. When young cells are formed in that situation,
the cytoblastema, which penetrates into the upper layers, is less
concentrated, and may therefore serve very well for the growth
of cells already formed, but not be capable of generating
new ones. This constitutes the distinction which was formerly
made between a growth by apposition and one by intussuscep-
tion ; (t growth by apposition" is a correct term, if it be
applied to the generation of new cells, and not to the growth
of those already existing, the new cells in the epidermis for
example, are formed only on its under surface, and are pushed
upwards when other new ones are formed beneath them;
but the new cells are generated throughout the entire thick-
ness of the organized tissues. The cells, however, grow in-
dividually by intussusception in both instances. The bones oc-
cupy, to a certain extent, a middle position between the organized
and unorganized tissues. The cartilage in the first instance
has no vessels, and the new cells are, therefore, formed in the
neighbourhood of the external surface only ; at a subsequent
period it receives vessels, which traverse the medullary or Haver-
sian canals, the latter, however, are not sufficiently numerous to
allow of the entire tissue becoming equably saturated with the
fluid parts of the blood, a process which would be still further
impeded by the greater firmness of cartilage and bone.
According to the above law, then, the formation of new
cytoblastema and new cells may take place partly upon the

SURVEY OF CELL-LIFE. 171

surface and partly around these medullary canals. Now, the
structure of bone becomes most simple, if we assume that,
in consequence of the firmness of the osseous substance, this
process goes on in layers, which do not completely coalesce
together. It must consist of a double system of layers, one
being concentric to each of the medullary canals, and the
other to the external surface of the bone. When the bone is
hollow, the layers must also be concentric to the cavity; and
when small medullary cavities exist in the place of canals,
as in the spongy bones, the layers must also be concentric to
them. The difference in the growth of animals and plants
also rests upon the same law. In plants, the nutritive fluid
is not so equably distributed throughout the entire tissues,
as it is in the organized tissues of animals, but is conveyed in
isolated fasciculi of vessels, widely separated from one another,
more after the manner of bone. These fasciculi of vessels are
also observed to be surrounded with small (most likely
younger) cells, so that, in all probability, the formation of
their new cells also takes place around these vessels, as it does
in bones around the medullary canaliculi. In the stem of
dicotyledonous plants the sap is conducted between the bark
and the wood, and on that account the new cells are generated
in strata concentric to the layers of the previous year. The
variety in the mode of growth, as to whether the new cells
are developed merely in separate situations in the tissue, or
equally throughout its whole thickness, does not, therefore,
constitute any primary distinction, but is the consequence of
a difference in the mode in which their nutritive fluid is
conveyed.

The generation of cells of a different character, such as fat-
cells, in the interior of a non-vascular tissue (in cartilage
which does not as yet contain vessels, for example), appears at
first sight to form an exception to the law just laid down. But
such is not really the case j the circumstance is capable of
two explanations, either the cytoblastema for this kind of
cells is furnished by the true cells of the tissue only when they
have attained a certain stage of their development, or, the
cytoblastema which penetrates into the depth of the tissue
contains the nutritive material for the true cells of the
tissue in a less concentrated state, whilst it is still sufficiently

172 SURVEY OF CELL-LIFE.

impregnated with the nutritive material for the other kind of
cells.

According to Schleiden, new cells are never formed in the
intercellular substance in plants ; in animals, on the contrary,
a generation of cells within cells is the less frequent mode, but
this does occur, and in such a way, that a threefold or four-
fold generation may take place in succession within one cell.
Thus, according to R. Wagner's observations (see the Supple-
ment), the Graafian vesicle appears to be an elementary cell ;
the ovum is developed within it in like manner as an element-
ary cell; within this, again, according at least to observations
made upon the bird's egg, cells are generated, some of which
contain young cells. It appears also, that a formation of
true cartilage-cells can sometimes take place within those
which already exist, and that young cells (fat-cells ?) may
be generated within them again. Several such examples
might be brought forward ; but by far the greater portion
of the cells of cartilage are formed in the cytoblastema on
the outside of the cells already present, and we never meet
with a generation of cells within cells in the case of fibre,
muscle, or nerve.

General phenomena of the formation of cells. Round
corpuscles make their appearance after a certain time in the
cytoblastema which, in the first instance, is structure-
less or minutely granulous. These bodies may either be
cells in their earliest condition (and some may be recognized
even at this stage), that is, hollow vesicles furnished with a
peculiar structureless wall, cells without nuclei, or they may
be cell-nuclei or the rudiments of cell-nuclei, round which cells
will afterwards be formed.

The cells without nuclei, or, more correctly, the cells in
which no nuclei have as yet been observed, occur only
in the lower plants, and are also rare in animals. For the
present, however, the following must be regarded as such,
viz. : the young cells contained within others in the chorda
dorsalis (see p. 13), the cells of the yelk -substance in the
bird's egg (p. 50), the cells in the mucous la}-er of the ger-
minal membrane of the bird's egg (p. 60), and some cells of
the crystalline lens (p. 88). PI. I, fig. 10, c, represents one

SURVEY OF CELL-LIFE. 173

of these cells without nuclei. Thus the mode of growth, in
this instance, is similar to that of the nucleated cells, after the
formation of their cell-membrane

By far the greater portion of the animal body, at least
ninety-nine hundredths of all the elementary parts of the bodii
of mammalia are developed from nucleated cells.

T/i e cell -nucleus is a corpuscle, having a very characteristic
form, by which it may in general be easily recognized. It
is rather round or oval, spherical or flat. In the majority of fully-
developed animal cells its average size would be about 0*0020-
00030 Paris inch ; but we meet with nuclei which are very
much larger, and others, again, much smaller than this. The
germinal vesicle of the bird's cg^ may be regarded as the
largest cell-nucleus ; the nuclei of the blood-corpuscles of
warm-blooded animals afford examples of very small cell-
nuclei. If the latter were but a very little smaller they would
escape observation altogether, and the blood-corpuscles would
then appear to be cells without nuclei. No other structure
can be detected in these verv small nuclei, nor can their cha-
racteristic form be further demonstrated. On the other hand,
that of the larger blood-corpuscles may be distinctly recog-
nized as a cell-nucleus.

The cell-nucleus is generally dark, granulous, often some-
what yellowish ; but some occur which are quite pellucid and
smooth. It is either solid, and composed of a more or less
minutely granulated mass, or hollow. Most nuclei of animal
cells exhibit more or less distinct trace of a cavity, at least,
their external contour is generally somewhat darker, and the
substance of the nucleus seems to be somewhat more com-
pact at the circumference. The nucleus may often be traced
through its progressive stages of development from a solid
body to a perfect vesicle ; this may be observed in the nuclei
of the cartilage-cells in the branchial cartilages of tadpoles.
The membrane of the cell-nucleus and its contents may be
distinguished in those which are hollow. The membrane is
smooth, structureless, and never of any remarkable thickness,
that of the germinal vesicle being the thickest. The con-
tents are either very minutely granulous, especially in the
small hollow cell-nuclei, or pellucid, as in the germinal
vesicle, and the larger nuclei in the cells of the branchial carti-

174 SURVEY OF CELL-LIFE.

lages of the tadpole, or larger corpuscles may be subsequently
formed in the interior of hollow nuclei, for instance, the
innumerable corpuscles in the germinal vesicle of the fish,
and fat-globules in the nucleus of the fat-cells in the cranial
cavity of fishes.

The nucleus, in most instances, contains one or two, more
rarely three or four small dark corpuscles, the nucleoli. Their
size varies from that of a spot which is scarcely discernible to
that of "Wagner's spot [macula germinativa) in the germinal vesi-
cle. Nucleoli cannot be distinctly recognized in all cell-nuclei.
They may be distinguished from the larger corpuscles, which are
sometimes developed in certain hollow nuclei, from the circum-
stance of their being formed at a much earlier period ; they
exist, indeed, before the cell-nucleus. They are placed eccen-
trically in the round nuclei, and in the hollow ones are dis-
tinctly seen to lie upon the internal surface of the wall. It is
very difficult to ascertain their nature ; it may also vary very
much in different cells. They sometimes appear to be capable
of considerable enlargement, as in the nuclei of the fat-cells in
the cranial cavity of the fish, and in such instances often have
the appearance of fat. According to Schleiden, hollow nucleoli
also frequently occur in plants.

Most cell-nuclei agree in the peculiarity of not being dis-
solved, or rendered transparent by acetic acid, at least not
rapidly so, whilst the cell-membrane of animal cells is in
most cases very sensitive to its action. Some cells, (such
as those of the yelk-cavity of the e^g} plate II, fig. 3,)
which have no perceptible nucleus of the ordinary form, ex-
hibit a globule having the appearance of a fat-globule, which
grows as the cell expands, though not in the same proportion,
and was probably formed previous to the cell. Whether such
a globule have the signification of a nucleus or not, must re-
main an undecided question.

The formation of the cell-nucleus. In plants, according to
Schleiden, the nucleolus is first formed, and the nucleus around
it. The same appears to be the case in animals. According
to the observations of R. Wagner on the development of ova
in the ovary of Agrion virgo,1 the germinal spot is first

1 See Wagner, Beitrage zur Geschichte der Zeugung und Entwickelung; Erster
Beitrag., tab. II, fig. 1.

SURVEY OF CELL-LIFE. 175

formed, and around that the germinal vesicle, which is the
nucleus of the ovum-cell, Eizelle.1 The youngest germinal
vesicle there represented by Wagner, appears to be hollow .
This is not generally the case, however, in the formation of
cell-nuclei. Plate III, fig. 1, e, appears to be a cell-nuclens
of a cartilage-cell in the act of forming. A small round
corpuscle is there seen, surrounded by some minutely gra-
nulous substance, whilst the rest of the cytoblastema i>
homogeneous. This granulous substance is gradually lost
around the object; at a subsequent period it begins to
be sharply defined, and then exhibits the form of a cell-
nucleus, which continues to grow for a certain period. (See
pi. Ill, fig. 1, a, b.) Such a nucleus usually appears solid
in the first instance, and many nuclei remain in this con-
dition ; in others, on the contrary, the portion of the sub-
stance situated nearest to the external surface continuallv

m

becomes darker, and not unfrequently at last forms a dis-
tinctly perceptible membrane, so that the nucleus is hollow
in such instances. The formative process of the nucleus
may, accordingly, be conceived to be as follows : A nucle-
olus is first formed ; around this a stratum of substance
is deposited, which is usually minutely granulous, but not as
yet sharply defined on the outside. As new molecules are
constantly being deposited in this stratum between those
already present, and as this takes place within a precise dis-
tance of the nucleolus only, the stratum becomes defined
externally, and a cell-nucleus having a more or less sharp con-
tour is formed. The nucleus grows by a continuous depo-
sition of new molecules between those already existing, that
is, by intussusception. If this go on equably throughout the
entire thickness of the stratum, the nucleus may remain solid ;
but if it go on more vigorously in the external part, the latter
will become more dense, and may become hardened into
a membrane, and such are the hollow nuclei. The circum-
stance of the layer generally becoming more dense on its
exterior, may be explained by the fact that the nutritive fluid
is conveyed to it from the outside, and is therefore more con-
centrated in that situation. Now if the deposition of the new

1 See the Supplement.

176 SURVEY OF CELL-LIFE.

molecules between the particles of this membrane takes place
in such a manner that more molecules are deposited between
those particles which lie side by side upon its surface than
there are between those which lie one beneath another in its
thickness, the expansion of the membrane must proceed more
vigorously than its increase in thickness, and therefore a con-
stantly increasing space must be formed between it and the
nucleolus, wherebv the latter remains adherent to one side of
its internal surface.

I have made no observations on the formation of nuclei with
more than one nucleolus. But it is easy to comprehend
how it may occur, if we conceive that two nucleoli may lie
so close together that the layers which form around them
become united before they are defined externally, and that by
the progressive deposition of new molecules, the external limi-
tation is so effected that two corpuscles are enclosed by it at
the same time, and then the development proceeds as though
only one nucleolus were present.

When the nucleus has reached a certain stage of develop-
ment, the cell is formed around it. The following appears to
be the process by which this takes place. A stratum of sub-
stance, which differs from the cytoblastema, is deposited upon
the exterior of the nucleus. (See pL III, fig. 1, d.) In the
first instance this stratum is not sharply defined externally,
but becomes so in consequence of the progressive deposition
of new molecules. The stratum is more or less thick, some-
times homogeneous, sometimes granulous ; the latter is most fre-
quently the case in the thick strata which occur in the forma-
tion of the majority of animal cells. We cannot at this period
distinguish a cell-cavity and cell-wall. The deposition of new
molecules between those already existing proceeds, however,
and is so effected that when the stratum is thin, the entire
layer — and when it is thick, only the external portion — be-
comes gradually consolidated into a membrane. The external
portion of the layer may begin to become consolidated soon
after it is defined on the outside; but, generally, the membrane
does not become perceptible until a later period, when it is
thicker and more defined internally ; many cells, however, do
not exhibit any appearance of the formation of a cell-mem-
brane, but they seem to be solid, and all that can be remarked

SURVEY OF CELL-LIFE. 177

is that the external portion of the layer is somewhat more
compact.

Immediately that the ccll-membranc has become consoli-
dated, its expansion proceeds as the result of the progressive
reception of new molecules between the existing ones, that is
to say, by virtue of a growth by intussusception, while at the
same time it becomes separated from the cell-nucleus. We
may therefore conclude that the deposition of the new mole-
cules takes place more vigorously between those which lie side
by side upon the surface of the membrane, than it does between
those which lie one upon another in its thickness. The inter-
space between the cell-membrane and cell-nucleus is at the
same time filled with fluid, and this constitutes the cell-con-
tents. During this expansion the nucleus remains attached
to a spot on the internal surface of the cell-membrane. If the
entire stratum, in which the formation of the cell commenced,
have become consolidated into a cell-membrane, the nucleus
must lie free upon the cell-wall ; but if only the external por-
tion of the stratum have become consolidated, the nucleus must
remain surrounded by the internal part, and adherent to a spot
upon the internal surface of the cell-membrane. It wrould seem
that the portion of the stratum which remains may be disposed
of in two ways : either it is dissolved and forms a part of the
cell-contents, in which case the nucleus will lie free upon the
cell- wall as before ; or it gradually becomes condensed into a
substance similar to the cell-membrane, and then the nucleus
appears to lie in the thickness of the cell-wall. This explains
the variety in the position of the nucleus with respect to the
ccll-membranc. According to Schleiden, it sometimes lies in
the thickness of the membrane in plants, so that its internal
surface, which is directed towards the cell-cavitv, is covered
by a lamella of the cell-walJ. In animals it also sometimes
appears to be slightly sunk in the cell-membrane j but I have
never observed a lamella passing over its inner surface ; on the
contrary, in almost all instances it lies quite free, adherent
only to the internal surface of the cell-membrane.

The particular stage of development of the nucleus at which
the cell commences to be formed around it varies very much.
In some instances the nucleus has already become a distinct

VI

178 SURVEY OF CELL-LIFE.

vesicle ere it occurs; the germinal vesicle, for example; in
others, and this is the most common, the nucleus is still solid,
and its development into a vesicle does not take place until a
later period, or perhaps the change never occurs at all. When
the cell is developed, the nucleus either remains stationary at
its previous stage of development, or its growth proceeds, but
not in proportion to the expansion of the cell, so that the
intermediate space between it and the cell-membrane, the cell-
cavity, is also constantly becoming relatively larger. If the
growth of a cell is impeded by the neighbouring cells, or if
the new molecules added between the existing particles of
the cell-membrane are applied to the thickening of the
cell- wall instead of to its expansion, it may occur that the
nucleus becomes more vigorously expanded than the cell, and
gradually fills a larger portion of the cell-cavity. An example
of this was brought forward at page 23, from the branchial
cartilages of the tadpole ; on the whole, however, such instances
are very rare. As the nuclei, in the course of their develop-
ment, and especially in such instances as that just mentioned,
continually lose their granulous contents and become pellucid,
and as in some cases, the germinal vesicle for example, other
corpuscles, such as fat -globules, &c, may be developed in these
contents of the nucleus (a circumstance which never occurs
with respect to the cell-cavities) it is often difficult to distin-
guish such enlarged nuclei from young cells. The presence
of two nucleoli is often sufficient to enable us to distinguish
such an enlarged hollow nucleus. The observation of the
stages of transition, between the characteristic form of the
cell-nucleus and these nuclei which so much resemble cells,
will also aid us in obtaining the information desired. As in
the case of the germinal vesicle, however, a positive decision
can only be obtained by demonstrating that such a nucleus
has precisely the same relation to the cell that an ordinary
cell-nucleus has ; that is to say, that such a nucleus is formed
before the cell, that the latter is formed as a stratum around
it, and that the nucleus is afterwards surrounded by the cell.
Whether the nucleus undergoes any further development, as
the expansion of the cell proceeds, or not, the usual result is
that it becomes absorbed. This does not take place, however,

SURVEY OF CELL-LIFE. 179

in all cases, for, according to Schleiden, it remains persistent
in most cells in the Euphorbiaccie, and the blood-corpuscles
may be quoted as an example to the same effect in animals.

The fact that many nuclei are developed into hollow resides,
and the difficulty of distinguishing some of these hollow nuclei
from cells, forms quite sufficient ground for the supposition
that a nucleus does not differ essentially from a cell ; that an
ordinary nucleated cell is nothing more than a cell formed
around the outside of another cell, the nucleus; and that
the only difference between the two consists in the inner
one being more slowly and less completely developed, after
the external one has been formed around it. If this descrip-
tion were correct, we might express ourselves with more pre-
cision, and designate the nuclei as cells of the first order, and
the ordinary nucleated cells as cells of the second order.
Hitherto we have decidedly maintained a distinction between
cell and nucleus ; and it was convenient to do so as long as
we were engaged in merely describing the observations. There
can be no doubt that the nuclei correspond to one another in
all cells ; but the designation, " cells of the first order," in-
cludes a theoretical view of the matter which has yet to be
proved, namely, the identity of the formative process of the
cell and the nucleus. This identity, however, is of the greatest
importance for our theory, and we must therefore compare
the two processes somewhat more closely. The formation
of the cell commenced with the deposition of a precipitate
around the nucleus ; the same occurs in the formation of the
nucleus around the nucleolus. The deposit becomes defined
externally into a solid stratum : the same takes place in the
formation of the nucleus. The development proceeds no
farther in many nuclei, and we also meet with cells which
remain stationary at the same point. The further development
of the cells is manifested either by the entire stratum, or only
the external part of it becoming consolidated into a membrane;
this is precisely what occurs with the nuclei which undergo
further development. The cell -membrane increases in its
superficies, and often in thickness also, and separates from
the nucleus, which remains lying on the wall ; the membrane
of the hollow cell-nuclei grows in the same manner, and the
nucleolus remains adherent to a spot upon the wall. A trans-

180 SURVEY OF CELL-LIFE.

formation of the cell-contents frequently follows, giving rise to
a formation of new products in the cell-cavity. In most of
the hollow cell-nuclei, the contents become paler, less granu-
lous, and in some of them fat -globules, &c, are formed. (See
pages 173, 4.) We may therefore say that the formation of cells
is but a repetition around the nucleus of the same process by
which the nucleus was formed around the nucleolus, the only
difference being that the process is more intense and complete
in the formation of cells than in that of nuclei.

According to the foregoing, then, the whole process of the
formation of a cell consists in this, that a small corpuscle (the
nucleolus) is the earliest formation, that a stratum (the nucleus)
is first deposited around it, and then subsequently a second
stratum (substance of the cell) around this again. The sepa-
rate strata grow by the reception of new molecules between
the existing ones, by intussusception, and we have here an illus-
tration of the law, in deference to which the deposition takes
place more vigorously in the external part of each stratum than
it does in the internal, and more vigorously in the entire ex-
ternal stratum than in the internal. In obedience to this law
it often happens that only the external part of each stratum
becomes condensed into a membrane (membrane of the nucleus
and membrane of the cell), and the external stratum becomes
more perfectly developed to form a cell, than the nucleus does.
When the nucleoli are hollow, which, according to Schleiden,
is the case in some instances in plants, perhaps a threefold
process of the kind takes place, so that the cell-membrane
forms the third, the nucleus the second, and the nucleolus the
first stratum. Probably merely a single stratum is formed
around an immeasurably small corpuscle in the case of those
cells which have no nuclei.

Varieties in the development of the cells in different tissues.
Although, as we have just seen, the formative process of the
cells is essentially the same throughout, and dependent upon a
formation of one or many strata, and upon a growth of those
strata by intussusception, the changes, on the other hand, which
the cells, when once formed, undergo in the different tissues,
are, in their phenomena at least, much more varied. They
may be arranged in two classes according as the individuality

SURVEY OF CELL-LIFE. 181

of the original cell is retained (independent cells), or as it is
more or less completely lost (coalescing cells, and cells which
undergo division).

The varieties which occur amongst the independent cells, are
partly of a chemical nature, and partly have reference to a dif-
ference in the growth of the cell-membrane, by which means
a change in the form of the cell may be produced.

The cell-membrane differs in respect to its chemical quali-
ties in different kinds of cells. That of the blood-corpuscles,
for instance, is dissolved by acetic acid, whilst that of the
cartilage-cell is not. The chemical composition of the cell-
membrane differs even in the same cell according to its age,
so that a transformation of the substance of the membrane
itself takes place in plants ; for, according to Schleiden, the
cell-membrane of the youngest cells dissolves in water, the
fully-developed cells not being acted upon by that fluid. The
simple cells are still more remarkable for their cell-contents.
One cell forms fat, another pigment, a third etherial oil ; and
here also a transformation of the cell-contents takes place. A
granulous precipitate is seen to form gradually in what was in
the first instance a pellucid cell, and this usually takes place first
around the cell-nucleus ; or, vice versa, during incubation, the
granulous (fatty) contents of the cells of the yelk-substance
gradually undergo partial solution. According to Schleiden,
this transformation of the substance of the cell-contents pro-
ceeds in accordance with a certain rule ; I have not made any
investigations upon the subject in animals.

