# Microscopical Researches/Introduction

MICROSCOPICAL RESEARCHES
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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 elementary structures, which, though they present various modifications, 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 Vascular 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 structure 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 particularly 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, ​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 phenomena 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 phenomena 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 expansion of existing cells; 3, the transformation of the cell-contents, and the thickening of the cell-wall; 4, the secretion and absorption 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 Müller’s Archiv, 1838, p. 137, plates 3 and 4.)
First, of the production of new cells. According to Schleiden, in Phenogamous plants, this process always (except as regards the cells of the Cambium,) takes place within the already mature cells, and in a most remarkable manner from out of the well-known cell-nucleus. On account of the importance of the ​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 structure — named by R. Brown, Areola or cell-nucleus, by Schleiden, Cytoblast — varies in its outline between oval and circular, according as the solid which it forms passes from the lenticular into the perfectly spheroidal figure. Its colour is mostly yellowish, 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 structure 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 alteration of form. In addition to these peculiarities of the cytoblast, already made known by Brown and Meyen, Schleiden has discovered in its interior a small corpuscle (see plate I, fig. 1, 4,) which, in the fully-developed cytoblast, looks like a thick ring, or a thick-walled hollow globule. It appears, however, to present a different appearance in different cytoblasts. Sometimes only the external sharply-defined circle of this ring can be distinguished, 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.”) 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 cytoblast ​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 gradually 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 cytoblasts 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 expressed 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 Musée, 1, 55; and the counter-observations of Schleiden, Müller’s Archiv, 1838, p. 161.) A mode of formation of new cells, different from the above described, 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, inasmuch as the young cells might escape observation in consequence 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 polyhedral from flattening against the neighbouring cells, or it is irregular 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 ​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 intussusception. (See Hugo Mohl’s “Erläuterung und Vertheidigung meiner Ansicht von der Structur der Pflanzen substanzen,” Tübingen, 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 observations 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 deposition 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 depositions 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 instances indubitable, for instance, in the pollen-tubes, (e. g. Phormium tenax.) Probably that extremely remarkable phenomenon of the motion of the fluid, which has now been observed in a great many cells of plants, is connected with the transformation of the cell-contents. In the Charæ, in which it is most distinct, a spiral motion may 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 operations 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 intercellular canals, and of a fluid containing sugar by the nectar-glands, &c. &c.
In all these processes each cell remains distinct, and main​ 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. Pl. 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 vessels, 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, however, nothing has been inferred from such individual points of resemblance. Every cell is not an analogous structure to a vegetable cell; and as to the polyhedral form, seeing that it necessarily 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 universally 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​searches upon the terminations of the nerves in the tail of the Larvae of frogs (Medic. Zeitung, 1837), I not only saw the beautiful cellular structure of the Chorda Dorsalis in these larvæ, but also discovered the nuclei in the cells. J. Müller 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. Müller 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. Müller, 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 larvæ, 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 opportunely 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 larve 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 suc​ceeded in rendering the origin of young cells from nuclei within the parent-cells in the branchial cartilages very probable, the matter was decided. Cells presented themselves in the anima 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 concerned these individual tissues, the desired evidence, that these cells correspond to the elementary cells of vegetables was furnished. 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, epithelium, 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. Schultz 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 signification 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 membrane was soon discovered to be composed entirely of cells, and ​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 vegetable 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.