We should also include under this head the formation of
the secondary deposits upon the internal surface of the cell-
membrane, so very frequently met with in plants. If a firm
cohering substance be formed from the cell-contents, it may
be deposited upon the internal surface of the cell-membrane.
In plants this deposition generally takes place in la vers, a
stratum being formed in the first instance upon the internal
surface of the cell-membrane, upon the internal surface of
that one a second, and so on until at last the entire cavity
may be almost filled by them. According to Valentin, these
surrounding deposits always take place in spiral lines which
are subject to great varieties in their arrangement, for there
may be one or many of them, and they may cither completely

182 SURVEY OF CELL-LIFE.

cover the internal surface of the cell-membrane, or not be in
contact with each other at all. I have not observed any such
secondary stratified depositions in animals.

The variations which may occur in the growth of the cell-
membrane in simple cells, depend upon the circumstance as to
whether or not the addition of new molecules takes place
equably at all parts of the cell-membrane. In the first case
the form of the cell remains unchanged, and the only other
distinction possible would be grounded upon the fact as to
whether the greater part of the new molecules were deposited
between the particles which lay side by side upon the super-
ficies of the cell-membrane, or between those which lay one
behind another in its thickness. The first mode of growth
produces an expansion of the cell-membrane, the effect of the
second is more especially to thicken it. Both modes are gene-
rally combined, but in such a manner that the expansion of the
cell-membrane prevails in most instances.

A great variety of modifications in the form of the cells may
be produced by the irregular distribution of the new mole-
cules. The globular, which is their fundamental form, may
be converted into a polyhedral figure, or the cells may become
flattened into a round or oval or angular tablet, or the expan-
sion of the cells may take place on one or on two opposite sides,
so as to form a fibre, and these fibres again may either be flat,
being at the same time in some instances serrated, or lastly,
the expansion of the cells into fibres may take place on dif-
ferent sides so as to give them the stellate form. Some of
these changes of form are, no doubt, due to mechanical causes.
Thus, for example, the polyhedral form is produced by the
close crowding of the spherical cells, and these, when separated
from one another, sometimes assume their round figure again;
such is the case with the velk-cells. Some of the other
changes would seem to be capable of explanation by exosmosis.
If, for example, the contents of a round cell be so changed,
that a fluid is generated in it which is less dense than the
surrounding fluid, the cell will lose some of its contents by
exosmosis, and must, therefore, collapse, and may become
flattened into a table as in the blood-corpuscles. Such expla-
nations, however, are unsatisfactory in by far the greatest num-
ber of instances, and we are compelled to assume, that the

SURVEY OF CELL-LIFE. 183

growth docs not necessarily proceed equably on all sides, but
that the new molecules may be deposited in greater abundance
in certain situations. Let us take the instance of a round
cell, the cell -membrane of which is already developed, and
suppose the deposition of new molecules to be confined to one
particular part of the cell-membrane, that part would become
expanded, and so a hollow fibre would grow forth from the cell,
the cavity of which would communicate with the cell-cavity.
The same result would take place, but more easily, if the new
molecules were disposed unequally previous to the period when
the external stratum of the precipitate, which is formed around
the nucleus, had become condensed into a distinctly perceptible
cell-membrane. The hollowing out of the fibre would then be
less perfect, and the growth of the fibre must advance, particu-
larly as regarded its thickness, before any manifest distinction
between wall and cavity could be perceived.

The cause of this irregular disposition cf the new molecules
may, in some instances, be due to circumstances altogether ex-
ternal to the cell. If, for instance, a cell lay in such a position
that one side of it was in contact with a concentrated nutri-
tive material, one could conceive that side of the cell growing
more vigorously, even though the force, which produces the
growth of the cell, should operate equably throughout the entire
cell. Such an explanation cannot, however, be received at all
in most instances, but we must admit modifications in the
principle of development of the cells, of such a nature, as that
the force, which affects the general growth of the cells, is
enabled to occasion an equable disposition of new molecules in
one cell, and an unequal one in another.

Amongst the changes which more or less completely deprive
the original cells of their individuality, are to be classed, in the
first place, the coalescence of the cell-walls with one another,
or writh the intercellular substance : secondlv, the division of
one cell into several ; and, thirdly, the coalescence of several
primary cells to form a secondary one.

A coalescence of the ccll-mcmbranc with the intercellular sub-
stance, or with a neighbouring cell-wall, appears to take place
in some cartilages for example. At first the cell-membrane has
a sharply-defined external contour, by degrees the boundary
line becomes paler, and at last is no longer perceptible with

184 SURVEY OF CELL-LIFE.

the microscope. We cannot, at present, lay down any general
law respecting the circumstances under which such a coalescence
occurs; it presupposes that the cell-membrane and intercellular
substance are homogeneous structures, and may perhaps always
take place when such a state exists.

As regards the subdivision of the cells, we have already seen
how a jutting out of the cell-membrane may be produced by
its more vigorous growth in certain situations. But a jutting
inwards into the cavity of the cell may also result from the
very same process. Now, if we imagine this jutting inwards to
take place in a circular form around a cell, as the consequence
of a partial increase in the force of its growth, it may proceed
to such an extent, that one cell may be separated into two,
connected together only by a short peduncle, which may after-
wards be absorbed. This would illustrate the most simple form
of subdivision in a cell. In the animal cells, however, which
undergo subdivision, that is, the fibre-cells, the process is more
complicated; firstly, because when an elongated cell subdivides,
it splits into many fibres; and, secondly, because the cells are so
very minute. The process, therefore, cannot for these reasons
be accurately traced, and the following is all that we can de-
tect : a cell becomes elongated on two opposite sides into several
fibres; from the angle, which the fibres "on either side form with
each other, a striated appearance gradually extends over the
body of the cell ; this formation of striae becomes more and more
distinct, until the body of the cell splits entirely into fibres.

The coalescence of several primary cells to form a secondary
cell is, to a certain extent, the opposite process to the last.
Several primary cells, of muscle for instance, are arranged close
together in rows, and coalesce into a cylinder, in the thickness
of which lie the nuclei of the primary cells. This cylinder is
hollow and not interrupted by septa, and the nuclei lie upon the
internal surface of its wall. These are the facts of the process,
so far as they have as yet been observed. One can form a
conception of so much as is yet required to render them com-
plete. If two perfectly-developed cells coalesce together, their
walls must first unite at the point of contact, and then the
partition-wall between the cavities must be absorbed. Nature,
however, does not by any means require that these acts should
occur at precisely defined periods. The coalescence may take

SURVEY OF CELL-LIFE. 185

place before the cell-wall and cell-cavity exist as distinct struc-
tures, somewhat in the following manner : the nuclei are formed
first, around them a new stratum of substance is deposited, the
external portion of which, in accordance with the course of
formation of an ordinary simple cell, would become condensed
into a cell-membrane. But in this instance the nuclei lie so
close together, that the strata forming around them and corre-
sponding to the cells, flow together, to form a cylinder, the ex-
ternal portion of which becomes condensed into a membrane,
just in the same manner as in simple cells, where merely the
external portion of the stratum formed around the nucleus,
becomes hardened on the outside into a membrane, in conse-
quence of the reception of new molecules. There is, therefore,
nothing in this which differs so very materially from the course
of development of a simple cell; indeed, we seemed to be com-
pelled to assume a similar process for the formation of the nuclei
furnished with two or more nucleoli. (See page 176.) It is
possible that there may be stages of transition between the
ordinary simple cell and these secondary cells. It has been
already mentioned at pages 117-118, that fat-cells occur in the
cranial cavity of fishes, many of which contain two nuclei.
It is possible that only one of them is the cytoblast of the
cell, and that the second is a nucleus which has formed subse-
quently ; but they resemble one another so completely in their
characteristic position on the cell-membrane (see pi. Ill, fig.
10,) that perhaps they may both be cytoblasts of a cell which
has been formed around both nuclei, in consequence of the ex-
ternal stratum of the precipitate having become condensed in
such a manner that the membrane enclosed both nuclei. Mean-
while observation affords no demonstrative proof on the sub-
ject, and the similarity in the position of these two nuclei may
be explained in another way. Fat thrusts all bodies which have
imbibed water towards the outside of the cell, in order that it
may assume its own globular form. If now a second nucleus
should form in one of these fat-cells, it will be thrust towards
the outside, and must gradually raise the cell-membrane into a
prominence. It may also be observed, that opportunities of
demonstrating the actual absorption of the fully- developed
partition-wall between two cells do occur in the spiral vessels of
plants.

186 THEORY OF THE CELLS.

THEORY OF THE CELLS.

The whole of the foregoing investigation has been con-
ducted with the object of exhibiting from observation alone the
mode in which the elementary parts of organized bodies are
formed. Theoretical views have been either entirely excluded,
or where they were required (as in the foregoing retrospect of
the cell-life), for the purpose of rendering facts more clear, or
preventing subsequent repetitions, they have been so presented
that it can be easily seen how much is observation and how
much argument. But a question inevitably arises as to the
basis of all these phenomena ; and an attempt to solve it will
be more readily permitted us, since by making a marked sepa-
ration between theory and observation the hypothetical may be
clearly distinguished from that which is positive. An hypo-
thesis is never prejudicial so long as we are conscious of the
degree of reliance which may be placed upon it, and of the
grounds on which it rests, Indeed it is advantageous, if not
necessary for science, that when a certain series of phenomena
is proved by observation, some provisional explanation should be
conceived that will suit them as nearly as possible, even though
it be in danger of being overthrown by subsequent observations;
for it is only in this manner that we are rationally led to new
discoveries, which either establish or refute the explanation. It
is from this point of view7 1 would beg that the following theory
of organization may be regarded ; for the inquiry into the source
of development of the elementary parts of organisms is, in fact,
identical with the theory of organized bodies.

The various opinions entertained with respect to the funda-
mental powers of an organized body may be reduced to two,
which are essentially different from one another. The first is,
that every organism originates with an inherent power, which
models it into conformity with a predominant idea, arranging
the molecules in the relation necessary for accomplishing certain
purposes held forth by this idea. Here, therefore, that which
arranges and combines the molecules is a power acting with a
definite purpose. A power of this kind wTould be essentially
different from all the powers of inorganic nature, because action

THEORY OF THE CELLS. 187

goes on in the latter quite blindly. A certain impression is
followed of necessity by a certain change of quality and quantity,
without regard to any purpose. In this view, however, the
fundamental power of the organism (or the soul, in the sense
employed by Stahl) would, inasmuch as it works with a definite
individual purpose, be much more nearly allied to the im-
material principle, endued with consciousness which we must
admit operates in man.

The other view is, that the fundamental powers of organized
bodies agree essentially with those of inorganic nature, that
they work altogether blindly according to laws of necessity and
irrespective of any purpose, that they are powers which are as
much established with the existence of matter as the phy-
sical powers are. It might be assumed that the powrers which
form organized bodies do not appear at all in inorganic nature,
because this or that particular combination of molecules, by
which the powers are elicited, does not occur in inorganic
nature, and yet they might not be essentially distinct from
physical and chemical powers. It cannot, indeed, be denied
that adaptation to a particular purpose, in some individuals
even in a high degree, is characteristic of every organism ;
but, according to this view, the source of this adaptation does
not depend upon each organism being developed by the opera-
tion of its own power in obedience to that purpose, but it
originates as in inorganic nature, in the creation of the matter
with its blind powers by a rational Being. AYe know, for
instance, the powers which operate in our planetary system.
They operate, like all physical powers, in accordance with
blind laws of necessity, and yet is the planetary system re-
markable for its adaptation to a purpose. The ground of
this adaptation does not lie in the powers, but in Him, who has
so constituted matter with its powers, that in blindly obeying its
laws it produces a whole suited to fulfil an intended purpose.
We may even assume that the planetary system has an indivi-
dual adaptation to a purpose. Some external influence, such ;i^
a comet, may occasion disturbances of motion, without thereby
bringing the whole into collision ; derangements may occur on
single planets, such as a high tide, &c., which are yet balanced
entirely by physical laws. As respects their adaptation to a
purpose, organized bodies differ from these in degree only;

188 THEORY OF THE CELLS.

and by this second view we are just as little compelled to
conclude that the fundamental powers of organization operate
according to laws of adaptation to a purpose, as we are in
inorganic nature.

The first view of the fundamental powers of organized bodies
may be called the ideological, the second the physical view.
An example will show at once, how important for physiology
is the solution of the question as to which is to be followed.
If, for instance, we define inflammation and suppuration to be
the effort of the organism to remove a foreign body that has
been introduced into it ; or fever to be the effort of the or-
ganism to eliminate diseased matter, and both as the result of
the " autocracy of the organism," then these explanations
accord with the teleological view. For, since by these pro-
cesses the obnoxious matter is actually removed, the process
which effects them is one adapted to an end ; and as the
fundamental power of the organism operates in accordance with
definite purposes, it may either set these processes in action
primarily, or may also summon further powers of matter to its
aid, always, however, remaining itself the " primum movens."
On the other hand, according to the physical view, this is just
as little an explanation as it would be to say, that the motion
of the earth around the sun is an effort of the fundamental
power of the planetary system to produce a change of seasons
on the planets, or to say, that ebb and flood are the reaction
of the organism of the earth upon the moon.

In physics, all those explanations which were suggested by
a teleological view of nature, as " horror vacui," and the like,
have long been discarded. But in animated nature, adaptation
— individual adaptation — to a purpose is so prominently
marked, that it is difficult to reject all teleological explanations.
Meanwhile it must be remembered that those explanations,
which explain at once all and nothing, can be but the last
resources, when no other view can possibly be adopted; and there
is no such necessity for admitting the teleological view in the
case of organized bodies. The adaptation to a purpose which
is characteristic of organized bodies differs only in degree from
what is apparent also in the inorganic part of nature ; and the
explanation that organized bodies are developed, like all the
phenomena of inorganic nature, by the operation of blind laws

THEORY OF THE CELLS. 189

framed with the matter, cannot be rejected as impossible.
Reason certainly requires some ground for such adaptation, but
for her it is sufficient to assume that matter with the powers
inherent in it owes its existence to a rational Being. Once esta-
blished and preserved in their integrity, these powers may, in
accordance with their immutable laws of blind necessity, verv
well produce combinations, which manifest, even in a high degree,
individual adaptation to a purpose. If, however, rational power
interpose after creation merely to sustain, and not as an imme-
diately active agent, it may, so far as natural science is concerned,
be entirely excluded from the consideration of the creation.

But the teleological view leads to further difficulties in the
explanation, and especially with respect to generation. If we
assume each organism to be formed by a power which acts
according to a certain predominant idea, a portion of this power
may certainly reside in the ovum during generation j but then
we must ascribe to this subdivision of the original power, at
the separation of the ovum from the body of the mother, the
capability of producing an organism similar to that which the
power, of which it is but a portion, produced : that is, we must
assume that this power is infinitely divisible, and yet that each
part may perform the same actions as the whole power. If,
on the other hand, the power of organized bodies reside, like
the physical powers, in matter as such, and be set free only
by a certain combination of the molecules, as, for instance,
electricity is set free by the combination of a zinc and copper
plate, then also by the conjunction of molecules to form an
ovum the power may be set free, by which the ovum is capable
of appropriating to itself fresh molecules, and these newly-
conjoined molecules again by this very mode of combination
acquire the same power to assimilate fresh molecules. The
first development of the many forms of organized bodies — the
progressive formation of organic nature indicated by geology —
is also much more difficult to understand according to the
teleological than the physical view.

Another objection to the teleological view may be drawn
from the foregoing investigation. The molecules, as we have
seen, are not immediately combined in various Mays, as the
purpose of the organism requires, but the formation of the
elementary parts of organic bodies is regulated by laws which

190 THEORY OF THE CELLS.

are essentially the same for all elementary parts. One can
see no reason why this should be the case, if each organism be
endued with a special power to frame the parts according to
the purpose which they have to fulfil : it might much rather
be expected that the formative principle, although, identical
for organs physiologically the same, would yet in different
tissues be correspondingly varied. This resemblance of the
elementary parts has, in the instance of plants, already led to
the conjecture that the cells are really the organisms, and that
the whole plant is an aggregate of these organisms arranged
according to certain laws. But since the elementary parts of
animals bear exactly similar relations, the individuality of an
entire animal would thus be lost ; and yet precisely upon the
individuality of the whole animal does the assumption rest, that
it possesses a single fundamental power operating in accordance
with a definite idea.

Meanwhile we cannot altogether lay aside teleological views
if all phenomena are not clearly explicable by the physical view.
It is, however, unnecessary to do so, because an explanation,
according to the teleological view, is only admissible when the
physical can be shown to be impossible. In any case it con-
duces much more to the object of science to strive, at least, to
adopt the physical explanation. And I would repeat that,
when speaking of a physical explanation of organic phenomena,
it is not necessary to understand an explanation by known
physical powers, such, for instance, as that universal refuge
electricity, and the like ; but an explanation by means of
powers which operate like the physical powers, in accordance
with strict laws of blind necessity, whether they be also to be
found in inorganic nature or not.

We set out, therefore, with the supposition that an organized
body is not produced by a fundamental power which is guided
in its operation by a definite idea, but is developed, according
to blind laws of necessity, by powers which, like those of
inorganic nature, are established by the very existence of
matter. As the elementary materials of organic nature are
not different from those of the inorganic kingdom, the source
of the organic phenomena can only reside in another combi-
nation of these materials, whether it be in a peculiar mode of
union of the elementary atoms to form atoms of the second

THEORY OF THE CELLS. 191

order, or in the arrangement of these conglomerate molecules
when forming either the separate morphological elementary
parts of organisms, or an entire organism. We have here
to do with the latter question solely, whether the cause of
organic phenomena lies in the whole organism, or in its sepa-
rate elementary parts. If this question can be answered, a
further inquiry still remains as to whether the organism or its
elementary parts possess this power through the peculiar mode
of combination of the conglomerate molecules, or through the
mode in which the elementary atoms are united into con-
glomerate molecules.

We may, then, form the two following ideas of the cause of
organic phenomena, such as growth, &c. First, that the cause
resides in the totality of the organism. By the combination
of the molecules into a systematic whole, such as the organism
is in every stage of its development, a power is engendered,
which enables such an organism to take up fresh material from
without, and appropriate it either to the formation of new
elementary parts, or to the growth of those already present.
Here, therefore, the cause of the growth of the elementary
parts resides in the totality of the organism. The other mode
of explanation is, that growth does not ensue from a power
resident in the entire organism, but that each separate ele-
mentary part is possessed of an independent power, an inde-
pendent life, so to speak ; in other words, the molecules in each
separate elementary part are so combined as to set free a
power by which it is capable of attracting new molecules, and
so increasing, and the whole organism subsists only by means
of the reciprocal1 action of the single elementary parts. So
that here the single elementary parts only exert an active
influence on nutrition, and totality of the organism may indeed
be a condition, but is not in this view a cause.

In order to determine which of these two views is the cor-
rect one, we must summon to our aid the results of the pre-
vious investigation. We have seen that all organized bodies
are composed of essentially similar parts, namely, of cells ;
that these cells are formed and grow in accordance with essen-

1 The word "reciprocal action" must here he taken in its widest sense, as
implying the preparation of material by one elementary part, which another requires
for its own nutrition.

192 THEORY OF THE CELLS.

tially similar laws j and, therefore, that these processes must,
in every instance, be produced by the same powers. Now, if we
find that some of these elementary parts, not differing from
the others, are capable of separating themselves from the
organism, and pursuing an independent growth, we may thence
conclude that each of the other elementary parts, each cell,
is already possessed of power to take up fresh molecules and
grow ; and that, therefore, every elementary part possesses a
power of its own, an independent life, by means of which it
would be enabled to develop itself independently, if the relations
which it bore to external parts were but similar to those in
which it stands in the organism. The ova of animals afford
us examples of such independent cells, growing apart from the
organism. It may, indeed, be said of the ova of higher animals,
that after impregnation the ovum is essentially different from
the other cells of the organism ; that by impregnation there
is a something conveyed to the ovum, which is more to it than
an external condition for vitality, more than nutrient matter ;
and that it might thereby have first received its peculiar
vitality, and therefore that nothing can be inferred from it with
respect to the other cells. But this fails in application to those
classes which consist only of female individuals, as well as with
the spores of the lower plants ; and, besides, in the inferior
plants any given cell may be separated from the plant, and
then grow alone. So that here are whole plants consisting
of cells, which can be positively proved to have independent
vitality. Now, as all cells grow according to the same laws,
and consequently the cause of growth cannot in one case lie
in the cell, and in another in the whole organism ; and since
it may be further proved that some cells, which do not differ
from the rest in their mode of growth, are developed indepen-
dently, we must ascribe to all cells an independent vitality, that
is, such combinations of molecules as occur in any single cell,
are capable of setting free the power by which it is enabled
to take up fresh molecules. The cause of nutrition and
growth resides not in the organism as a whole, but in the
separate elementary parts — the cells. The failure of growth
in the case of any particular cell, when separated from an
organized body, is as slight an objection to this theory, as it
is an objection against the independent vitality of a bee, that

THEORY OF THE CELLS. 193

it cannot continue long in existence after being separated
from its swarm. The manifestation of the power which resides
in the cell depends upon conditions to which it is subject only
when in connexion with the whole (organism).

The question, then, as to the fundamental power of orga-
nized bodies resolves itself into that of the fundamental powers
of the individual cells. We must now consider the general
phenomena attending the formation of cells, in order to dis-
cover what powers may be presumed to exist in the cells to
explain them. These phenomena may be arranged in two
natural groups : first, those which relate to the combination of
the molecules to form a cell, and which may be denominated the
plastic phenomena of the cells ; secondly, those which result
from chemical changes either in the component particles of the
cell itself, or in the surrounding cytoblastema, and which may
be called metabolic phenomena (to ^Eraj3oA(/o)i', implying that
which is liable to occasion or to suffer change).

The general plastic appearances in the cells are, as we have
seen, the following : at first a minute corpuscle is formed,
(the nucleolus) ; a layer of substance (the nucleus) is then pre-
cipitated around it, which becomes more thickened and ex-
panded by the continual deposition of fresh molecules between
those already present. Deposition goes on more vigorously at
the outer part of this layer than at the inner. Frequently the
entire layer, or in other instances the outer part of it only,
becomes condensed to a membrane, which may continue to take
up new molecules in such a manner that it increases more
rapidly in superficial extent than in thickness, and thus an
intervening cavity is necessarily formed between it and the
nucleolus. A second layer (cell) is next precipitated around
this first, in which precisely the same phenomena arc repeated,
with merely the difference that in this case the processes, espe-
cially the growth of the layer, and the formation of the space
intervening between it and the first layer (the cell-cavity), go
on more rapidly and more completely. Such were the pheno-
mena in the formation of most cells; in some, however, there
appeared to be only a single layer formed, while in others (those
especially in which the nucleolus was hollow) there were three.
The other varieties in the development of the elementary parts
were (as we saw) reduced to these — that if two neighbouring

13

194 THEORY OF THE CELLS.

cells commence their formation so near to one another that the
boundaries of the layers forming around each of them meet at
any spot, a common layer may be formed enclosing the two
incipient cells. So at least the origin of nuclei, with two or
more nucleoli, seemed explicable, by a coalescence of the first
layers (corresponding to the nucleus), and the union of many
primary cells into one secondary cell by a similar coalescence
of the second layers (which correspond to the cell). But the
further development of these common layers proceeds as though
they were only an ordinary single layer. Lastly, there were
some varieties in the progressive development of the cells, which
were referable to an unequal deposition of the new molecules
between those already present in the separate layers. In this
wav modifications of form and division of the cells were ex-
plained. And among the number of the plastic phenomena in
the cells we may mention, lastly, the formation of secondary
deposits ; for instances occur in which one or more new layers,
each on the inner surface of the previous one, are deposited on
the inner surface of a simple or of a secondary cell.

These are the most important phenomena observed in the
formation and development of cells. The unknown cause,
presumed to be capable of explaining these processes in the
cells, may be called the plastic power of the cells. We will, in
the next place, proceed to determine how far a more accurate
definition of this power may be deduced from these phenomena.

In the first place, there is a power of attraction exerted in
the very commencement of the cell, in the nucleolus, which
occasions the addition of new molecules to those already pre-
sent. We may imagine the nucleolus itself to be first formed
by a sort of crystallization from out of a concentrated fluid.
For if a fluid be so concentrated that the molecules of the
substance in solution exert a more powerful mutual attraction
than is exerted between them and the molecules of the fluid
in which they are dissolved, a part of the solid substance must
be precipitated. One can readily understand that the fluid
must be more concentrated when new cells are being formed in
it than when those already present have merely to grow. For
if the cell is already partly formed, it exerts an attractive force
upon the substance still in solution. There is then a cause
for the deposition of this substance, which does not co-operate

THEORY OF THE CELLS. 10.0

when no part of the cell is yet formed. Therefore, the greater
the attractive force of the cell is, the less concentration of the
fluid is required ; while, at the commencement of the forma-
tion of a cell, the fluid must be more than concentrated. Bui
the conclusion which may be thus directly drawn, as to the
attractive power of the cell, may also be verified by observation.
Wherever the nutrient fluid is not equally distributed in a
tissue, the new cells are formed in that part into which the
fluid penetrates first, and where, consequently, it is most con-
centrated. Upon this fact, as we have seen, depended the
difference between the growth of organized and unorganized
tissues (see page 169). And this confirmation of the foregoing
conclusion by experience speaks also for the correctness of the
reasoning itself.

The attractive power of the cells operates so as to effect the
addition of new molecules in two ways, — first, in layers, and
secondly, in such a manner in each layer that the new mole-
cules are deposited between those already present. This is
only an expression of the fact ; the more simple law, by which
several layers are formed and the molecules are not all de-
posited between those already present, cannot yet be explained.
The formation of layers may be repeated once, twice, or thrice.
The growth of the separate layers is regulated by a law, that
the deposition of new molecules should be greatest at the part
where the nutrient fluid is most concentrated. Hence the
outer part particularly becomes condensed into a membrane
both in the layer corresponding to the nucleus and in that
answering to the cell, because the nutrient fluid penetrates
from without, and consequently is more concentrated at the
outer than at the inner part of each layer. For the same
reason the nucleus grows rapidly, so long as the layer of the
cell is not formed around it, but it either stops growing-
altogether, or at least grows much more slowly so soon as
the cell -layer has surrounded it ; because then the latter
receives the nutrient matter first, and, therefore, in a more
concentrated form. And hence the cell becomes, in a general
sense, much more completely developed, while the nucleus-
layer usually remains at a stage of development, in which
the cell-layer had been in its earlier period. The addition of
new molecules is so arranged that the layers increase more

196 THEORY OF THE CELLS.

considerably in superficial extent than in thickness ; and thus
an intervening space is formed between each layer and the
one preceding it, by which cells and nuclei are formed into
actual hollow vesicles. From this it may be inferred that
the deposition of new molecules is more active between those
which lie side by side along the surface of the membrane, than
between those which lie one upon the other in its thickness.
Were it otherwise, each layer would increase in thickness, but
there would be no intervening cavity between it and the pre-
vious one, there would be no vesicles, but a solid body com-
posed of layers.

Attractive powder is exerted in all the solid parts of the cell.
This follows, not only from the fact that new molecules may
be deposited everywhere between those already present, but
also from the formation of secondary deposits. When the
cavity of a cell is once formed, material may be also attracted
from its contents and deposited in layers ; and as this depo-
sition takes place upon the inner surface of the membrane
of the cell, it is probably that which exerts the attractive in-
fluence. This formation of layers on the inner surface of the
cell-membrane is, perhaps, merely a repetition of the same
process by which, at an earlier period, nucleus and cell were
precipitated as layers around the nucleolus. It must, how-
ever, be remarked that the identity of these two processes
cannot be so clearly proved as that of the processes by which
nucleus and cell are formed ; more especially as there is a
variety in the phenomena, for the secondary deposits in plants
occur in spiral forms, while this has at least not yet been de-
monstrated in the formation of the cell-membrane and the
nucleus, although by some botanical writers the cell-membrane
itself is supposed to consist of spirals.

The power of attraction may be uniform throughout the
whole cell, but it may also be confined to single spots ; the
deposition of new molecules is then more vigorous at these
spots, and the consequence of this uneven growth of the cell-
membrane is a change in the form of the cell.

The attractive power of the cells manifests a certain form of
election in its operation. It does not take up all the substances
contained in the surrounding cytoblastema, but only particular
ones, either those which are analogous with the substance

THEORY OF THE CELLS. 197

already present in the ecll (assimilation), or such as differ from
it in chemical properties. The several Layers grow by assimi-
lation, but when a new layer is being formed, different material
from that of the previously-formed layer is attracted : for the
nucleolus, the nucleus and cell-membrane are composed of
materials which differ in their chemical properties.

Such are the peculiarities of the plastic power of the cells,
so far as they can as yet be drawn from observation. lint
the manifestations of this power presuppose another faculty of
the cells. The cytoblastema, in which the cells are formed,
contains the elements of the materials of which the cell is
composed, but in other combinations : it is not a mere solu-
tion of cell- material, but it contains only certain organic
substances in solution. The cells, therefore, not only attract
materials from out of the cytoblastema, but they must have
the faculty of producing chemical changes in its constituent
particles. Besides which, all the parts of the cell itself may be
chemically altered during the process of its vegetation. The
unknown cause of all these phenomena, which we comprise
under the term metabolic phenomena of the cells, we will
denominate the metabolic potver.

The next point which can be proved is, that this power is
an attribute of the cells themselves, and that the cytoblastema
is passive under it. Vie may mention vinous fermentation1

1 I could not avoid bringing forward fermentation as an example, because it is
the best known illustration of the operation of the cells, and the simplest represen-
tation of the process which is repeated in each cell of the living body. Th<
who do not as yet admit the theory of fermentation set forth by Cagniard-Latonr,
and myself, may take the development of any simple cells, especially of the Bpores,
as an example; and we will in the text draw no conclusion from fermentation
which cannot be proved from the development of other simple cells which grow
independently, particularly the spores of the inferior plants. We have every con-
ceivable proof that the fermentation-granules are fungi. Their form is that of fungi;
in structure they, like them, consist of cells, many of which enclose other young cells.
They grow, like fungi, by the shooting forth of new cells at their extremities; they
propagate like them, partly by the separation of distinct cells, and partly by the gene-
ration of new cells within those already present, and the bursting of the parent-cells.
Now, that these fungi are the cause of fermentation, follows, first, from the constancy
of their occurrence during the process; secondly, from the cessation of fermentation
under any influences by which they are known to be destroyed, especially boiling heat,
arseniatc of pota8S, &C. ; and, thirdly, because the principle which excites the pro,
of fermentation must be a substance which is again generated and in< by the

198 THEORY OF THE CELLS.

as an instance of this. A decoction of malt will remain for
a long time unchanged ; but as soon as some yeast is added
to it, which consists partly of entire fungi and partly of a
number of single cells, the chemical change immediately ensues.
Here the decoction of malt is the cytoblastema; the cells clearly
exhibit activity, the cytoblastema, in this instance even a boiled
fluid, being quite passive during the change. The same occurs
when any simple cells, as the spores of the lower plants, are
sown in boiled substances.

In the cells themselves again, it appears to be the solid
parts, the cell-membrane and the nucleus, which produce the
change. The contents of the cell undergo similar and even
more various changes than the external cytoblastema, and it is
at least probable that these changes originate with the solid
parts composing the cells, especially the cell-membrane, because
the secondary deposits are formed on the inner surface of the
cell-membrane, and other precipitates are generally formed in
the first instance around the nucleus. It may therefore, on the
whole, be said that the solid component particles of the cells
possess the power of chemically altering the substances in con-
tact with them.

The substances which result from the transformation of the

process itself, a phenomenon which is met with only in living organisms. Neither do
I see how any further proof can possibly be obtained otherwise than by chemical ana-
lysis, unless it can be proved that the carbonic acid and alcohol are formed only at
the surface of the fungi. I have made a number of attempts to prove this, but they
have not as yet completely answered the purpose. A long test-tube was filled with
a weak solution of sugar, coloured of a delicate blue with litmus, and a very small
quantity of yeast was added to it, so that fermentation might not begin until several
hours afterwards, and the fungi, having thus previously settled at the bottom, the fluid
might become clear. When the carbonic acid (which remained in solution) commenced
to be formed, the reddening of the blue fluid actually began at the bottom of the tube.
If at the beginning a rod were put into the tube, so that the fungi might settle upon
it also, the reddening began both at the bottom, and upon the rod. This proves,
at least, that an undissolved substance which is heavier than water gives rise to
fermentation ; and the experiment was next repeated on a small scale under the
microscope, to see whether the reddening really proceeded from the fungi, but the
colour was too pale to be distinguished, and when the fluid was coloured more
deeply no fermentation ensued ; meanwhile, it is probable that a reagent upon car-
bonic acid may be found which will serve for microscopic observation, and not
interrupt fermentation. The foregoing inquiry into the process by which organized
bodies are formed, may perhaps, however, serve in some measure to recommend this
theory of fermentation to the attention of chemists.

THEORY OF THE CELLS. 199

contents of the cell are different from those which are produced
by change in the external cytoblastema. What is the cause
of this difference, if the metamorphosing power of the cell-
membrane be limited to its immediate neighbourhood merel;

Might we not much rather expect that converted substance
would be found without distinction on the inner as on the
outer surface of the cell-membrane? It might be said that the
cell-membrane converts the substance in contact with it without
distinction, and that the variety in the products of this con-
version depends only upon a difference between the convertible
substance contained in the cell and the external cytoblastema.
But the question then arises, as to how it happens that the
contents of the cell differ from the external cytoblastema. If
it be true that the cell-membrane, which at first closely sur-
rounds the nucleus, expands in the course of its growth, so as
to leave an interspace between it and the cell, and that the
contents of the cell consist of fluid which has entered this
space merely by imbibition, they cannot differ essentially from
the external cytoblastema. I think therefore that, in order to
explain the distinction between the cell-contents and the ex-
ternal cvtoblastema, we must ascribe to the cell-membrane not
only the power in general of chemically altering the substances
which it is either in contact with, or has imbibed, but also of
so separating them that certain substances appear on its inner,
and others on its outer surface. The secretion of substances
already present in the blood, as, for instance, of urea, by the
cells with which the urinary tubes are lined, cannot be ex-
plained without such a faculty of the cells. There is, however,
nothing so very hazardous in it, since it is a fact that different
substances are separated in the decompositions produced by
the galvanic pile. It might perhaps be conjectured from tl
peculiarity of the metabolic phenomena in the cells, that a
particular position of the axes of the atoms composing the cell-
membrane is essential for the production of these appearances.
Chemical changes occur, however, not only in the cytobla-
stema and the cell-contents, but also in the solid parts of
which the cells arc composed, particularly the cell-membrane.
Without wishing to assert that there is any intimate connexion
between the metabolic power of the cells and galvanism, I may
yet, for the sake of making the representation of the process

200 THEORY OF THE CELLS.

more clear, remark that the chemical changes produced by a
galvanic pile are accompanied by corresponding changes in the
pile itself.

The more obscure the cause of the metabolic phenomena in
the cells is, the more accurately we must mark the circum-
stances and phenomena under which they occur. One condi-
tion to them is a certain temperature, which has a maximum
and a minimum. The phenomena are not produced in a
temperature below 0° or above 80° It. ; boiling heat destroys
this faculty of the cells permanently; but the most favorable
temperature is one between 10° and 32° R. Heat is evolved
by the process itself.

Oxygen, or carbonic acid, in a gaseous form or lightly con-
fined, is essentially necessary to the metabolic phenomena of
the cells. The oxygen disappears and carbonic acid is formed,
or vice versa, carbonic acid disappears, and oxygen is formed.
The universality of respiration is based entirely upon this
fundamental condition to the metabolic phenomena of the
cells. It is so important that, as we shall see further on,
even the principal varieties of form in organized bodies are
occasioned by this peculiarity of the metabolic process in the
cells.

Each cell is not capable of producing chemical changes in
every organic substance contained in solution, but only in par-
ticular ones. The fungi of fermentation, for instance, effect
no changes in any other solutions than sugar ; and the spores
of certain plants do not become developed in all substances.
In the same manner it is probable that each cell in the animal
body converts only particular constituents of the blood.

The metabolic power of the cells is arrested not only by
powerful chemical actions, such as destroy organic substances
in general, but also by matters which chemically are less un-
congenial ; for instance, concentrated solutions of neutral salts.
Other substances, as arsenic, do so in less quantity. The meta-
bolic phenomena may be altered in quality by other substances,
both organic and inorganic, and a change of this kind may re-
sult even from mechanical impressions on the cells.

Such are the most essential characteristics of the funda-
mental powers of the cells, so far as they can as yet be deduced
from the phenomena. And now, in order to comprehend dis-

THEORY OF THE CELLS. 201

tinctly in what the peculiarity of the formative process of a
cell, and therefore in what the peculiarity of the essential
phenomenon in the formation of organized bodies consists, we
will compare this process with a phenomenon of inorganic
nature as nearly as possible similar to it. Disregarding all
that is specially peculiar to the formation of cells, in order to
find a more general definition in which it maybe included with
a process occurring in inorganic nature, we may view it as a
process in which a solid body of definite and regular shape i -
formed in a fluid at the expense of a substance held in solu-
tion by that fluid. The process of ciystallization in inorganic
nature comes also within this definition, and is, therefore, the
nearest analogue to the formation of cells.

Let us now compare the two processes, that the difference
of the organic process may be clearly manifest. First, with
reference to the plastic phenomena, the forms of cells and
crystals are very different. The primary forms of crystals
are simple, always angular, and bounded by plane surfaces ;
they are regular, or at least symmetrical, and even the very
varied secondary forms of crystals are almost, without exception,
bounded by plane surfaces. But manifold as is the form of
cells, they have very little resemblance to crystals ; round
surfaces predominate, and where angles occur, they are never
quite sharp, and the polyhedral crystal-like form of many cells
results only from mechanical causes. The structure too of cells
and of crystals is different. Crystals are solid bodies, composed
merely of layers placed one upon another ; cells are hollow
vesicles, either single, or several inclosed one within another.
And if we regard the membranes of these vesicles as layers,
there will still remain marks of difference between them and
crystals ; these layers are not in contact, but contain fluid be-
tween them, which is not the case with crystals; the layers in
the cells are few, from one to three only; and they differ from
each other in chemical properties, while those of crystals con-
sist of the same chemical substance. Lastly, there is also a
great difference between crystals and cells in their mode of
growth. Crystals grow by apposition, the new molecules are
set only upon the surface of those already deposited, but cells
increase also by intussusception, that is to say, the new mole-
cules arc deposited also between those already present.

202 THEORY OF THE CELLS.

But greatly as these plastic phenomena differ in cells and
in crystals, the metabolic are yet more different, or rather they
are quite peculiar to cells. For a crystal to grow, it must be
already present as such in the solution, and some extraneous
cause must interpose to diminish its solubility. Cells, on the
contrary, are capable of producing a chemical change in the
surrounding fluid, of generating matters which had not pre-
viously existed in it as such, but of which only the elements
were present in another combination. They therefore require
no extraneous influence to effect a change of solubility ; for if
they can produce chemical changes in the surrounding fluid,
they may also produce such substances as could not be held in
solution under the existing circumstances, and therefore need
no external cause of growth. If a crystal be laid in a pretty
strong solution, of a substance similar even to itself, nothing
ensues without our interference, or the crystal dissolves com-
pletely : the fluid must be evaporated for the crystal to in-
crease. If a cell be laid in a solution of a substance, even
different from itself, it grows and converts this substance
without our aid. And this it is from which the process going
on in the cells (so long as we do not separate it into its several
acts) obtains that magical character, to which attaches the idea

of Life.

From this we perceive how very different are the j)henomena
in the formation of cells and of crystals. Meanwhile, however,
the points of resemblance between them should not be over-
looked. They agree in this important point, that solid bodies
of a certain regular shape are formed in obedience to definite
laws at the expense of a substance contained in solution in a
fluid; and the crystal, like the cell, is so far an active and posi-
tive agent as to cause the substances which are precipitated to
be deposited on itself, and nowhere else. We must, therefore,
attribute to it as well as to the cell a power to attract the sub-
stance held in solution in the surrounding fluid. It does not
indeed follow that these two attractive powers, the power of
crystallization — to give it a brief title — and the plastic power
of the cells are essentially the same. This could only be ad-
mitted, if it were proved that both powers acted according to
the same laws. But this is seen at the first glance to be by
no means the case : the phenomena in the formation of cells

THEORY OF THE CELLS. 203

and crystals, arc, as we have observed, very different, even if
we regard merely the plastic phenomena of the cells, and Leave
their metabolic power (which may possibly arise from some
other peculiarity of organic substance) for a time entirely out

of the question.

Is it, however, possible that these distinct ions arc only
secondary, that the power of crystallization and the plastic
power of the cells are identical, and that an original difference
can be demonstrated between the substance of cells and that
of crystals, by which wc may perceive that the substance of
cells must crystallize as cells according to the lavs by which
crystals are formed, rather than in the shape of the ordinary
crystals? It may be worth while to institute such an inquiry.

In seeking such a distinction between the substance of eel Is
and that of crystals, we may say at once that it cannot con-
sist in anything which the substance of cells lias in common
with those organic substances which crystallize in the ordinary
form. Accordingly, the more complicated arrangement of the
atoms of the second order in organic bodies cannot give rise to
this difference ; for we see in sugar, for instance, that the mode
of crystallization is not altered bv this chemical composition.

Another point of difference by which inorganic bodies are
distinguished from at least some of the organic bodies, is
the faculty of imbibition. Most organic bodies are capable
of being infiltrated by water, and in such a manner that it
penetrates not so much into the interspaces between the ele-
mentary tissues of the body, as into the simple struct ore]
tissues, such as areolar tissue, &c. ; so that they form an
homogeneous mixture, and we can neither distinguish par-
ticles of organic matter, nor interspaces filled with water. T
water occupies the infiltrated organic substances, just as \\
present in a solution, and there is as much difference between
the capacity for imbibition and capillary permeatioi
there is between a solution and the phenomena of capillary
permeation. When water soaks through a layer of glue, we
do not imagine it to pass through pores, in the common
of the term; and this is just the condition of all Bubstani
capable of imbibition. They possess, then a doul

nature, they have a definite form like solid bodies; but 1:
fluids, on the other hand, they are also permeable by anythin

i r

204 THEORY OF THE CELLS.

held in solution. As a specifically lighter fluid poured on one
specifically heavier so carefully as not to mix with it, yet gra-
dually penetrates it, so also, every solution, when brought into
contact with a membrane already infiltrated with water, bears
the same relations to the membrane, as though it were a solu-
tion. And crystallization being the transition from the fluid to
the solid state, we may conceive it possible, or even probable,
that if bodies, capable of existing in an intermediate state
between solid and fluid could be made to crystallize, a con-
siderable difference would be exhibited from the ordinary mode
of crystallization. In fact, there is nothing, which we call a
crystal, composed of substance capable of imbibition ; and even
among organized substances, crystallization takes place only in
those which are capable of imbibition, as fat, sugar, tartaric
acid, &c. The bodies capable of imbibition, therefore, either
do not crystallize at all, or they do so under a form so different
from the crystal, that they are not recognized as such.

Let us inquire what would most probably ensue, if material
capable of imbibition crystallized according to the ordinary
laws, what varieties from the common crystals would be most
likely to show themselves, assuming only that the solution has
permeated through the parts of the crystal already formed,
and that new molecules can therefore be deposited between
them. The ordinary crystals increase only by apposition ; but
there may be an important difference in the mode of this
apposition. If the molecules were all deposited symmetrically
one upon another, we might indeed have a body of a certain
external form like a crystal ; but it would not have the struc-
ture of one, it would not consist of layers. The existence
of this laminated structure in crystals presupposes a double
kind of apposition of their molecules ; for in each- layer the
newly-deposited molecules coalesce, and become continuous
with those of the same layer already present ; but those mole-
cules which form the adjacent surfaces of two layers do not
coalesce. This is a remarkable peculiarity in the formation of
crystals, and we are quite ignorant of its cause. We cannot
yet perceive why the new molecules, which are being deposited
on the surface of a crystal (already formed up to a certain
point), do not coalesce and become continuous with those
already deposited, like the molecules in each separate layer,

THEORY OF THE CELLS. 205

instead of forming, as they do, a new layer ; and why this new
layer does not constantly increase in thickness, instead of pro-
ducing a second layer around the crystal, and so on. In the
meantime we can do no more than express the fact in the form
of a law, that the coalescing molecules axe deposited rather along
the surface beside each other, than in the thickness upon one
another, and thus, as the breadth of the layer depends upon the
size of the crystal, so also the layer can attain only a certain
thickness, and beyond this, the molecules which are being de-
posited cannot coalesce with it, but must form a new layer.

If we now assume that bodies capable of imbibition could
also crystallize, the two modes of junction of the molecules
should be shown also by them. Their structure should also
be laminated, at least there is no perceptible reason for
a difference in this particular, as the very fact of layers
being formed in common crystals shows that the molecules
need not be all joined together in the most exact manner
possible. The closest possible conjunction of the molecules
takes place only in the separate layers. In the common
crystals this occurs by apposition of the new molecules on
the surface of those present and coalescence with them. In
bodies capable of imbibition, a much closer union is possible,
because in them the new molecules may be deposited by intus-
susception between those already present. It is scarcely,
therefore, too bold an hypothesis to assume, that when bodies
capable of imbibition crystallize, their separate layers would
increase by intussusception; and that this does not happen in
ordinary crystals, simply because it is impossible.

Let us then imagine a portion of the crystal to be formed :
new molecules continue to be deposited, but do not coalesce
with the portion of the crystal already formed; they unite with
one another only, and form a new layer, which, according to
analogy with the common crystals, may invest either the whole
or a part of the crystal. We will assume that it invests the
entire crystal. Now, although this layer be formed by the
deposition of new molecules between those already present in-
stead of by apposition, yet this does not involve any change in
the law, in obedience to which the deposition of the coalescing
molecules goes on more vigorously in two directions, that is,
along the surface, than it docs in the third direction corre-

20G THEORY OF THE CELLS.

sponding to the thickness of the layer; that is to say, the
molecules which are deposited by intussusception between those
already present, must be deposited much more vigorously be-
tweeuthose lying together along the surface of the layer than
between those which lie over one another in its thickness.
This deposition of molecules side by side is limited in common
crystals by the size of the crystal, or by that of the surface on
which the layer is formed ; the coalescence of molecules there-
fore ceases as regards that layer, and a new one begins. But
if the layers grow by intussusception in crystals capable of
imbibition, there is nothing to prevent the deposition of more
molecules between those which lie side by side upon the sur-
face, even after the lamina has invested the whole crystal ; it
may continue to grow without the law by which the new mole-
cules coalesce requiring to be altered. But the consequence is,
that the layer becomes, in the first instance more condensed,
that is, more solid substance is taken into the same space;
and afterwards it will expand and separate from the completed
part of the crystal so as to leave a hollow space between itself
and the crystal ; this space fills with fluid by imbibition, and
the first-formed portion of the crystal adheres to a spot on its
inner surface. Thus, in bodies capable of imbibition, instead
of a new layer attached to the part of the crystal already
formed, we obtain a hollow vesicle. At first this must have the
shape of the body of the crystal around which it is formed,
and must, therefore, be angular, if the crystal is angular. If,
however, we imagine this layer to be composed of soft sub-
stance capable of imbibition, we may readily comprehend how
such a vesicle must very soon become round or oval. But the
first formed part of the crystal also consists of substance capable
of imbibition, so that it is very doubtful whether it must have
an angular form at all. In common crystals atoms of some
one particular substance are deposited together, and we can
understand how a certain angular form of the crystal may re-
sult if these atoms have a certain form, or if in certain axes
they attract each other differently. But in bodies capable of
imbibition, an atom of one substance is not set upon another
atom of the same substance, but atoms of water come between;
atoms of water, which are not united with an atom of solid
substance, so as to form a compound atom, as in the water of

THEORY OF THE CELLS. 207

crystallization, but which exist in some other unknown manner
between the atoms of solid substance. It is not possible,
therefore, to determine whether that part of the crystal which
is first formed must have an angular figure or not.

An ordinary crystal consists of a number of laminae ; when
so small as to be but just discernible, it lias the form which
the whole crystal afterwards exhibits, at least as far as regards
the angles ; we must therefore suppose that the first layer is
formed around a very small corpuscle, which is of the same
shape as the subsequent crystal. We will call this the pri-
mitive corpuscle. It is doubtful what may be the shape of
this corpuscle in the crystals which are capable of imbibition.
The first layer, then, is formed around the corpuscle in the
way mentioned; it grows by intussusception, and thus forms a
hollow, round or oval vesicle, to the inner surface of which the
primitive corpuscle adheres. As all the new molecules that arc
being deposited may be placed in this layer without any altera-
tion being required in the law which regulates the coalescence
of the molecules during crystallization, we must conclude that
it remains the only layer, and becomes greatly expanded, so as
to represent all the layers of an ordinary crystal. It is, how-
ever, a question whether there may not exist some reasons why
several layers can be formed. We can certainly conceive such
to be the case. The quantity of the solid substance that must
crystallize in a given time, depends upon the concentration of
the fluid; the number of molecules that may, in accordance with
the law already mentioned, be deposited in the layer in a given
time depends upon the quantity of the solution which can
penetrate the membrane by imbibition during that time. If
in consequence of the concentration of the fluid there must be
more precipitated in the time than can penetrate the mem-
brane, it can only be deposited as a new layer on the outer
surface of the vesicle. When this second layer is formed, tin1
new molecules are deposited in it, and it rapidly becomes ex-
panded into a vesicle, on the inner surface of which the first
vesicle lies with its primitive corpuscle. The first vesicle now
either does not grow at all, or at any rate much more slowly,
and then only when the endosmosis into the cavity of the
second vesicle proceeds so rapidly that all that might be pre-
cipitated while passing through it, is not deposited. The second

208 THEORY OF THE CELLS.

vesicle, when it is developed at all, must needs be developed
relatively with more rapidity than the first; for as the solution
is in the most concentrated state at the beginning, the necessity
for the formation of a second laver then occurs sooner : but
when it is formed, the concentration of the fluid is diminished,
and this necessity occurs either later or not at all. It is pos-
sible, however, that even a third, or fourth, and more, may be
formed ; but the outermost layer must always be relatively the
most vigorously developed ; for when the concentration of the
solution is only so strong, that all that must be deposited in a
certain time, can be deposited in the outermost layer, it is all
applied to the increase of this layer.

Such, then, would be the phenomena under which substances
capable of imbibition would probably crystallize, if they did so
at all. I say probably, for our incomplete knowledge of crys-
tallization and the faculty of imbibition, does not as yet admit
of our saying anything positively a 'priori. It is, however,
obvious that these are the principal phenomena attending the
formation of cells. They consist always of substance capable
of imbibition ; the first part formed is a small corpuscle, not
angular (nucleolus), around this a lamina is deposited (nucleus),
which advances rapidly in its growth, until a second lamina
(cell) is formed around it. This second now grows more quickly
and expands into a vesicle, as indeed often happens with the
first layer. In some rarer instances only one layer is formed ;
in others, again, there are three. The only other difference in
the formation of cells is, that the separate layers do not con-
sist of the same chemical substance, while a common crystal
is always composed of one material. In instituting a com-
parison, therefore, between the formation of cells and crystal-
lization, the above-mentioned differences in form, structure,
and mode of growth fall altogether to the ground. If crystals
were formed from the same substance as cells, they would pro-
bably, in these respects, be subject to the same conditions as
the cells. Meanwhile the metabolic phenomena, which are
entirely absent in crystals, still indicate essential distinctions.

Should this important difference between the mode of for-
mation of cells and crystals lead us to deny all intimate con-
nexion of the two processes, the comparison of the two may
serve at least to give a clear representation of the cell-life.

THEORY OF THE CELLS. 209

The following may be conceived to be the state of the matter :
the material of which the cells are composed is capable of
producing chemical changes in the substance with which it is
in contact, just as the well-known preparation of platinum
converts alcohol into acetic acid. This power is possessed by
every part of the cell. Now, if the cytoblastema be so changed
by a cell already formed, that a substance is produced which
cannot become attached to that cell, it immediately crystallizes
as the central nucleolus of a new cell. And then this con-
verts the cytoblastema in the same manner. A portion of that
which is converted may remain in the cytoblastema in solution,
or may crvstallize as the commencement of new cells : another
portion, the cell-substance, crystallizes around the central cor-
puscle. The cell- substance is either soluble in the cytoblastema,
and crystallizes from it, so soon as the latter becomes saturated
with it ; or else it is insoluble, and crystallizes at the time of
its formation, according to the laws of crystallization of bodies
capable of imbibition mentioned above, forming in this manner
one or more layers around the central corpuscle, and so on.
If we conceive the above to represent the mode of the formation
of cells, we regard the plastic power of the cells as identical
with the power by which crystals grow. According to the
foregoing description of the crystallization of bodies capable of
imbibition, the most important plastic phenomena of the cells
are certainly satisfactorily explained. But let us see if this
comparison agrees with all the characteristics of the plastic
power of the cells. (See above, p. 19 A et seq.)

The attractive power of the cells does not always operate
symmetrically; the deposition of new molecules may be more
vigorous in particular spots, and thus produce a change in the
form of the cell. This is quite analogous to what happens in
crystals; for although in them an angle is never altered, then-
may be much more material deposited on some surfaces than
on others ; and thus, for instance, a quadrilateral prism may be
formed out of a cube. In this case new layers are deposited on
one, or on two opposite sides of a cube. Now, if one layer in
cells represent a number of layers in a common crystal, it may
be easily perceived that instead of several new layers being
formed on two opposite surfaces of a cell, the one Layer would
grow more at those spots, and thus a round cell would be elou-

14

210 THEORY OF THE CELLS.

gated into a fibre ; and so with the other changes of form. Divi-
sion of the cells can have no analogue in common crystals,
because that which is once deposited is incapable of any further
change. But this phenomenon may be made to accord with the
representation of crystals capable of imbibition, just as wrell as
the coalescence of numerous cells in the manner described at
page 184 does. And if we ascribe to a layer of a crystal capa-
ble of imbibition the power of producing chemical changes in
organic substances, we can very well understand also the origin
of secondary deposits on its inner surface as they occur in cells.
For if, in accordance with the laws of crystallization, the lamina
has become expanded into a vesicle, and its cavity has become
filled by imbibition with a solution of organic substance, there
may be materials formed by means of the converting influence
of the lamina, which cannot any longer be held in solution.
These may, then, either crystallize within the vesicle, as new
crystals capable of imbibition under the form of cells; or if
they are allied to the substance of the vesicle, they may so
crystallize as to form part of the system of the vesicle itself:
the latter may occur in two ways, the new matters may be
applied to the increase of the vesicle, or they may form new
layers on its inner surface from the same cause which led to
the first formation of the vesicle itself as a laver. In the cells
of plants these secondary deposits have a spiral arrangement.
This is a very important fact, though the laws of crystallization
do not seem to account for the absolute necessity of it. If,
however, it could be mathematically proved from the laws of
the crystallization of inorganic bodies, that under the altered
circumstances in which bodies capable of imbibition are placed,
these deposits must be arranged in spiral forms, it might be
asserted without hesitation that the plastic power of cells and
the fundamental powers of crystals are identical.

We come now, however, to some peculiarities in the plastic
power of cells, to which we might, at first sight, scarcely expect
to find anything analogous in crystals. The attractive power
of the cells manifests a certain degree of election in its opera-
tion ; it does not attract every substance present in the cyto-
blastema, but only particular ones ; and here a muscle-cell,
there a fat-cell, is generated from the same fluid, the blood.
Yet crystals afford us an example of a precisely similar pheno-

THEORY OF THE CELLS. 211

menon; and one which has already been frequently adduced as
analogous to assimilation. If a crystal of nitre he placed in
a solution of nitre and sulphate of soda, only the nitre crystal-
lizes; when a crystal of sulphate of soda is put in, only the
sulphate of soda crystallizes. Here, therefore, there occurs just
the same selection of the substance to be attracted.

We observed another law attending the development of the
plastic phenomena in the cells, viz. that a more concentrated
solution is requisite for the first formation of a cell than for
its growth when already formed, a law upon which the differ-
ence between organized and unorganized tissues is based. In
ordinary crystallization the solution must be more than satu-
rated for the process to begin. But when it is over, there
remains a mother lye, according to Thenard, which is no
longer saturated at the same temperature. This phenomenon
accords precisely with the cells ; it shows that a more con-
centrated solution is requisite for the commencement of
crystallization than for the increase of a crystal already
formed. The fact has indeed been disputed by Thomson;
but if, in the undisputed experiment quoted above, the crystal
of sulphate of soda attracts the dissolved sulphate of soda
rather than the dissolved nitre, and vice versa, the crystal of
nitre attracts the dissolved nitre more than the dissolved sul-
phate of soda, it follows that a crystal does attract a salt held
in solution, because the experiment proves that there are de-
grees of this attraction. But if there be such an attraction
exerted by a crystal, then the introduction of a crystal into a
solution of a salt, affords an efficient cause for the deposition
of this salt, which does not exist when no crystal is introduced.
The solution must therefore be more concentrated in the latter
case than in the former, though the difference be so slight as
not to be demonstrable by experiment. It would not, how-
ever, be superfluous to repeat the experiments. In the in-
stance of crystals capable of imbibition, this difference may be
considerably augmented, since the attraction of molecules may
increase perhaps considerably by the penetrating of the solution
between those already deposited.

We see then how all the plastic phenomena in the cells
may be compared with phenomena which, in accordance with
the ordinary laws of crystallization, would probably appear if

212 THEORY OF THE CELLS.

bodies capable of imbibition could be brought to crystallize.
So long as the object of such a comparison were merely to
render the representation of the process by which cells are
formed more clear, there could not be much urged against it ;
it involves nothing hypothetical, since it contains no explana-
tion ; no assertion is made that the fundamental power of the
cells really has something in common with the power by which
crystals are formed. We have, indeed, compared the growth
of organisms with crystallization, in so far as in both cases
solid substances are deposited from a fluid, but we have not
therefore asserted the identity of the fundamental powers. So
far we have not advanced beyond the data, beyond a certain
simple mode of representing the facts.

The question is, however, whether the exact accordance of
the phenomena would not authorize us to go further. If the
formation and growth of the elementary particles of organisms
have nothing more in common with crystallization than merely
the deposition of solid substances from out of a fluid, there is
certainly no reason for assuming any more intimate connexion
of the two processes. But we have seen, first, that the laws
which regulate the deposition of the molecules forming the
elementary particles of organisms are the same for all ele-
mentary parts ; that there is a common principle in the deve-
lopment of all elementary parts, namely, that of the formation
of cells ; it was then shown that the power which induced the
attachment of the new molecules did not reside in the entire
organism, but in the separate elementary particles (this we
called the plastic power of the cells); lastly, it was shown that
the laws, according to which the new molecules combine to
form cells, are (so far as our incomplete knowledge of the laws
of crystallization admits of our anticipating their probability)
the same as those by which substances capable of imbibition
would crystallize. Now the cells do, in fact, consist only of
material capable of imbibition; should we not then be justi-
fied in putting forth the proposition, that the formation of the
elementary parts of organisms is nothing but a crystallization
of substance capable of imbibition, and the organism nothing
but an aggregate of such crystals capable of imbibition ?

To advance so important a point as absolutely true, would
certainly need the clearest proof; but it cannot be said that

THEORY OF THE CELLS. 213

even the premises which have been set fort li have in all [joints

the requisite force. For too little is still known of the cause
of crystallization to predict with safety (as was attempted above)
what would follow if a substance capable of Imbibition were to

crystallize. And if these premises were allowed, there are two
other points which must be proved in order to establish tin-
proposition in question: 1. That the metabolic phenomena of
the cells, which have not been referred to in the foregoing
argument, are as much the necessary consequence of the faculty
of imbibition, or of some other peculiarity of the substance of
cells, as the plastic phenomena are. 2. That if a number of
crystals capable of imbibition are formed, they must combine
according to certain laws so as to form a systematic whole,
similar to an organism. Both these points must be clearly
proved, in order to establish the truth of the foregoing view.
But it is otherwise if this view be adduced merel as an hypo-
thesis, which may serve as a guide for new investigations. In
such case the inferences are sufficiently probable to justify
such an hypothesis, if only the two points just mentioned can
be shown to accord with it.

With reference to the first of these points, it would certainly
be impossible, in our ignorance as to the cause of chemical
phenomena in general, to prove that a crystal capable of im-
bibition must produce chemical changes in substances sur-
rounding it; but then we could not infer, from the manner
in which spongy platinum is formed, that it would act so
peculiarly upon oxygen and hydrogen. But in order to
render this view tenable as a possible hypothesis, it is only
necessary to see that it may be a consequence. It cannot be
denied that it may : there are several reasons for it, though
they certainly are but weak. For instance, since all cells
possess this metabolic power, it is more likely to depend on a
certain position of the molecules, which in all probability is
essentially the same in all cells, than on the chemical com-
bination of the molecules, which is very different in different
cells. The presence, too, of different substance's on the Inner
and the outer surface of the cell-membrane (see above, pa.
109) in some measure implies that a certain direction of the
axes of the atoms may be essential to tin' metabolic pheno-
mena of the cells. I think, therefore, that the cause of the

214 THEORY OF THE CELLS.

metabolic phenomena resides in that definite mode of arrange-
ment of the molecules which occnrs in crystals, combined with
the capacity which the solution has to penetrate between these
regularly deposited molecules (by means of which, presuming
the molecules to possess polarity, a sort of galvanic pile will
be formed), and that the same phenomena would be observed
in an ordinary crystal, if it could be rendered capable of imbi-
bition. And then perhaps the differences of quality in the
metabolic phenomena depend upon their chemical composition.
In order to render tenable the hypothesis contained in the
second point, it is merely necessary to show that crystals capable
of imbibition can unite with one another according to certain
laws. If at their first formation all crystals were isolated, if
they held no relation whatever to each other, the view would
leave entirely unexplained how the elementary parts of or-
ganisms, that is, the crystals in question, become united to
form a whole. It is therefore necessary to show that crystals
do unite with each other according to certain laws, in order to
perceive, at least, the possibility of their uniting also to form
an organism, without the need of any further combining
power. But there are many crystals in which a union of this
kind, according to certain laws, is indisputable ; indeed they
often form a whole, so like an organism in its entire form,
that groups of crystals are known in common life by the names
of flowers, trees, &c. I need only refer to the ice-flowers on the
windows, or to the lead- tree, &c. In such instances a number
of crystals arrange themselves in groups around others, which
form an axis. If we consider the contact of each crystal with
the surrounding fluid to be an indispensable condition to the
growth of crystals which are not capable of imbibition, but that
those which are capable of imbibition, in which the solution can
penetrate whole layers of crystals, do not require this condition,
we perceive that the similarity between organisms and these
aggregations of crystals is as great as could be expected with
such difference of substance. As most cells require for the
production of their metabolic phenomena, not only their pe-
culiar nutrient fluid, but also the access of oxygen and the
power of exhaling carbonic acid, or vice versa ; so, on the other
hand, organisms in which there is no circulation of respiratory
fluid, or in which at least it is not sufficient, must be developed

THEORY OF THE CELLS. 215

in such a way as to present as extensive a surface as possible
to the atmospheric air. This is the condition of plants, which
require for their growth that the individual cells should come
into contact with the surrounding medium in a similar manner,
if not in the same degree as occurs in a crystal tree, and in
them indeed the cells unite into a whole organism in a form
much resembling a crystal tree. But in animals the circulation
renders the contact of the individual cells with the surrounding
medium superfluous, and they may have more compact fornix
even though the laws by which the cells arrange themscb.
are essentially the same.

The view then that organisms are nothing but the form
under which substances capable of imbibition crystallize, ap-
pears to be compatible with the most important phenomena of
organic life, and may be so far admitted, that it is a possible
hypothesis, or attempt towards an explanation of these pheno-
mena. It involves very much that is uncertain and paradoxical,
but I have developed it in detail, because it may serve as a
guide for new investigations. For even if no relation between
crystallization and the growth of organisms be admitted in
principle, this view has the advantage of affording a distinct
representation of the organic processes ; an indispensable re-
quisite for the institution of new inquiries in a systematic
manner, or for testing by the discovery of new facts a mode
of explanation which harmonizes with phenomena already
known.

SUPPLEMENT

(referred to AT p. IG)
ON THE SIGNIFICATION OF THE GERMINAL VE8ICLE.

When treating of the different parts of the ovum, in the
foregoing work, it was found impossible to give a positivi
solution to the question as to whether the germ-vesicle was a
young cell or the nucleus of the yelk-cell. Most of the facts
before us were in favour of the latter view ; but if this were the
correct one, the yelk-cell ought to be developed around the
previously existing vesicle in such manner, that it in the
first instance closely encompassed the latter, and afterwards
became gradually expanded. This decisive observation was
wanting, and the researches communicated by R. Wagner, in
his ' Prodromus/ rather tended to show that, in the formation
of the ovum around the germinal vesicle, the membrane was
not formed immediately around the vesicle, but that it inclosed
at the same time a quantity of the granular mass in which the
germ-vesicle lies. I was not at that time acquainted with a
work of Wagner's, which contained the facts necessary to a
solution of the question, viz. his f Bcitrage zur Geschichte
der Zeugung unci Entwickelung., Erster Bcitrag:' from the
' Mathematisch-physikalischen Klasse der Konigl. Baierschen
Acad, der Wissenschaften in Munchen/ Speaking of the
ovaries of insects, Wagner says, at page J 5 : — " At the spot
where the oviduct widens, the granular mass, which resembles
the vitelline mass, becomes more plentiful; the separate germ-
vesicles seem to be imbedded in it. I have so represented it
in the ' Prodromus/ fig. 18. Lately, however, it has appeared
to me, as though the germ-vesicles with their germinal spots
were actually already surrounded by a chorion and a perfectly
pellucid yelk." The accompanvin- illustration from Agrion
virgo exhibits clearly how thai which Wagner calls chorion.

218 SUPPLEMENT.

or the cell-membrane of the yelk-cell, closely encompasses the
germ-vesicle at first and then gradually expands, while between
it and the vesicle a transparent fluid collects ; in which, at a
later period, a turbidness commences, occurring first in the
neighbourhood of the germ-vesicle. Wagner had thus dis-
covered in the course of his observations that the details of
the process were just what must have been expected according
to the theory of the unity of the principle of development for
all elementary particles of the organism. That the germ-
vesicle is the nucleus of the yelk-cell appears to me therefore
to be scarcely dubitable. The illustration given by Wagner
also shows that the germinal spot is first developed, then the
germ-vesicle around it, and around this again the yelk-cell. It
is not surprising that granulous contents may form within
the germ-vesicle at a subsequent period, since the same thing
occurs in the indubitable nucleus of the adipose cells of the
fish, and the formation of the cell is probably nothing more
than a repetition of that same process around the nucleus, by
means of which the nucleus was originally formed around the
nucleolus.

REMARKS

UPON A STATEMENT PUT FORTH BY PROFESSOR VALENTIN,
RESPECTING PREVIOUS RESEARCHES ON THE SUBJECT OF
THIS WORK.

After I had finished this Treatise, I received the first part
of Wagner's 'Lehrbuch der Physiologic/1 Leipzig, 1839;
which was just then issuing from the press, and which con-
tained (at page 132) an outline of the development of the
animal tissues, communicated by Professor Valentin. The
author introduces the subject with some historical remarks,
in which he represents my researches as giving an essen-
tial completeness to the analogies between animal tissues
and vegetable cells which had been previously pointed out,
more particularly by himself. There are very many ways
of drawing a comparison between two objects, and simi-
litudes may be discovered which are opposed to the whole
internal construction of the things in which they are observed.
Everything, therefore, depends upon the sort of comparison
drawn. If Valentin's historical representation be justified, the
idea of a comparison, similar in its kind to that on which my
researches are based, must have a previous existence in his
earlier investigations. I have endeavoured to analyse the
fundamental idea of my investigation in the commencement
of the Third Section of this treatise ; it was this — that one
common principle of development forms the basis of all the
elementary particles of organisms. It originated in a com-
parison being drawn between a cartilage-cell and a vegetable
cell, in such sense, that the molecules are united together for
the formation of both of them, in accordance with similar laws,
since in both instances a nucleolus is first formed; around this

1 Rudolph Wagner's Elements of Physiology, translated by R. Willis, m.h.. p. 214.

220 REMARKS UPON A STATEMENT

a nucleus, aud around this again a cell. The accordance in the
mode of development of two so different elementary particles,
first led to the deduction of the principle of a similar mode of
formation for all elementary particles, and then to its proof by
observation. Therefore, what we have to decide is, first,
whether the idea of comparing an animal elementary structure
with a vegetable cell, with reference to a similar mode of de-
velopment, does occur in Valentin's earlier observations ; and,
secondly, whether Valentin has recognised the principle which
is contained in the similar mode of development of two ele-
mentary particles which, in a physiological sense, are very dis-
similar. In my preface I have given a brief historical sketch
of the subject from my own point of view, and Valentin's
remarks do not convince me of the necessity of making any
alteration in it. Impartiality, however, requires that Valentin's
representation should follow this statement, and I therefore
append the passages cited by him, word for word, from his
works :

" In my first Mstogenetic researches, I observed certain pecu-
liar granules lying in a transparent gelatinous substance, as the
primordial matter of all the tissues. I pointed out the difference
between these granules in the serous and mucous layers, at the
period of the earliest separation of the layers from one another,
hi the vascular layer I found large globules or cells, which, in
respect to their form and juxtaposition, I compared, as early as
the year 1835, with vegetable cellular tissue. (Entwickelimgs-
geschichte, 287. The vascular layer seems to be composed of large
globules having a mean diameter of 0*001013 Paris inch, which are
perfectly transparent in their interior, and so closely crowded toge-
ther, that they are flattened against one another at many of their points of
contact, and assume an hexagonal form like the cellular tissue of plants.)
/ also first directed attention to the resemblance in form of the
cartilages in which ossification was commencing, and particularly
{from observations made in conjunction with Purkinje) of the
branchial cartilage of the tadpole to the vegetable cellular tissue.
(lb. 209-10. The cartilages of the labyrinth present a variety of form
whilst passing through the process of ossification, which differs very
essentially from most of the other cartilages of the body, which will be
described at greater length presently. In place of the ordinary car-
tilage-corpuscle, they contain large bodies which are not so well defined
in form, most of them furnished with linear boundaries, being roundish,

PUT FORTH BY VALENTIN. 221

semilunar, tetrahedral, or polyhedral in shape, with a mean diameter
of from 0*000405, to 0*000650 Paris inch. Bui bo soon aa they

ossify, the calcifying portion, or that which is already ossified, consists
of a tissue of beautiful six-sided prisms (Balken), closely resembling
vegetable cellular tissue, upon and within which an- small grannies of
a round figure, with a diameter of about 0*000152 l'aris inch. The la
described form, was observed both by Purkinje ami myself long since
in the cartilages of the tadpole also, especially in the branchial arches.)
1 described the round cells of the globules with their interposed
cellular substance from the chorda dorsal is of young embryo
(lb. 157. Although the external appearance of the chorda dorsalis
clearly presents a certain resemblance to a cartilage, the microscopii
investigation of its structure most distinctly disproves similarity. In
every instance in which it is present, it consists of an external, symme-
trical, perfectly transparent envelope and globules of variable size-, but
always very numerous, and lying closely packed together. A gelatinous
and perfectly transparent mass occupies the interspaces left between
them. These globules are largest in fishes and amphibia, smaller in
birds, and smallest in mammalia." In the second passage, which

Valentin cites on this point (Repertor. i, 187), the research
of J. Miiller, which I have noticed at page 7 in this treatise,
are referred to and quoted, the following also is from the sunn-
source: — "which (chorda dorsalis) the reporter (Valentin) has also
observed in foetal pigs of eight lines in length, in the form of a thick
cord lying within the cartilaginous vertebrae, its internal structure, in
the embryos of mammalia, birds, and amphibia, being, according to
his observations, essentially similar to the permanent analogous forma-
tions of the cartilaginous fishes.) Soon after this J. Mailer, from
his own independent invest ig at ions, gave a more detailed <
planation of the cells in the spinal cord of fishes {Myxinoidens
74, $c.) In the epithelia, which Purkinje and Raschkow
(Meletem. c. mammal, dent. evol. 12), as well as I (Nov. act.

ac. N. C. vol. xviii, p. 1.96) These (the tuft-like groups of the

choroid plexus) do not lie free, but they, as well as the connecting
granulous membrane, are covered with a very delicate ami transparent
epithelium, the separate globules of which have the mosl regular six-
sided cell-border, and are perfectly colourless and transparent. Each
of them, however, contains, in the mass in its interior, a dark round
nucleus, or formation, which reminds the observer of the nucleus oc-
curring in the cells of the epidermis, the pistil, &C, in the vegetable
kingdom. In man, whose choroid plexus exhibit- a more blackish or
dark colour even to the naked eve, the epithelium itself has a Mimlar
formation to that just described", but the centre of each cell contains
in its exterior a round pigment-globule, corresponding to the central
point of the situation of the nucleus in its interior. Similar pigment-
globules exist in most birds, but not being bo regularly deposited, it is

222 REMARKS UPON A STATEMENT

more difficult to detect the cell-shaped and more rounded globules,
although they are quite as certainly present. When the object has
not been at all damaged, the cells, and especially the pigment- globules
adhering to the outside, exhibit an arrangement like that of the vege-
table cells in general, and particularly in the earliest stages in the
formation of the leaf, that is, a disposition corresponding to spiral lines
projected on the surface in accordance with the strictest rules)

compared to the cellular tissue of plants, I chose, expressly

(1. c. 77 . Each of these globules (ganglion-globules), wherever ob-
served, has an external, more or less distinct, areolar tissue-like envelope,
and contains a parenchymatous mass proper to itself, an independent
nucleus or kernel (nucleus oder Kern), which again encloses a
second roundish, transparent nucleus) on account of this re-
semblance in form, the uniform appellation of the nucleus
{Kernes), just as I afterwards described the nucleolus which was

observed by me. (Repertor. i, 143. In every cell without exception
there is a somewhat smaller and more compact nucleus of a round or
oval form. It usually occupies the centre of each cell, consists of a
minutely granulous substance, but encloses a well-defined, round cor-
puscle, which thus forms a sort of second nucleus within it.) In the

study of the epithelia, prosecuted particularly by Henle and

myself, there was no want of analogies with vegetable cellular

tissue, the individuality of the cell-parietes was also distinctly

demonstrated. (lb. 284. Roundish, hexagonal, flat, aud tolerably
thin cells lie (in the external skin of the proteus) close upon one
another, disposed in regular arrangement, and always connected
together with their lateral edges and angles in mutual correspondence.
The interior of these delicate bodies is filled by a granulous or yel-
lowish mass, which represents a sort of nucleus. But the separate
granules of this nucleus, however closely they may lie together, may
be accurately distinguished from one another. With a very strong
magnifying power, each one of these granules may be seen to be more
transparent in its centre than it is in its periphery. It may then
also be most distinctly ascertained, that the somewhat delicate parietes
of each cell are perfectly isolated from the central cavity. No trace
of granules or fibres can be observed on the walls themselves ; there is
merely a clear, transparent, vitreous, and homogeneous mass.) I had

also remarked that the nuclei {pigment -vesicles) were the parts

first formed in the pigment of the choroid coat (Entwickelungs-

geschichte, 194. The following is the mode in which, according to
my observations, the stratum of pigment is formed in man, mammalia,
and birds ; separate, round, colourless, and transparent corpuscles are
first deposited upon the internal surface of the substance they are to
cover, in the earliest period (up to the tenth week) these corpuscles in
the human subject measure from 0*000355 to 0-000405 Paris inch in
diameter. They are the future pigment-corpuscles or pigment- vesicles.

PUT FORTH BY VALENTIN. 223

Pigment-globules of a black colour are soon, however, developed on
their periphery, so that the corpuscles or vesicles jusl mentioned are
transparent in their centre when they have ceased to be so, and have
become dark on their circumference. It is plain that von Ammor) and
R. Wagner have seen this condition as well as myself. The globules

are so small from the commencement, that they This proc

of deposition of the black-coloured globules upon the pigment-cor-
puscles goes on afterwards continuously, and to such an extent that
the latter are enveloped and covered on all sides by them, and are only
rendered visible when the globules are removed by pressure or washing.);
and I compared the pigment-cells with the cellular tissue of

plants. (Repertor. ii, 245. The pigment here (in the choroid) has
the same character which it has in most other parts of the body, that
is, a round, clear, transparent, and colourless nucleus, or the pigment-
molecules lie closely crowded together around a pigment-vesicle. These
heaps of pigment composed of pigment-vesicles, and the molecules of
pigment deposited around them, are extended outsidewise, and in man,
the dog, the rabbit, the horse, the ox, and such like, form unequal
pentagons or hexagons, wrhich are placed close together in a similar
manner to the cells of the parenchymatous cellular tissue of plants.
Langenbeck de retina, 38.) Schwann gave an essential complete-
ness to these analogies, by showing that the gelatinous primordial
mass of the tissues ivas composed of cells, that the bodies im-
bedded in it are nuclei, and that these and the cells oft <n exhibit
analogous laws of development. (Froriep's Notizen, 1838,
Mikroskopische Untersucliungen iiber die Struktur dor Tbicre
und Pflanzen, Heft i, 1838.) As early as 1837 / had observed
the cells of the germinal membrane in the ovum of sepia, with
their nuclei and nucleoli, and the areas surrounding them, and
had communicated my researches in a letter to Breschet. Shortly
after 1 became acquainted with Schwann's first communication I
commenced the investigation of the subject. The chief results
of my inquiries are contained in the following communication. I
have, at the same time, referred to the correspond///// passages
in the first part of Schwann's treatise, which I have received
this day.,}

I will only add that the second part also, (consisting of sh< i
8 to 13, and Plates III and IV,) therefore the whole of the
portion of my treatise containing the observations, had appeared
previous to Valentin's researches, and had been communicated to
the Parisian Academy in the year 1838; a remark which docs
not appear altogether superfluous, since Professor Wagner has

224

REMARKS UPON A STATEMENT, ETC.

communicated an epitome of my observations (which I sent to
him four weeks after he had requested it from me) in his
Physiology, with the remark that it had arrived later than
the observations of Valentin. Moreover, even my first com-
munications in Froriep's Notizen contained the fundamental
laws for the formation of all the tissues, and the details also
respecting by far the most of them.

b

Fiff. 2

i

b

b ...

a, ...

I

d...

•■ c

r

a

a. .

_Fu/3.

a

9

T20 10.

r^

Fig. 7.

(

Fig 6

r

- 1
t

■a.

h

/-N

c

Fig 12

Fig. 11.

Fig. 14

T

■

EXPLANATION OF THE PLATES.

Where no other measurement is given, the figure re-
presents the object magnified about 450 diameters, line -ar
measurement.

PLATE I.

Fig. 1. Parenchymatous cellular tissue, with cell-nuclei from
an onion, magnified 290 times.

2. Matrix of the pollen of Rhipsalis salicornoides.

3. Do. do.

I am indebted to the kindness of Dr. Schleiden for the last two delineations.

4. Cells from the chorda dorsalis of Cyprinus erythroph-

thalmus.

5. Cartilage from the point of a branchial ray, from the

same.

6. Cartilage from the middle of a branchial ray, from the

same.

7. Cartilage from the root of a branchial ray, from the

same.

8. Branchial cartilage from the larva of Rana esculcnta.

9. Cranial cartilage (ethmoid bone) from the larva of

Pelobates fuscus.

10. Cells from the crystalline lens of a foetal pig four

inches long.

11. An isolated nucleus of the cells of the crystalline lens.

12. Cells from the crystalline lens of the same fectu-, ex-

hibiting their prolongation into the fibres of the Lena.

13. Fibres from the innermost layers of the lens of a pike.

14. Cell from the epidermis of a species of grass.

L5

226 EXPLANATION OF THE PLATES.

PLATE II.

Fig. 1. Ovum of a goat, after Krause (Miiller's Archiv, 1837,
PI. I, fig. 5).

2. Cells from the yelk-cavity of a mature hen's egg.

3. Cells from the interior of an egg measuring a line and

a half in diameter, taken from the ovary of a hen.

4. Portion of the germinal membrane of a mature hen's

egg before incubation, viewed from above.

5. Portion of the germinal membrane from a hen's egg

after sixteen hours' incubation. It is folded in such
a manner that the external surface or serous layer
forms the margin.

6. Cells from the serous layer of the same germinal mem-

brane in the neighbourhood of the area pellucida,
after separation of the mucous layer.

7. Cells from the mucous layer of the same germinal

membrane on the outside of the area pellucida.

8 and 9. Pigment-cells of different kinds and stages of
development, from the tail of the tadpole.

10. Cells from the interior of the shaft of a fully deve-

loped wing-feather of the raven.

11. Earlier stages of development of the same, from the

portion of the shaft of an immature feather which
has not as yet become hard.

12. Cell-nuclei, from the same, around which no cells have

as yet formed.

13. Flat cells splitting into fibres, from the cortex on the

side of the shaft of a raven's feather in progress of
formation.

/•>■

t

^

9 (

■

o

Fig •/

■i,

o

-.

>-'' — ■ •

Oj

'

^~

O O

J

8

■

Schitr-

.

Fin. 2

'} v^

\

3

Tip 7.

-

■

Fig. 9.

Fin. 10.

' 11.

: 12

Fig. 13.
O

| i

^o

EXPLANATION OF THE PLATES.

PLATE III.

Fig. 1. From • the point of a branchial cartilage of Etana
esculenta. The lower margin of the delineation
exhibits the natural border of the cartilage.

2. Cartilage from the ilium of a foetal pig five inches

long, after the application of acetic acid.

3. Enamel fibres from immature teeth of a fcctal pig.

I. Cells from the surface of the enamel membrane.

5. Fibres which compose the substantia propria of the

human tooth, isolated by maceration for two days
in dilute hydrochloric acid.

6. Fibre-cells from the areolar tissue lying beneath the

superficial muscles of the neck of a foetal pig mea-
suring seven inches.

7. A more fully developed cell of areolar tissue.

8. Cells from the gelatinous substance between the cho-

rion and amnion of a foetal pig seven inches long.

9. Larger and very pale cells from the areolar tissue of

the orbital cavity of the same foetus.

10. Fat-cells from the cranial cavity of the young of
Cyprinus erythrophthalmus.

II. Fibre-cells from the tendo achillis of a foetal pig three

and a half inches long.

12. From the middle coat of the aorta of a foetal pig

measuring seven inches in length.

13. Cells from the interior of the quadratus lnmborum

muscle of a foetal pig three and a half inches Ion.

228 EXPLANATION OF THE PLATES.

PLATE IV.

Fig. 1. Dorsal muscles of a foetal pig three and a half inches
long.

2. The fibre c from the previous figure, after the applica-

tion of acetic acid.

3. From the brachial muscles of a foetal pig seven

inches long.

4. Primitive muscular fasciculus from the cockchafer.

5. Muscular fasciculus from a pike.

6. A portion of the ischiatic nerve of a foetal pig mea-

suring four inches.

7. Fasciculus of nervous fibres from the brachial plexus

of a foetal pig four inches in length.

8. Single nervous fibres : a, from the nervus trigeminus

of a foetal pig measuring six inches and a half; b,
c, d3 from the nervus ischiadicus of the same.

9. Nervous fibre from the vagus of a calf.

10. Ganglion -globules from the lowest ganglia of the

sympathetic of a frog.

11. Capillary vessels in the tail of the tadpole.

12. Ideal representation of the formation of the capillary

vessels in the area pellucida of a hen's egg.

r>

■

— -

—

■

7

-i

CONTRIBUTIONS TO PHYTOGENESIS,

TRANSLATED FROM THE GERMAN

OF

DR. M. J. S C H L E I D E N,

l'KOFESSOR OF BOTANY IN THE UNIVERSITY OF Jh S I

CONTRIBUTIONS TO PIIYTOGENESIS.1

The general fundamental law of human reason, its undc-
viating tendency to unity in its acquisition of knowledge, has
always been evinced in the department which treats of or-
ganized bodies as fully as in all other branches of science;
and manifold have been the endeavours to establish the ana-
logies between the two great divisions of the animal and
vegetable kingdoms. But eminent as the men have been who
have devoted their attention to this subject, it cannot be
denied that all attempts which have been hitherto made with
this view must be regarded as entirely unsuccessful. If, in-
deed, the fact has of late been pretty generally admitted, still
the reason of the circumstance has not always been quite
correctly apprehended and put forth in its full precision and
clearness. The cause of this, however, is, that the idea of
individual, in the sense in which it occurs in animal nature,
cannot in any way be applied to the vegetable world. It 1-
only in the very lowest orders of plants, in some Alga and
Fungi for instance, which consist only of a single cell, that \\ e
can speak of an individual in this sense. But every plant
developed in any higher degree, is an aggregate of fully indi-
vidualized, independent, separate beings, even the cells them-
selves.

Each cell leads a double life: an independent one, pertain-
ing to its own development alone; and another incidental, in

1 [These first appeared in Mailer's Archiv fur Anatomie and Physi Pi II,

1838. But as they have been republished with some additional notes In a collected
edition of Schleiden's papers, entitled ' Beitrage zur Botanik,' I have made use of the
latter work as my text; with the exception of the notes, I believe it corresponds
precisely with the paper in Mailer's Archivj which, it is also right I should state,
has been already most faithfully translated bj Mr. Francis, in Taylor's 'Scientinc
Memoirs/ vol. ii, Part VI.— Translator.]

232 CONTRIBUTIONS TO

so far as it lias become an integral part of a plant. It is,
however, easy to perceive that the vital process of the indi-
vidual cells must form the very first, absolutely indispensable
fundamental basis, both as regards vegetable physiology and
comparative plrysiology in general; and, therefore, in the very
first instance, this question especially presents itself: how does
this peculiar little organism, the cell, originate ?

The great importance of the subject is the only excuse I
can adduce for venturing at the present moment to publish the
following remarks, feeling as I do only too well convinced
that more extended researches can alone impart to them their
proper scientific value. Perhaps, however, I may succeed by
these remarks in drawing attention to this very important
subject.

Since no real advance in science results from the attempt
to explain natural phenomena hypothetically, and least of all,
where all the conditions for the erection of a tenable hypo-
thesis, namely, guiding facts, are wanting, I may omit all
historical introduction ; for, so far as I am acquainted, no
direct observations exist at present upon the development of
the cells of plants. Sprenger's pretended primitive cells have
long since been shown to be solid granules of amylum. To
enter upon Raspail's work appears to me incompatible with
the dignity of science. Whoever feels any desire to do so,
may refer to the work itself.

The only work connected with this subject, the highly dis-
tinguished one by Mirbel, I shall have occasion to refer to
subsequently, since even he does not make any allusion to the
process of cell-formation. It is to be regretted that Meyen,
who perhaps has studied vegetable anatomy more comprehen-
sively than any one up to the present time, should have con-
fined himself almost exclusively to the investigation of deve-
loped forms, and not yet have brought the formative process
itself in any degree within the sphere of his enquiries. I still
have many doubts, the solution of which I had hoped to have
found in his Physiology, but hoped in vain.

It was Robert Brown who, with his comprehensive natural
genius, first realized the importance of a phenomenon, which,
although observed previously by others, had yet remained
totally neglected. He found, in the first instance, in a great

PIIYTOGENESIS.

many of the cells in the epidermis of the Orchidea, an opaque
spot, named by him areola, or nucleus of the cell. JIc subse-
quently pursued this phenomenon in the earlier stages of the
pollen-cells, in the young ovulum, in the tissue of the stigma,
not only in the Orclndcce, but also in many other Monocotyle-
dons, and even in some Dicotyledon*.

As the constant presence of this areola in the cells of verv
young embryos and in the newly-formed albumen could not
fail to strike me in my extensive investigations into the deve-
lopment of the embryo, it was very natural that the consider-
ation of the various modes of its occurrence should lead to the
thought, that this nucleus of the cell must hold some close
relation to the development of the cell itself. I consequently
directed my attention particularly to this point, and was for-
tunate enough to see my endeavours crowned with success.

Before, however, I proceed to the communication of these
observations, I must first give a somewhat more detailed
description of the nucleus. As I have to treat of a peculiar
and, I think, universal elementary organ of vegetables, I do
not consider it necessary to apologise for applying a definite
name to this body, and therefore call it Cytoblast (kvtoc,
fiXaaToq) in reference to its function, which will be described
hereafter.

This formation varies in its outline from oval to circular,
according as the solid which it forms passes from the lenticular
into the perfectly spheroidal figure. I have found the oval
and flat cytoblasts more frequently in Monocotyledons, in the
albumen and pollen ; the globular chiefly in the Dicotyledons,
and in the leaf, stem, articulated hairs, and similar structures ;
no exclusive rule, howrever, can be laid down on this point.

The colour of the cytoblast is in general yellowish, but it
sometimes passes into an almost silvery white. I remarked it
as being most transparent in the albumen of sonic wat. I
plants, in the unripe pollen, in some Orc/tidcw, and also in the
rudiments of the leaf of Crassula portulaca. Its excessive
transparency renders it scarcely perceptible in the spores of
some Helvelloids. It is coloured by iodine, according to its
various modifications, from a pale yellow to the darkest brown.

It varies considerably in size. It is in general largest in
Monocotyledons, and in the albumen; and smallest in DtCO-

234 CONTRIBUTIONS TO

tyledons, in the leaf, stem, and their metamorphosed parts.
The largest which I have seen measured 00022 Paris inch in
diameter (in Fritillaria pyrenaica) ; the smallest, in the em-
bryonal extremity of the pollen-tube of Linum pallescens, from
0*00009 to 0-0001 Paris inch. In the albumen of Abies excelsa
I found the average of several admeasurements of examples,
which appeared of equal size, to be 0-00034-0-00059-000079.
In the young leaves of Crassida portulaca, 0-0003 ; and in the
albumen of Pimelea drupacea, 0-00095-0-001055. Little im-
portance, however, can, on the whole, be attached to these
admeasurements, since they increase and diminish, and we
cannot determine in what period of its existence the cytoblast
may be at the time.

Its internal structure is in general granulous, without, how-
ever, the granules, of which it consists, being very clearly dis-
tinct from each other. Its consistence is very variable, from
such a degree of softness as that it almost dissolves in water,
to a firmness which bears a considerable pressure of the com-
pressorium without alteration of form. The more recent its
formation, the softer it is; and this also applies to cases in
which its existence is merely transitory. It is denser and more
sharply defined when it endures throughout the whole vital
process of the plant as a permanent tissue, as in the Orchidete.

These peculiarities have been more or less fully described
by R. Brown (Organs and Mode of Fecundation in Orchidese
and Asclepiadese ; Linn. Trans. 1833, p. 710), and recently by
Meyen (Physiologie, &c, Bd. I, p. 207). A phenomenon, how-
ever, has escaped both of these most acute observers, which I
am notwithstanding disposed to regard as one of the most
essential. In very large and beautifully developed cytoblasts,
for example, in the recently formed albumen of Phormium
tenax and Cham&dorea schiedeana (pi. I, fig. 5), there is ob-
served (whether sunk in the interior or on its surface, is not
yet clear to me) a small, sharply defined body, which, judging
from the shadow that it casts, appears to represent a thick
ring, or a thick-walled hollow globule. In examples which are
not so well developed, only the external sharply defined circle of
this ring can be observed, and in its centre a dark point ; for
example, in the stipes of the embryo of Limnanthes Douglasii,
Orchis latifolia (pi. I, fig. 21), Pimelea drupacea (figs. 14, 15).

PHYTOGENESIS.

In still smaller cytoblasts it appears only as a sharply i ir-
cuinscrihcd spot; this is most frequently the case, as in the
pollen of Richardia cethiopica, in the young embryo of lAnum
pallescens, and in almost all Orchidea (fig. 1G) ; or, lastly,
only a remarkable small dark point is observed. I have not,
as yet, succeeded in discovering it in the very smallest and
most transitory cytoblasts (in the leaves of Dicotyled ru for
instance). I have also found two in some very rare c but

they occurred as exceptions to the general rule, and alu a\ -
where the majority exhibited the simple nucleus; for example,
in Chanuedorea schiedeana (figs. 6, 7), Seca/e cereale, Pimelea
drupacea (fig. 1-1) ; in the two latter I have sometimes found
even three (fig. 15). The observations I have made upon all
plants in which it was possible to trace the entire process of
formation completely, lead to the conclusion, that these small
bodies are formed earlier than the cytoblast (pi. I, figs. 1, 'J);
and I am almost inclined to conjecture that they are not alto-
gether unallied to the nuclei which Fritsche has shown to
exist in starch, and may probably indeed be identical with
them.1 The size of this corpuscle also varies considerably,
from the extent of half the diameter of the cytoblast to the
most minute point, whose size could not be measured in con-
sequence of the thread in the diaphragm of the microscope
exceeding it so much in thickness. In the albumen of Ahics
excelsa I found it to average from 0-000015- 0-00009. ~> Paris
inch; in Pimelea drupacea, from 0-00029-0-0003. Sometimes
it appears darker, at others brighter, than the remaining mass
of the cytoblasts. In general it has more consistency than
the rest of the cytoblast, and continues sharply defined alt or
that has been changed by pressure into an amorphous mass, as
in Pimelea drupacea for example.

There is a second point, on which I must say a few words,
in order to be enabled to express myself more briefly hereafter
without being unintelligible, which relates to the different
inorganic substances that occur during the vital process oi
plants, and pertain to the scries of starch and woody tibi I

make no pretensions whatever to a complete enumeration of all

1 More accurate investigation of the structure <>f the starch granule* ha- ihown
this supposition to he quite untenable.

23(5 CONTRIBUTIONS TO

the substances which differ in a chemical sense; and just as
little do I require that chemists should approve all my terms
and characteristics (independent of this, perfection at the pre-
sent time would be an impracticable task) ; I shall merely
notice in a few words the most important modification s, their
consequence and signification in the course of the develop-
ment of vegetable organization, in order to avoid repetitions
in future.

In the plant starch appears almost to take the place of
animal fat. It is superfluous nutritive material, which is de-
posited for future use; and we therefore usually find it in
places where a new formative process is to commence after a
short repose, or where a too luxuriant life has generated a
superabundance of nutritive material. It has of late been the
subject of such deep research that it is unnecessary for me to
enter upon it more fully ; I will merely refer the reader to
the most recent and practical summary of the results in
Meyen's Physiologie, Bd. I, p. 190, &c.

The starch is sometimes supplanted by a semi-granulous
substance ; for instance, in pollen, the albumen of some plants,
and frequently in the cells of the leaf, as matrix of the
chlorophylle. It is chiefly distinguished by its occurrence in
irregular, granulous forms, which have no internal structure,
and from its being coloured a brownish -yellow or brown by
tincture of iodine. This substance, which I shall call mucus,
is probably identical with that of which the cytoblasts are com-
posed, and with the small granules in gum, which I shall pre-
sently mention. Meyen has already remarked the probability
of the first supposition (Physiologie, Bd. I, p. 208).

But when the starch is to be employed in new formations,
it becomes dissolved, in a manner as yet quite unknown in
chemistry, into sugar or gum, the latter sometimes appearing to
pass into the former, or vice versa. The sugar appears in the
form of a perfectly transparent fluid, which is almost as clear
as water, is not rendered turbid by alcohol, and receives from
tincture of iodine only so much colour as corresponds to the
strength or weakness of the solution of the reagent.

The gum appears as a somewhat yellowish, more consistent,
and less transparent fluid, which is coagulated into granules by
tincture of iodine, assuming a pale yellow permanent colour.

PHYTOGENESIS. _.;7

In the further progress of organization (in which process the
gum is always the last, immediately preceding fluid), a quantity
of exceedingly minute granules appear in it, most of which,
on account of their minuteness, look like mere black points.
Iodine then seems to colour the fluid a somewhat darker
yellow. The granules, however, when their size is sufficiently
large to render their colour perceptible, become of a dark
brownish-yellow under its influence.

It is in this mass that organization always takes place, and
the youngest structures are composed of another distinct, per-
fectly transparent substance, which presents an homogeneous
colourless mass when subjected to pressure; when dried it
imbibes water and swells; it is not at all affected by tincture
of iodine, nor does it ever imbibe it ; after pressure it appears
as colourless as before, and is so completely transparent as to
be altogether invisible when not surrounded by coloured or
opaque bodies. This substance frequently occurs in plants (for
example, in great quantity, together with a little starch, in
peculiar large cells in the tubers of Orchis) ; for brevity's sake
I shall call it vegetable gelatine ; and am inclined to class
under this head, as mere slight modifications, pectine, the basis
of gum tragacanth, and many of those substances which are
usually enumerated under the term vegetable mucus.

It is this gelatine which is ultimately converted by new
chemical changes into the actual cellular membrane, or struc-
tures which consist of it in a thickened state, and into the
material of vegetable fibre.

I now pass on to our subject itself. There are two situa-
tions in the plant in which the formation of new organization
may be observed most easily and clearly, in consequence of
there being cavities closed by a simple membrane, viz. in the
large cell, which subsequently contains the albumen of the
seed, the embryonal sac, and in the extremity of the pollen-
tube, from which the embryo itself is developed. The em-
bryonal sac never contains starch originally, but probably, in
most instances, the saccharine solution (which gives the Bweet
taste to unripe pod-fruits and the Cerealia), or gum.

The pollen, on the contrary, always contains starch, or the
above-mentioned granulous mucus representing it, as an essen-
tial constituent part. The so-called vegetable spermatozoa

238 CONTRIBUTIONS TO

will, probably, on more accurate investigation, be mostly re-
duced to one of these substances. These substances, however,
soon become dissolved, and converted either into sugar or gum ;
both changes take place at times, even before the pollen-grain
has commenced to send forth tubes upon the stigma, frequently
during the gradual descent of the pollen -tube through the
style to the ovule ; so that in some cases unaltered starch may
still be found even in the embryonal extremity.

At both these situations the before -mentioned minute
mucus -granules are very soon developed in the gum, upon
which the solution of gum, hitherto homogeneous, becomes
clouded, or when a larger quantity of granules is present,
more opaque. Single, larger, more sharply defined granules
next become apparent in the mass (fig. 2, the upper part) ;
and very soon afterwards the cytoblasts appear (fig. 2, the
lower part), looking like granulous coagulations around the
granules. The cytoblasts, however, grow considerably in this
free state ; and I have observed, in Fritillaria pyrenaica for
instance, a gradual expansion from 0'00084< to 0*001 Paris inch.

So soon as the cytoblasts have attained their full size, a
delicate transparent vesicle rises upon their surface. This is
the young cell, which at first represents a very flat segment of
a sphere, the plane side of which is formed by the cytoblast,
and the convex side by the young cell, which is placed upon
it somewhat like a watch-glass upon a watch. In its natural
medium it is distinguished almost by this circumstance alone,
that the space between its convexity and the cytoblast is per-
fectly clear and transparent, and probably filled with a watery
fluid, and is bounded by the surrounding mucus - granules
which have been aggregated together at its first formation,
and are pressed back by its expansion, as I have endeavoured
to represent it in plate XV, figs. 4, 5, 6. But if these young
cells be isolated, the mucus-granules may be almost entirely
removed by shaking the stage. They cannot, however, be
observed for any length of time, for in a few minutes they
become completely dissolved in distilled water, leaving only
the cytoblasts behind. The vesicle gradually expands and be-
comes more consistent (fig. 1, b), and, with the exception of the
cytoblast, which always forms a portion of it, the wall now con-
sists of gelatine. The entire cell then increases beyond the

PIIYT0GENES1S. ,!i

margin of the cytoblast, and quickly becomes so large thai the
latter at last merely appears as a small body enclosed in one
of the side walls. At the same time the young cell frequently

exhibits highly irregular protrusions (fig, 1, c), a proof that
the expansion by no means proceeds uniformly from one point.
During the progressive growth of the cell, and evidently arisi]
from the pressure of the neighbouring objects, the form be-
comes more regular, and then also frequently passes into that
of the rhomboidal dodecahedron, so beautifully defined a priori
by Kieser. (Compare fig. 1, from b to e, with fig. S.) The
cytoblast is still always found enclosed in the cell-wall, in
which situation it passes through the entire vital process of the
cell which it has formed, if it be not, as is the case in cells
which are destined to higher development, absorbed either in
its original place, or after having been cast off as a usel<
member, and dissolved in the cavity of the cell. So far as I
could observe, it is only after its absorption that the formation
of secondary deposits commences upon the inner surface of
the cell-wall (fig. 9).

As a general rule, it is rarely that the cytoblast accom-
panies the cell which it formed through its entire vital proce
nevertheless, it is,

1. Characteristic of the families of the Orchidete and CacU
that in them a portion of their cellular tissue remains in a
lower stage of development during the entire period of life.

2. In various plants it occurs that cellular tissue, which fa
merely a transitory signification, is not perfectly developed.
but retains the cytoblast, and is absorbed together with it at a
subsequent period. Yet I have also remarked that the latter
in the middle period of its existence lost much of its distinct-
ness and sharpness of outline, which, however, reappeared when
absorption commenced; for example, in the nucleus of tin
ovule of Abies excelsa, Tulipa sylvestris, and Daphne alpina. It
is most extraordinary that some physiologists should haw frit
prepared to deny the fact, that absorption takes place in plant
since even very considerable portions of cellular tissue of the
nucleus of the ovule, for instance, become completely fluid
again, and are received into the common mass of the sap. It
is true this only takes place so long as the cell still consist
of the simple original membrane, ami i^ not so far advanced

240 CONTRIBUTIONS TO

in its individual development that its wall is thickened hy
secondary deposits.

3. The cytoblasts also remain persistent in the pollen-gra-
nules in some rare instances ; such is the case in some, perhaps
in all the Abietince. The lenticular cytoblast has already been
observed by Fritsche in Larix europaa, but the true nature
of it was not recognised.

4. Lastly, many hairs, particularly such as exhibit motions
of the sap within their cells, retain the cytoblasts (c,/, fig. 25).
It is at the same time remarkable, and a proof of the close re-
lation which the cytoblast bears to the whole vital activity of
the cell, that the little currents which frequently cover the
entire wall like a network, always proceed from and return to
it, and that when in statu integro it is never situated without
the currents (fig. 25).

I have observed the above-described development of the cells
throughout its entire course in the albumen of Chamcedorea
schiedeana, Phormium tenax, Fritillaria pyrenaica, Tulipa sylves-
triSjElymus arenarius, Secale cereale, Leucoji spec, Abies excelsa}
Larix europcea, Euphorbia pallida, Ricinus leucocarpa, Momordica
elaterium, and in the embryonal extremity of the pollen-tube
of Linum pallescens, Oenothera crassipes, and many other plants.
It was in the summer of 1837, after this treatise had been
written, that I first began to examine the Leguminosce, and
found to my surprise that these plants, so constantly investi-
gated and everywhere employed as illustrations for the history
of vegetable development, afforded the most beautiful and ready
opportunities for the study of this process, which had been
overlooked by all observers. No one, however, had considered
the saccharine fluid contained in the embryonal sac as worthy
of examination.

Without exactly tracing the entire course of the formation of
the cells through all its details, I found the cell-nuclei, previous
to the appearance of the cells, floating loose in the fluid in
very many plants. Finally, I have not met with a single ex-
ample of newly-developed cellular tissue, the cambium excepted,
in which the cytoblasts were wanting. I therefore consider that
I am justified in assuming the process above described to be
the universal law for the formation of the vegetable cellular
tissue in the Phanerogamia.

PIIYTOGKNESIS. 241

My observations arc much more limited with respect to the
Cryptogamia; nevertheless, I found the cytoblasts in the sporidia
of the Helvelloids, where, however, in consequence of their great

transparency, they arc only perceptible with a very strong
magnifying power, and after the field has been much darkened.
I have seen them in the large yellowish cells in the interior of
the so-called anthers in Char a vulgaris, I also observed their
development into cells in the sporules of Marchantia poly-
morpha, one of which, pushing the original wall of the sporule
before it, forms the long capillary root (pi. I, figs. 18-20).

It is evident from the foregoing, that the cytoblast can new i
lie free in the interior of the cell, but is always enclosed in the
cell- wall, and (so far as we can learn from the observation of
those cytoblasts which are sufficiently large to allow of this
very difficult investigation) in such a manner that the wall of
the cell splits into two laminae, one of which passes exterior,
and the other interior to the cytoblasts. That upon the inner
side is generally the more delicate, and in most instances only
gelatinous, and is also absorbed simultaneously with the cyto-
blast (figs. 8, 16, 21). In making a section, they are some-
times detached and scattered over the object, which might lead
to the supposition that they lay free. It is probable also that
subsequently, when absorption commences, they do become dis-
engaged from their connexion with the cell-wall, and a Blight
touch may then be sufficient to move them from this position.
The cell-wall is often considerably thickened in their neighbour-
hood, especially when they are somewhat globular; for instanc
in the pollen-tube, which has become cellular in certain Orchid'
(figs. 16, 20).

Meyen, who should always be consulted with reference to
anatomical questions, has endeavoured, in his Physiologic, vol. i,
p. 45, &c., to establish the opinion, that the cell is formed
of spiral fibres which lie closely one upon another, founding
his view in a most ingenious manner upon his own beautiful
observations on the relations of structure in fully-developed
cells. My direct observation, which may easily he repeated
by every one, shows, it is true, quite a different mode of forma-
tion; I must, however, bring the facts related by Meyen into
unison with my discovery, in order not to permit an apparent
contradiction to remain unresolved.

L6

242 CONTRIBUTIONS TO

Meyen himself correctly observes, when treating of those
spiral tubes whose very narrow fibres lie close upon one
another, that an enveloping membrane could not indeed be
observed, but that this by no means justified our concluding
on its absence. For if the thickenings of the cell-walls which
are formed in most, perhaps in all, cases in spiral lines, in
those instances in which they make their appearance early,
whilst the original cell-wall itself is yet in statu nascentia and
soft, become firmly connected with the latter; and if at the
same time the separate coils of the spiral fibre lie perfectly
close one upon another, so that with our present microscopes
no space remains perceptible between them, — it naturally fol-
lows that on tearing the entire membrane (the so-called un-
rolling of the spiral vessels), the fracture in the direction of
the coils of the fibre must be so sharp that our instruments
could not possibly show the inequalities. At the same time
it should be remembered that the original cell-membrane,
especially in long hair-cells, frequently undergoes so great an
expansion that it must at last become infinitely delicate, so
that even the thinnest and apparently most simple cell-wall
does not exclude the possibility of its being composed of the
original membrane and the secondary deposit. If, then, we
proceed from those spiral cells and vessels whose coils are so
far distant from one another as to admit of no doubt with respect
to the existence of an external enveloping membrane, and if
we trace the presence of this membrane through all the forms
of the constantly approximating coils of the fibre, until only
the feebleness of our optical instrument renders further direct
observation impossible, the laws of sound analogy require that
we should, in such instances, also admit the presence of a
similar membrane. There is yet a more direct mode of proof,
namely, the investigation of the history of the development.

It is an altogether absolute law, that every cell (setting
aside the cambium for the present) must make its first appear-
ance in the form of a very minute vesicle, and gradually
expand to the size which it presents in the fully -developed
condition ; an extended investigation of this formative process
also invariably shows that a cell never exhibits a trace of
spiral formation, discoverable either from its aspect, or
on tearing it, previous to its complete development, i.e. before

PHYTOGENESIS. 243

it has absorbed the cytoblast. Id all spiral cells, particularly
such as exhibit detached fibres, we find the walls of the fully-
developed cells to be perfectly simple at the commencement.
For instance, I remarked this in the outer parchment-like laj
of all aerial roots.1 Meyen discovered the spiral fibres in
Oncidium altissimum, Acropera Loddigerii, Brassavola cordata,
Cyrtopodium speciosum, Aerides odorata, Epidendron elongatum,
Cattleya Forbesii, Colax Harrison)}, and Pothos crassinervia.
This is still more evident in the true cortical layer of tho
aerial roots, where I discovered in Colaxy Cyrtopodiumt and
Acropera the far more beautifully developed and much broader
spiral fibres. There is no trace of them to be found in quit
young aerial roots, and their formation pertains decidedly to
a process of lignification.

We find further evidence that the spiral fibres do not occur
until a subsequent period in the pericarp of the Casuarin
the cells of which, previous to or shortly after impregnation,
do not evince a trace of spiral formation. Meyen, in his
Physiologie, has taken too little notice of these iibre-cells in
the envelopes of many seeds, which is the more to be re-
gretted, as these interesting and sometimes extremely pretty
formations promise some explanation respecting the physiology
of the cell-life, especially if the opportunity should occur of
investigating the individual development of several of them
accurately. I may be permitted to communicate a lew obser-
vations on this subject.

Their occurrence is more extensive than is generally sup-
posed. They are found in the hairs of the pericarp in some
Composite, where they were found by Lessing in Perdicium
taraxaci and Senecio flaccidus, and by myself in Trichocline
humilis and heterophylla.

1 Meyen, in his Pbytotomie, p. 1G3, called this an outer cortical layer, which ^.^
situated on the true epidermis of the aerial roots. Some doubts have recentlj been
raised as to the correctness of this view. It may. however, be slmosl incontestably
proved, since the cellular layer, which Meyen calls epidermis, possess) I actual
mata, which, in consequence of their being covered, usually indeed occur onl) in a
rudimental form, frequently exhibit a more compUcated structure, although deviating
only in appearance, as hx Ae rides odorata, but often likewise appeal of quite the
ordinary form, as in Pothos crassinervia. Moreover it was not Dutrochet, as would
seem from Meyen's Physiologie, p. is, hut Link, who firsl drew ittention to this
laver.

244 CONTRIBUTIONS TO

They occur in the epidermis of the pericarp in many Labiates,
as in Ziziphora, Ocymum ; in most Salvias, for instance, Urn-
bata, hispanica, Spielmanni, &c. ; and lastly, in Horminum
pyrenaicum. My uncle Horkel was familiar with them in all
these many years ago; Baxter noticed and published their
occurrence in Salvia verbenacea only. I can add to these
Dracocephalum moldavica.

R. Brown discovered them in the parenchyma of the peri-
carp in the Casuarinoe, and I in the spongy inflated cellular
tissue in Picridium vulgare, where they mostly occur in a
reticular form, and present an extremely beautiful appear-
ance.

Horkel also discovered them in the epidermis of the seed
itself in the Polemoniacece long before Lindley made known
their presence in Collomia linearis. They occur in Collomia,
Gilia, Ipomopsis, Polemonium, Cantua, Caldasia, and perhaps in
the entire family, with the exception of Phlox, with which
genus Leptosiphon, in which are the first indications of them,
is closely allied. Horkel had also studied them in the seeds
of Hydrocharis, where they occur in the highest degree of deve-
lopment, long before Nees von Esenbeck published the fact.
Robert Brown mentions them in the Orchidece, which statement
I find confirmed as to most of our native species of Orchis.
I have also discovered very beautiful spiral fibre-cells in the
epidermis of the seed of Momordica elaterium, and a very deli-
cate reticular formation of fibres in Linaria vulgaris, Datura
stramonium, in Salvias, and in several other Labiates ; probably
it is common to the whole family.

Lastly, they occur, according to HorkeFs discovery, in the
parenchyma of the integuments of the seed in Cassyta and
Punica.

Whether these formations be studied in their individual
development in a single species, or in their progressive stages
in a series of allied plants, some highly interesting general re-
sults will be obtained in either case. The universal and alto-
gether absolute fact at which we first arrive is, that the fibres
are never formed free, but are developed in the interior of
cells ; and that the walls of these cells in the young state are
simple, and generally very delicate. Corda's statement re-
specting spiral cells without an enveloping membrane (Ueber

PHYTOGENESIS. 245

Spiral faserzellen , &c., pp. 7, 8) is bused upon inaccurate ob-
servation.

These cells are at first generally filled with starch; rarely
with mucus or gum. The starch always passes into the latter
substance in the progress of development ; and this 11 con-
verted into jelly, the change, as it would seem, taking place
from without inwards. This jelly finally is converted at its
outer surface into vegetable fibre, following the direction of a
spiral line, the coils of which are sometimes narrower, some-
times wider. When these forms are observed in their different
stages of development and in their various conditions, the idea
involuntarily forces itself upon the mind that the spiral forma-
tion is the result of a spiral movement of a fluid on the walls
of cells between them and the central jelly. Ilorkcl once
actually observed the motion of small globules between the
coils of the fibre in progress of formation in Hydrocharit.

The great variety in the appearance of the fibres seems to
depend upon the period of their origin, and on modification in
the chemical changes of the formative material. It probably
depends solely upon the former circumstance whether the spiral
fibre lies free in the cell, when it is formed very late, or
whether it is blended with the membrane of the cell, if its
development commence at a period when the cell-membrane
itself is yet very soft and gelatinous, and may consequently
become agglutinated to the fibre, which is likewise still in a
gelatinous state.1 This is the case in Casuarvna, Cassytha,
Hydrocharis, Trichocline, Orchis, &c. ; in most cases, however,
the cell-wall is too far developed to unite with the fibre,
and the latter then lies loose in the interior of the cell. In
rarer instances the material is almost entirely applied to the
formation of the fibre (always indeed when the fibre coalesces
with the wall), for example, in Salvia Spielmanni, M<>-
mordica elaterium. I have reason to suppose that this com-
plete consumption almost always takes place in spiral reatl Is,
and is the cause of their subsequently conveying only air.
More frequently, however, one or more fibres an1 formed ; but
then a great portion of the jelly has still remained uncon-

1 Subsequent researches, have produced important modification* in tbifl opinion.
Consult my essay on the Spiral Formations in Vegetable Cells. Ploi 18 ' Nea

21, 22, PI.

246 CONTRIBUTIONS TO

sumed, which, when the cell is moistened with water, comes
forth in form of an intestine (wie ein Darm hervortritt), and in
swelling expands itself over the fibres, thus appearing to sur-
round them ; this is the case in most Salvia and Polemoniacecs,
in Senecio flaccidus, Ocyinum polystachyum and polycladum
{Lumnitzera, Jacq.) There is an intermediate form between
this and the former, when the jelly itself forms a broad
spirally-wound band, which appear upon its surface to be com-
posed of innumerable delicate fibres ; their occurrence in this
state is very beautifully shown in Perdicium Taraocaci and
Zizipliora. A still less advanced stage of development exhibits
merely a cylinder or cone of gelatine in the interior of the
cell, the surface of which, however, is marked with delicate
spiral lines. This is seen in some Salvia, in S. verticillata for
example, and in Leptosiphon androsaceum. Finally, the lowest
stage of development is where the gelatinous cylinder, which
is furnished with spiral strise, has a cavity in its interior con-
taining starch, which has not as yet undergone decomposi-
tion : this instructive phenomenon is found in Dracocepjhalum
moldavica, Ocymum basilicum, and some allied species. In illus-
tration of the above, consult plate 2, figs. 1-10, with their
exjnanations.

Before quitting the subject of spiral fibre, I will merely
add, what indeed has been of late admitted by every good
observer, that the only difference between spiral cell and spiral
vessel consists in the dimensions, although constant transitions
may be observed between them just as well as between the cells
of the liber and the parenchyma ; and consequently, as regards
this doctrine at least, there is no longer any place for natural-
philosophical phantasies about the arrestment of ideal forms of
higher types, and such like empty words. That which forms a
liber-cell out of a round cell, the preponderating expansion of an
organ lengthwise, is also that which transforms the spiral cells
(the vermiform bodies) into spiral vessels. The function of the
spiral fibre, however, is, as every candid vegetable physiologist
will certainly admit, entirely unknown to us at the present
time. It is certain that spiral vessels and spiral cells occur in
the living plant quite as frequently filled with sap (in the
younger vegetating portions) as with air (in the older organs
which have attained their full size) ; and it is this which has

PHYTOGENESIS. 247

given rise to the conflicting views of authors. Hut the Bame
also occurs in all cells under certain circumstances, and the
influence of the spiral fibre remains meanwhile altogether ob-
scure and unexplained. Perhaps the foregoing may render it
probable that the spiral is everywhere only a secondary varia-
tion of form in the product of the vital power (the fibrin) pro-
duced by a different tendency of the vital activity of the coll,
so soon as this is compelled, as a certain stage of its develop-
ment, to give up its independent individuality, and enter ;is an
integral portion into the complex of the entire plant.

I also think that we may venture, in conclusion, to deduce
from the data above enumerated, that this indication of a spiral
formation is the surest sign that we have no longer anything
to do with the simple cell-membrane.

I now return, after this somewhat lengthy digression, to my
subject. The process of cell-formation, which I have ju-1
endeavoured to describe in detail, is that which I have observed
in most of the plants which I have investigated. There are,
however, some modifications of this process which make the
observation of many parts very difficult, and sometimes indeed
render it impossible, although, notwithstanding this, the law
remains undisturbed and universally valid, because analogy
requires it, and we can fully explain the causes of the impossi-
bility of direct observation.

The difficulties which I now notice depend especially upon
the physical and chemical properties of the substance which
precedes the formation of cells. The materials enumerated
above are to be regarded as scarcely anything more than sepa-
rate facts, which, for the purpose of giving a general view and
rendering the classification more easy, I have intentionally
selected from the organic chemical processes of vegetable life,
which are constantly in operation, and with which we are as
yet totally unacquainted. Almost all these materials con-
stantly exist together in the living plant, and it is merely
their preponderance in a greater or Lesser degree which enables
us to sav that the cell contains ainvlinn or gum, and BO forth,
Onlv towards the termination of the individual life of tin-
cells do we find them filled with a le^s number of different
substances; the cells which contain ethereal oil are probably
the onlv instances in which we find but a Bingle one.

248 CONTRIBUTIONS TO

If we now assume a cell to be completely filled with a
transparent solution of sugar in which there is rapidly gene-
rated just so much gum, as may form, by an equally quick
conversion into jelly, a delicate cell-membrane, the exist-
ence of which we cannot possibly recognise with the micro-
scope, in consequence of the similar refracting power of the
wall, the contents, and the surrounding medium ; it then be-
comes exceedingly probable that a number of such formative
processes may go on which escape our observation, and become
known to us only in their results, when, after the absorption
of the parent-cell, we suddenly find two new ones in its place.
If, on the other hand, our attention has been previously directed
to this process, we have, in the application of reagents, espe-
cially iodine, which is quite indispensable to the physiological
botanist, several means of rendering it visible in instances
where it is suspected to be going forward. Gradual transition
to the completely invisible processes are readily found by more
extended investigation ; I will just mention one of the most
difficult instances which I have met with, by way of example.
It occurs in the germination of the sporules of Marchantia poly-
morpha. Only a few, generally only from two to four of the
cell-nuclei which appear in the sporules, serve for the formation
of cells.; the others become quickly enveloped with chlorophyll,
and are thus withdrawn from the vital process. The transparent
fluid, however, in which these cytoblasts float, passes through
the remaining stages of the metamorphosis into cell-membrane
only just at the boundary of the latter, and with such rapidity
that the exceedingly delicate young cells cannot be distin-
guished by anything else than a minute, generally more or less
uninterrupted circle of infinitely small, black granules, and by
a scarcely perceptible greater transparenc}7 of the contents of
the newly-formed cells in comparison with that of the parent-
cell, and finally, under the most favorable circumstances, by the
spot at which the newly-developed cells come into contact, the
point of juncture being still covered by the membrane of the
parent-cell. (PI. I, figs. 18-20.) This may perhaps be general
in the Cryptogamia, and especially in water plants, and probably
Mohl's division of the cells of Conferva may be thus explained.

If we consider, however, that there are undoubtedly many
plants, among which the Fungi and infusorial Algcz should pro-

PHYTOGENESIS. 249

bably be classed more especially, in which we arc, as yet .it
least, totally unacquainted with the cytoblasts, in conseqi enoe
of tlicir absolute minuteness and transparency; if we further
bear in mind that the nucleolus in the cell-germ, even
in the larger cytoblasts, frequently appears immeasurably
small, or even entirely escapes the eye with the highi -st mag-
nifying power j and, lastly, if we deduce from what has been
previously stated, that nevertheless this granule, which can do
longer be rendered perceptible, probably furnishes in the suit-
able medium a sufficing cause for the formation of a cytobla-t
which serves as an introduction to the whole formative process
of the cells ; then, indeed, we are forced to confess that the
imagination obtains ample latitude for the explanation in every
case of the generation of infusorial vegetable structure, even
without the aid of a deus ex machina (the y cue ratio spontanea).
But my present object is to communicate only facts and their
immediate consequences, and not to dream; I will therefore
rather add a few more observations on the growth of the
plant.

What is meant by to grow ? is a question to which every child
quickly replies, " when I am getting as big as father." There
is truth in this answer, but not sufficient to satisfy science.
Words have no value in themselves, but arc like coin, merely
tokens of a value not exhibited in specie, in order to facilitate
commerce. And to carry the simile further, insecurity in this
intellectual property, and frequently bankruptcy results, if
this coinage has not its unchangeable, accurately-determined
standard ; in a word, the utility of a scientific expression de-
pends upon the accurate definition of the idea on which it is
based. Unfortunately the perplexity of our social relations hat
caused us to forget entirely the original meaning of mon<
the sign has become to us the thing itself; may some good
genius protect us from similar mistakes in our intellectual life.
We must here be on our guard against two dangerous roCK
first, when we transfer words from one science to another,
without first accurately testing whether they tit their new
situation as respects all their accompanying significations al
and, secondly, when we voluntarily lose sight of the significa-
tion of a word consecrated by the spirit of the language and
its historical development, and employ it without fui

250 CONTRIBUTIONS TO

mony in compound words, where perhaps, at the most, only
some unessential part of its signification suits.

Thus E. Meyer, for example (Linnsea, vol. vii, p. 454), after
repeating the well-known experiments of Duhamel, lays down
this position : " the law of the longitudinal growth of the
internodes is to grow in a direction from above downwards."
He requires this position for his theory, and must consequently
defend it in every way, although he himself confesses that this
reversed growth must appear paradoxical to every one of his
readers. He would never have arrived at this position if he
had more accurately analysed the word " grow" (with which
animal physiology had rendered him familiar), with reference
to its applicability to the plant; he would soon have discovered
that the generation of new cells, and so far the actual growth
of the plant, constantly takes place in its outermost portions
in an upward direction, and that his very simile of the building
up a voltaic pile is exceedingly well adapted to refute himself.
The experiments of Duhamel and Meyer would have no fur-
ther result than to show that the inferior, that is, the earliest
generated, older cells of the internode possess a greater capa-
bility to extend in the longitudinal direction, and retain this
power longer than the younger cells.

We have an excellent illustration of the second point in the
proposition so frequently expressed of late, that the stem of
the plant is composed of the coalesced petioles. The word
" coalesce" [verwachsen, to grow together) has possessed, how-
ever, from time immemorial, both in ordinary life and in
science, the signification that two or more originally and
naturally separate parts have become by the j)rocess of growth
either abnormally or, under certain circumstances, normally
united. If therefore the word " coalesce" be applied to the
stem of the plant, an organ, which, in every period of its ex-
istence, under all forms of its appearance, is a simple and
undivided one, and at the origin of the plant even constantly
appears earlier than the leaves with their petioles, it certainly
involves a mischievous abuse of language, by which science
itself can gain nothing, and will even lose in the estimation
of the intelligent non-professional man, who sees through such
a play upon words. What would the zoologist say were we to
regard the trunk as a coalescence of the extremities.

PIIYTOGENESIS. 25 1

I return then to my question : what is the meaning of to
grow? In hackneyed phrase we are told, " To grow signift
increase of the mass of an individual, and takes place in the
inorganic world by juxtaposition, in the organic by intus-
susception." Have we gained anything for vegetable physi-
ology by this reply ? I think not. If the plant is to grow
by intussusception, then I say it consists of an aggregate of
single, independent, organic molecules, the cells j it increases
its mass by new cells being deposited upon those already ex-
isting i consequently by juxtaposition. But the single cell in
the progress of its expansion, which frequently reaches an
enormous bulk in comparison with its original size (I will
merely remind the reader of the pollen-tubes), also increases
in substance in the interior of its membrane, and by this
means also the mass of the entire plant is increased ; it con-
sequently grows by intussusception also. Finally, after a certain
period the cell deposits new organic material in layers upon its
primitive membrane ; thus another form of juxtaposition, which
still, however, belongs to the cycle of vegetable vitality. It
hence becomes readily apparent that, in respect to scientific
botany, the idea "grow" still requires a new foundation in
order to be capable of being applied with certainty.

Of the three instances just cited, the second and third
belong more to the individual life of the cells, and are of
secondary importance only, as respects the idea of the whole
plant, regarded as an organism composed of a certain number of
cells. The plant considered in its totality increases its mass, that
is, the number of the cells composing it, in the first way only.
We must therefore here discriminate three processes essen-
tially distinct from each other in a physiological sense, which,
when strictly regarded, scarcely find an analogy in the other
kingdoms of nature.

1. The plant grows, that is, it produces the number of cells

allotted to it.

2. The plant unfolds itself by the expansion and develop-
ment of the cells already formed. It is this phenomenon
especially, one altogether peculiar to plants, which, because it
depends upon the fact of their being composed of cells, can
never occur in any, not even the most remote form in crystals
or animals.

252 CONTRIBUTIONS TO

3. The walls of the fully- developed cells become thickened
by the deposition of new matter in layers, a process which, in
accordance with the old rule, a potiori fit denominatio, may be
most aptly termed the lignification of the plant.

If, in respect to the growth of the plant, we now hold to
the literal sense conveyed under No. 1, then this question
must arise, — Where are the new cells formed? Here three
instances comprise all possible replies. Namely, the new cells
are either formed outside on the surface of the entire previous
mass, or in its interior ; and in that case again either in the
intercellular spaces or in the cells themselves ; quartum non
datur.

Mirbel, in two extremely ingenious and profound memoirs
on the Marchantia polymorpha, which he presented to the
French Academy in 1831 and 1832 (p. 32), has expressed the
opinion, that all the three cases just now mentioned as possible
do actually occur in plants. Without intending here to anti-
cipate what follows, I must remark that only one case (the
formation of new cells within the old ones) appears to be
proved by his direct observations. The second case is merely
a conclusion drawn, and the germination of the sporules of the
Marchantice, which was to elucidate the third case, has been
observed by me to be quite different, as I have already repre-
sented above.

Finally, however, we have yet to examine whether the differ-
ence of the organs may not establish such a physiological
difference of growth as may merit our attention. We may
distinguish here four instances. We observe : 1. The develop-
ment of the plants in the upward direction [in puncto veye-
tationis, C. Fr. Wolff). 2. The elongation downwards. We
thus comprise the formation of the necessary organs of the
plant, the stem, the leaves (with their metamorphoses), and the
root. 3. We have to keep in view the production of accidental
organs, for example, bulbs, &c. And, 4. We find an annual
thickening of the axile formations, the development of the
woody stem.

Let us now see which of the three possible modes of forma-
tion of new cells is actually realised in each of the cases just
enumerated. I have already explained how the new cells are
developed in the embryonal sac ; in other words, within a large

PHYTOGENESIS. 253

cell. A similar process occurs in the embryonal end of the
pollen-tube, consequently in a highly elongated cell; I shall
now proceed to describe the further development of the embryo.
After the first cells, generally few in number, are formed, they
rapidly expand to such an extent that they fill the pollen-tut)
which soon ceases to be perceptible as the original enveloping
membrane; but at the same time several cytoblasts origin)
in the interior of each of these cells, and generate new eel
on the rapid expansion of which the parent-cells also < to

be visible and become absorbed. The same process is repeated
indefinitely. But since the newlv-gcneratcd cells have con-
tinually less room to expand, and therefore constantly beco?
smaller, the previous transparency is soon lost in consequence
of the continual production of new cytoblasts in the interior,
and the tissue becoming more and more compressed; and from
this stage to the perfect completion of the embryo we are eon-
ducted by the clearly logical inference that the process thus
introduced continues the same, since no new force comes into
operation which could induce us to assume a sudden variation
of the vital action, more especially as we soon meet with the
same manifestation of the vegetative power again.

Meanwhile the seed germinates, and the embryo becomes a
plant; and then indeed the question may arise, — Does the pro-
cess of life continue the same thenceforward in the interne-
and foliaceous organs? Now we are here very quickly con-
vinced of the negative, that is, that a formation of new cells on
the surface of the existing organs does not take place. The
surface is always smooth, and generally covered in a very early
state with a kind of epidermis, the outer layer being more
transparent and almost as clear as water; and we never find
even an indication of a newly-formed cell upon the surface.

But if the embryo be the type of the entire plant, and the
latter do not present anything which is not a repetition of its
organs, if we have found the growth of the embryo to con-i>t
only in the formation of cells within cells, we may then expect
to find the same result also in the process of the growth of the
whole plant. It is especially a foliaceous organ, the anther,
which has hitherto been studied and followed in its develop-
ment by many celebrated men (particularly well by Mirbel
and here it is quite decided that the increase of cells takes

254 CONTRIBUTIONS TO

place within the old ones. It is also certain that in this case
the formative process accords with that above described. R.
Brown and Meyen have enumerated many instances where
they observed the cytoblast in very young pollen-cells. In
Pinus, Abies, Podostemon, Lupinus and others, I have traced
the development of the pollen after Mirbel perfectly; I have
distinctly observed the cell-nuclei and their development into
new cells within one another in Abies, never having missed the
cytoblast in young cells.

Now if the pollen-grains be nothing more than converted
leaf-parenchyma, if the anther be merely a metamorphosis of
the leaf, we may certainly infer inversely that the process
which we have observed in it, and which characterized the
formation of the embryo and cotyledons (as prototypes of the
leaf) will be again found in all foliaceous organs. For the
same reason which was stated with respect to the later stages
of the development of the embryo, actual observation is infi-
nitely difficult in this case. I have nevertheless examined a
great many buds in reference to this point, and have most
decidedly convinced myself of the identity of the process both
in the constantly elongating apex of the axis, and in the leaves
which always originate somewhat beneath it. Succulent plants,
the Aloinece and Crassulacece, are best adapted for this purpose.
Crassula Portulaca seemed to me most advantageous, for in it
I first succeeded in separating some cells from their connexion,
in whose interior young cells were already developed, without,
however, entirely filling the parent-cell. But having once be-
come familiar with the subject, I was afterwards able to detect
these individualities from amongst the apparently semi-organised
chaos in all other plants. Another circumstance indeed pre-
sents itself here, which renders the subject much more difficult
than in the case of the embryo. For, independently of the
minuteness of the cells, their walls, in those parts of the plant
which are just newly formed, still consist merely of jelly, and
are so delicate that it is exceedingly difficult to separate the
parts intended for examination without completely destroying
the organization. (Compare plate I, figs. 22-4.)

This process is more easily perceptible in articulated hairs,
and in such as have a head consisting of several cells, where
the same appearances which I have so frequently observed in

PIIYTOGENESIS.

_ : . > . >

the young embryo, and sucli as Mirbel has so beautifully
described in the development of the gemma' in the cups of
Marchantia, may be readily and beautifully seen; for example,
in the common potato. Meyeu lias also made Bimilai ob-
servations, although he still expresses himself with some doubt
on the subject. (Wiegmann's Archiv, \K:)7, vol. ii, p. 22.)

It is not until after as many cells are formed as the organ
requires for its completion that the cell-walls become firmer,
and then commences the unfolding of the organ by the mere
expansion of the cells already formed.

But I must here enter somewhat more into detail, in order
to explain the probable origin of the vascular bundles and
epidermis. At a somewhat early period a stripe of more trans-
parent cells is defined in the axis of the leaf which is in the
act of formation, within which no more new ones are deve-
loped, and these cells soon considerably exceed in size those of
the remaining mass, which are constantly becoming smaller
by continual division. These cells are the basis of the future
vascular bundle which forms the midrib of the leaf; for whilst
the parenchymatous cells subsequently expand in every direc-
tion, these are developed in their longitudinal dimension only,
and are thus enabled, although fewer in number, to follow the
expansion of the other cells in the longitudinal direction of the
leaf. It is not till a later period that these cells, in conse-
quence of a difference in the depositions in their interior, be-
come distinguished into spiral vessels and cells of the libt r.
The spiral vessels are always first perceptible in the new In-
formed parts, and in the entire bud also, in the immediate
neighbourhood of the old, previously-formed spiral vessels; and
they proceed in this manner downwards from the stem into
the new parts. I do not understand therefore what is intended
when the fibres of the stem are regarded as descending from
the buds; one might just as well conceive the river to run
from the ocean to its source.

A similar process occurs in the development of the Bide
nerves of leaves. The formation of new cells generally ceac
at an early period in the outermost layers of cells. The cells
there are soon filled with a limpid fluid, and, by the expansion
of the subjacent parenchyma, naturally become superficial, flat,
and expanded.

25G CONTRIBUTIONS TO

The cells of the vascular bundle and of the epidermis
appear in this way to be less potentialized, — are as it were
cells of lower dignity than those of the parenchyma; and
perhaps this physiological peculiarity is connected with the
fact, that they more rarely secrete peculiar chemical substances,
but for the most part become thickened only by depositions
within their walls of new vegetable fibrous (or more correctly
membranous) substance. I cannot forbear venturing some
suggestions in this place, which are perhaps less closely con-
nected with the subject of this memoir, but which may possibly
at some future time be of importance for the understanding of
the entire plant. Let us recapitulate the process of growth of
the plant just now represented. A simple cell, the pollen-tube,
is its first foundation. Within this, cells are generated; in them
new cells are developed, and so forth, throughout the entire
life. But here the above-mentioned mode of the origin of the
vascular bundles and of the epidermis in relation to the paren-
chyma would indicate, that the lower the dignity of the cell,
the greater power does it possess, in the first place, of expand-
ing and extending in length, and the less capacity does it
possess, in the second place, of forming peculiar finer sub-
stances in its interior. If now the potentialization (poten-
zirung) of the cells proceed throughout the entire growth of
the plant, there thence results a constantly advancing approxi-
mation of organs otherwise kept asunder, and continually rising
ennoblement of the substances developed in the cells. Conse-
quently, the lower parts of the internodes will appear to be
more elongated than the upper ; the leaves and young shoots
{summitates herbarum, Pharmacol.) to contain nobler saps than
the stem ; the members become shortened as they approach
nearer to the upper terminal point of the plant, the leaves
come closer together, and the result of the continually
increasing potentialization of the cell, of the constantly dimi-
nishing expansion in length, of the constantly advancing ap-
proximation of the lateral organs, of the constantly rising
ennoblement of the substances developed, is, finally, the flower
in its individualised distinctness, with its splendour of colour,
its perfume, and its mysterious capacity of determining, by
means of its juices, a single cell to be developed afresh into an
independent plant, and to pass anew though the same cycle.

PHYTOGBNBSIS.

I return, after tins digression, to my subject. So far I be-
lieve I have demonstrated tolerably conclusively, and in accord-
ance with nature, that the entire growth of the plan! ' consists
only of a formation of cells within cells. Let as now pasfl on
to the root. I can contribute but very little to the explana-
tion of this part of the subject; for I have not as yet succeeded
in arriving at any satisfactory result, from the somewhat limited
researches which I have instituted; for instance, 1 have been
altogether unsuccessful in deciding the question as to wheth
a fluid is secreted at the extremity of the radicle, in which
new cells are developed. On the other hand, it is certain that
there exists in the extremity of the root a concavo-convex
mass (a meniscus) of cellular tissue, in which the process of
cell-formation takes place in the same manner as in the pai
of the plants which grow in the ascending direction. A chief
cause of the elongation of the root consequently consists in
this, — that new cells are continually formed in the interior of
the existing cells, on the convex side of that mass of cells,
while on the concave side, the cells already formed expand
simultaneously, and chiefly indeed in the longitudinal direction,
and in this way constantly push the extremity of the root
before them.

The third case, the formation of the accidental organs of the
plant, I must entirely pass over, as I am altogether unpro-
vided with any personal observations upon the subject. Pro-
bably, however, the process here is the same as in the previous
cases, forMeyen (Physiologie, vol. i, p. 209) observed the cell-
nuclei in germinating tubers of Orcludece. Analogy also leads to
a similar conclusion, since all these parts are nothing more than
morphological modifications of organs which have been already
treated of in this memoir. The fourth point, however, still
remains for discussion, namely, the increase in thickness
plants which form woody stems (Dicotyledons). The origin ami
signification of cambium is the nut on which so many young
phytologists have already broken their milk-teeth, the Gordiau
knot which so many botanical Alexanders have cut instead of
untying, and the enigma, for the solution of which almost all
the Corypheei of our science have laboured with more or I

1 I beg to observe, that generalh throughout the entire memoir pfacni

plants onlv arc referred to.

258 CONTRIBUTIONS TO

success. My researches also with respect to this newly -arising
formative layer between bark and wood are by no means
concluded.

Before, however, I proceed to communicate my observations
on this subject, it is necessary once more to take up the ques-
tion of the individuality of plants. I have already remarked
above that, in the strictest sense of the word, only the separate
cell deserves to be called an individual. If we go a step
farther, we might define each axis with its lateral organs to be
individual beings. If, however, we disregard this circumstance
of the plant being composed of cells and similar axes, and con-
ceive the term individual, as applied to the organic world, to
signify a body which cannot be divided into two or more simi-
lar ones without the abolition of its idea of totality, and whose
vital process has a fixed point of commencement and termi-
nation in definite periodicity, it thence follows that the her-
baceous (planta annua) and the true biennial plants, which
flower in the second year, and then die off entirely, are the only
ones which can be regarded as individuals in the vegetable
kingdom. The idea of individual life also necessarily requires
as a characteristic that individual death should be a condition
of the organization itself. But where such a death does not
take place as a final termination from internal necessity, as an
internal preconditioned cessation of the organizing force, there
also must individuality be out of the question* This is the
case, however, only in the above-mentioned plants, and from
them solely, therefore, as from the prototype, must we set out,
in all researches into the nature and life of the vegetable
organism.

In order to facilitate the transition to what is to follow, I will
now proceed to the exposition of the two different modes of
propagation. It either takes place by a process which has
hitherto been called impregnation in plants, and to which a
sexual difference has been ascribed (Wiegmann's Archiv, 1837,
vol. i, p. 290, &c), or by division ; the plant, for instance, deve-
loping on itself a perfectly similar individual, and then at an
appointed time dismissing it. This latter, the formation of so-
called bulbilli, &c. occurs, together with the former, in only a
small number of plants. We must, however, make ourselves
somewhat more intimately acquainted with it. This formation,

PIIYTOGENESIS. 2 »9

for instance, docs not always take place in such a manner that
the parent plant separates itself entirely from lluin, and scat-
ters them about singly, but it most frequently forms, previous
to its own individual death, a peculiar organ, which places the
offspring in a peculiar vital connexion with one another, and at
the same time serves as a reservoir for a certain quantity of
nutritive material, by which the first development of th<
young individuals is facilitated. But in mosl cas< a this organ
is merely a metamorphosis of some other one with which we
are already familiar, for example, the stem or the root, or, as
in the potato, the axillary buds; and no scientific person has
therefore ever hesitated to speak of these things as mere /tor-
tious of a plant, which continue to live as connecting member
between the younger individuals after the death of that one
which has generated them. On the other hand, a different
course has been taken, where stem and root simultaneously, and
therefore almost the entire totality of the plant, take part in
the formation ; and although the result in this case may per-
haps be that there can be no question at all of an hetero-
morphy of a known portion of a plant, still the physiological
identity in the signification of this and the former part has
not been maintained with precision, and the view has thus
been obscured.

Most manuals are silent upon this subject, as though it
were quite self-evident that the tree was to be regarded as
the perfect plant j and I believe it not difficult to prove that,
where vegetable physiology still lies very deep in error, this par-
ticular misconception is solely in fault. Two entirely distinct
ideas have here been confounded, namely, the highest Btage of
development to which vegetable life can raise itself, ami the
type upon which the idea of the individual must be based. If,
then, the first of these ideas may be maintained with regard to
the tree, still the application of the second to it fails com-
pletely in every respect, as has been very correctly asserted
before by E. Meyer (Linmca, vii, p. 42 1). It necessarily per-
tains to the notion of a plant, that it produces foliaceous organs
on its stem, yet there is no tree which has leaves. Paradoxical
as this may sound, it is still not the Less true It is a feet, of
which certainly no botanist is ignorant, that no Lignified part
of a plant, even though it be only in its second year, is capable

260 CONTRIBUTIONS TO

of producing a leaf; but the direct consequence is by no means
so generally acknowledged, that for that very reason the woody
stem cannot come under the idea of plant. Much confusion
has arisen in our physiology from the error of regarding the
tree as a sing J e plant, the ideal definition of root, stem, bud,
&c. have become very vague, and bitter controversies have
been carried on with respect to the functions of these parts,
which could have no result, because the one party spoke of
this, the other of that, this one of the stalk, the other of the
stem, this of root-fibrils, that of ligneous root-substance.

The so-called lignified root is, however, just as little a root,
as the lignified stem is still a stalk, but both together form an
inseparable, and, moreover, altogether purely accidental organ
for the plant, which has secreted the annual individual upon its
surface, in order to bring into connexion, by means of a single
organized membrane, the whole sum of the newly formed
young individuals. The tree corresponds precisely to the
polypidom, and it appears to me to be as unsound to set out
from it as the type in plants, as it would be for the zoologist
to take a Gorgonia as the ideal of animal individuality. This
analogy, however, is in no way weakened by the circumstance,
that we meet with this woody stem most frequently in precisely
the highest developed plants ; but, on the contrary, it is natural
that, if the animal kingdom in a certain measure receive the
vegetative part of its character from the vegetable kingdom,
this should connect itself by the lowest stage of animals to the
highest plants, whilst even this vegetative half of the vital phe-
nomena in the higher animals is in like manner purified and
ennobled by its individuality constantly gaining in independence.

With this explanation of the woody stem (the root included),
it will henceforward appear by no means remarkable that this
organ (as if it were a mere organized, soil) can generate upon
every part of its surface young vegetable individuals j that is,
buds, so soon as it is in a condition to convey nutritive material
to those buds from any part, whether it correspond apparently
to the former root or to the stem ; while this refined idea of
the plant conducts to the law, that in the regular course of
vegetation, neither root nor internode, but only the axilla of
the leaf, is capable of generating a bud, i. e., a new axis with
lateral organs.

PHYTOGENESIS. 261

Hut the following remarks, which in nature (who never, lil.
a bad artist without a plan, fluctuates between the mod oppo-
site methods) would be, in the usual mode of tr< ating it, an Inex-
plicable contradiction and an absolute miracle, will serve for the

decisive establishment of tins view .

So soon as the secretion of this organized mass, the wood,
takes place, for instance, we suddenly miss the influence of the
law of formation, which, until then, had without exception
directed the growth of the entire plant in all its pari Hen ,
so far as we are at present acquainted with the subject, there is
no formation of cells within cells, here no expansion on all sid( -
of the originally minute vesicle occurs, there is here no cyto-
blast upon which the young might be developed ; but beneath
the outermost layer of cells, which are comprised in the term
bark, an organisable fluid is poured out, as it were-, into a single,
large, intercellular space, which fluid, as it seems, consolidates
quite suddenly throughout its entire extent into a new, alto-
gether peculiarly-formed tissue of cells, which arc deposited
one upon another, the so-called prosenchyma. Here, more-
over, there is decidedly no formation of vascular bundles from
cells of lower dignity, for all of them originate simultaneously
and of their full size; and what has been called (spiral) vessels
of the wood, is something which differs immensely from tin-
spiral vessels of herbaceous plants, both in respect of their
origin, and probably of their physiological signification also.
In like manner, no result has been obtained from the con-
troversies which have been sometimes carried on with great
warmth respecting the function of spiral vessels, nor could
any be gained, because each party meant the spiral vessels of
herbaceous plants, or of the wood, ad libitum, completely
losing sight of the possibility that the two might ho verj
different things. If, for instance, we examine the cambium in
the earliest period at which it begins to acquire organisation,

1 This position has undergone essential modifications, in consequence of subsequent
researches which I have made with respect to the cambium, and which proved thai
a cambium, in the sense in which it had been previouslj used in physiology, nan
as denoting an amorphous formative fluid between the \\<><mI and bark, had no « I
enceatall; that the wood and the bark, on thi form one uninterrupted

continuity, and their margin i* merelj denoted bj a Layei <»f di ly-walled, gela-
tinous cellular tissue.

262 CONTRIBUTIONS TO

we find that it consists throughout of gelatinous prosenchy-
matous cells -which perfectly resemble one another. Shortly
afterwards, some separate longitudinal rows of these cells ap-
pear to have increased somewhat in breadth, which is the only
circumstance that distinguishes them from the adjacent mass.
As development advances, we observe that some dark spots
appear upon the walls of some of these expanded cells, which
we soon recognise to be small, flat air-bubbles, that have been
formed between their walls and those of the neighbouring cell.
Gradually all the expanded cells which are so disposed one
upon the other are changed in this way : the air-bubble gra-
dually appears more sharply defined, assuming the circular or
oval figure ; and there appears in its centre a smaller circle
which constantly becomes more distinct, and which originates
in the following manner : when the deposition of new masses
takes place upon the inner wall of the cell, the parts corre-
sponding to the outer air-bubble remain free from the deposit,
thus forming a small canal which traverses the newly-deposited
mass. We now recognise the fully- developed porous vessel,
the partition walls between each two superincumbent cells ap-
pearing at the same time to be more or less absorbed. This
history of the formation of the porous vessels, which may
readily be observed in limes and willows, greatly contradicts
the general notion that the porous canals serve to facilitate the
communication of the sap. As the air -bubble is first formed
on the outside of the wall, it renders the passage of the sap at
that spot impossible, and for this reason the origin of the
porous canal might be most readily and naturally explained as
a local atrophy of the cell-wall. At the same time the above
shows that the distinction between fir-wood and that of trees
which bear leaves, in respect to anatomical structure, cannot
be of such vast physiological importance; since, with similar ele-
ments and development, the distinction is really based on the
larger or smaller number of cells that are converted into porous
vessels.

There are still, however, a great many gaps to fill up. In
particular the origin of the medullary rays, and their relation to
the wood; the formation of the new bark; and, lastly, the origin
of the buds in the body of the wood, are so many questions
for extended researches, to the execution of which, however, we

PHYTOGENESIS. 263

may look forward at no distant time, when we consider the

ardent and gratifying zeal which has been awakened and
cherished, especially amongst our contemporaries, in favour of
the sound and scientific study of the anatomy and physiology
of plants.

I have attempted in this Memoir, so far as lay in my power,
to solve many interesting questions in Vegetable Physiology, or,
by more accurate definitions of the subject, to advance nearer
to a future solution. May these observations meet with a
friendly reception at the hands of the vegetable physiologists
of Germany, and be speedily improved upon and extended.

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EXPLANATION OF THE PLATES,

SCII LEI DEN'S THE AT I SI..

PLATE I.

Fig. 1. Cellular tissue from the embryo -sac of Chamadorea
Schiedeana in the act of formation, a. The inner-
most mass, consisting of gum with intermingled
mucous granules and cytoblasts. b. Newly formed
cells, still soluble in distilled water, c-e. Further
development of the cells, which, with the exception
of the cytoblasts, may still coalesce, under slight
pressure, into an amorphous mass.

2. The formative substance from fig. 1, a, more highly

magnified, gum, mucous granules, nuclei of the
cytoblasts, and cytoblasts.

3. A single and as yet free cytoblast, still more highly

magnified.

4. A cytoblast with the cell forming upon it.

5. The same, more highly magnified.

6. The same. The cytoblast here exhibits two nuclei,

and is delineated in

7. isolated after the destruction of the cell by pressure.

8. The same cellular tissue in a higher degree of deve-

lopment than that represented by fig. 1, e. The
contiguous cell-walls have already united. Bj
making a transverse section, it may be distinctly
perceived that the cytoblast is enclosed in the cell-
wall.

9. Cells from a delicate transverse section of the almost

matured albumen.

266 EXPLANATION OF THE PLATES.

Fig. 10. Common partition-wall between two cells from fig. 9,
under a higher magnifying power. The stratiform
depositions may be observed at b, and the porous
canals produced by their local failure at «. I could
distinctly enumerate, from nine to twelve layers
which had been deposited within fourteen days.

11. A sporule from Rhizina Icevigata Fries, with the

cytoblast.

12, 13, 14. Different cytoblasts from the embryo-sac of

Pimelea drupacea before the appearance of cells.

15. A young cell with its cytoblast, from the same. The

latter in this instance presents the unusual number
of three nucleoli.

16. A portion of the emb^onal end of the pollen-tube

projecting from the ovulum in Orchis Morio, within
which, towards the upper part, cells have been
already developed. At the lower part, the original
pollen -tube may still be distinguished. The almost
globular cytoblasts are, in this instance, distinctly
enclosed in the cell-wall.

17. Embryonal end of the pollen-tube from Linum pal-

lescens, together with an appended lobule of the
embryo-sac (a). The process of the formation of
cells is commencing. Above, a young cell with its
cytoblast is already perceptible, beneath this several
cell-nuclei are seen floating free.

18. 19, 20. Commencing germination in the sporules of

Marchantia polymorpha. Compare the text, p. 248.

21. Portions of the pollen-tube which have become cel-
lular, from Orchis latifolia, in the highest stage of
development; the investment of the pollen-tube is
no longer perceptible. The cytoblast is enclosed in
the cell-wall, just as in fig. 16.

22 and 23. Two isolated cells from the terminal shoot
(]junctum vegetationis, Wolff) of Gasteria racemosa;
22 exhibits two free cytoblasts; 23, two newly-
formed cells within the original cell.

D. H. HILL UBRARY

North Carolina State College

.--

a,

9

10

4.J. Scfhleidert. ad.na"

EXPLANATION OF THE PLATES. 267

Fig. 24. A very young leaf of Crasnda portulacai the five
cells which solely compose it being still surrounded

by a parent-cell.

25. Three cells from an articulated hair of potato, with
a retiform current of mucus upon their walls. In
the central cell the direction of the currents is par-
tially indicated by arrows.

In all the instances in which I have observed the movements in the cells of phae-
nogamous plants, I have constantly found the moving matter to consist of a yellowish
mucous fluid, perfectly insoluble in distilled water, and mixed with minute black
granules, but differing entirely from the other aqueous sap of the cells ; and even
when the currents were so small as to appear merely as excessively minute delicate
lines of black points, I succeeded with higher magnifying powers in distinguishing
the yellowish mucous fluid, especially when aided by the favorable circumstance
(which not unfrequently occurs) of the current becoming arrested by some impedi-
ment, which causes a somewhat larger quantity of the moving material to accumu-
late, and is generally followed either by a change in the direction, or a division of
the current.

PLATE II.

Fig. 1. Cells from the epidermis of the pericarp of Ocymum
basilicum, moistened with water, so that the mucous
globule has expanded, and torn the outer cell-wall
(a) from the side walls (/;).

2. Cells from the pericarp of the epidermis of Ziziphora

dasyantha.

3. Cells from the pericarp of the epidermis of Salvia m r-

ticillata.
1. Cells from the pericarp of the epidermis of Salvia

Horminum.
5. Cells from the pericarp of the epidermis <>t' Sahia

Spielmanni.
2 3 4 and 5, a, exhibit the remains of the side-walls of

the ruptured cells.
G. A portion of the epidermis (a) and of the integument

(/,) of the ovule of ( ollomia cocciiu a. The epiderm

cells contain merely granules of starch.

268 EXPLANATION OF THE PLATES.

Fig. 7. The epidermis-cells of the half-ripe seed of the same
plant, for the most part containing gum; at a, some
still undecomposed starch.

8. The same cells from the same seed nearly ripe. Beau-

tiful spiral fibres have been formed from the con-
tents, which are entirely consumed.

9. Cells of the epidermis of the seed of Leptosiphon andro-

saceum, moistened with water, so that the cone of
jelly has come forth, a. The remains of the cell-
walls.

10. Cells from the ej)iderinis of the seed of Hydrocharis

Morsus ranee. In the lower part of the cells, where
they are connected together, the spiral coils take a
direction different from that in the upper and free
part.

For the figures in Plate II consult the text, pp. 243-6.

THE END.

C. AND J. ADLARD, PRINTERS,
BARTHOI OMEW CLOSE.

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