The above facts show, first of all, two things:—1. That the cell-membrane in the mantle of Phallusia does not, as stated by Kolliker and Lowig, consist of cellulose, but rather that it behaves towards iodine and sulphuric acid, as well as towards caustic potass, exactly like an animal substance nitrogenous; 2. That the homogeneous, or only in the second layer, slightly fibrous interstitial substance, is composed of tolerably pure cellulose.
In Clavellina Kölliker and Löwig found cells in a lamina of the mantle, similar to those in Phallusia, also imbedded in an interstitial substance; in the tunic of Salpa these cells are wanting, the cellulose substance contains nuclei and crystals; in Pyrosoma, they found in the structureless tunic, only isolated ramified cells; the structureless membrane of Diazona is penetrated according to them by elongations of the fleshy tunic of the animal. In the tunic of Didemnum, the same observers again found cells, of which the membrane, though incrusted with carbonate of lime, was soluble in boiling potass; in Aplidium they found similar cells in the interstitial substance, and here also the membrane of the cells was soluble in the caustic potass—only the interstitial substance remaining. In Botryllus, according to them, the internal layer consists of delicate fibres, which, like the rest of the homogeneous substance, in which nuclei and crystals occur, resist the action of hydrochloric acid and of potass; the nuclei are soluble in potass; the crystals insoluble in acid; branched channels, dilated at the extremity, which exist in this instance, are regarded by Kölliker and Löwig as processes of the fleshy tunic.
[The author then details his experiments on the mantle of Cynthia microcosmus, and proves the existence of cellulose in it in a fibrous form, mixed with another substance soluble in caustic potass, of which the outer epidermis appears to be wholly composed. But whether the fibres are composed of pure cellulose, and the second nitrogenous element is simply deposited between them, or whether the latter also pervades the substance of the fibres themselves, he is unable to determine. The cellulose, however, in Cynthia microcosmus appears to differ in some respects from that in the tunic of Phallusia mamillaris, inasmuch as it is coloured blue by ioduretted chloride of zinc, which the latter is not.
In this part of his paper the author describes a mode of procuring thin slices of very soft or yielding substances, by including the latter tightly between two pieces of cork and cutting thin slices of the whole with a razor. The structure of the mantle in the undescribed Ascidian from Chili appears to be very similar to that of the Cynthia last described.] He then proceeds:—
Although the methods pursued by us respectively, were very different, yet the results of my observations coincide in great measure with those of Kdlliker and Lbwig. In only one principal point do I differ from them: the membrane of the large cells in the mantle of Phallusia is not composed of cellulose. It behaves exactly like animal membrane, and is probably nitrogenous, and would therefore represent the primordial sac of the vegetable cell, which exhibits precisely the same chemical re-actions.
It seems to me that the observers just quoted had not seen the membrane of these cells, indicated by the delicate folds described above, as they adduce as a distinction between these cells and those of a plant, the coalescence of their walls, composed of cellulose, with the homogeneous interstitial substance. In Didemnum candidum, it is true, they observed not only the membrane, but also that it was soluble in caustic potass; and consequently in this case it could not be composed of cellulose. That I did not meet with isolated cells in the mantle of Cynthia as Kölliker and Lbwig did, does not surprise me, those observers not having found a trace of such cells in the mantle of Phallusia gelatinosa, whilst in another specimen they noticed nuclei and indications of these cells. It seems, therefore, as if the latter belonged to a definite period of the animal’s life..
Kölliker and Lbwig at the end of their paper refer to the history of the development of the embryo of certain Ascidians, given by Milne Edwards; from which they conclude;—
That the external structureless tunic of the embryo afterwards forms the mantle of the adult animal, consisting of cellulose; 2. That this tunic, which subsequently contains nuclei, fibres, &c., is the product of the cells formed by the segmentation of the yelk. They believe also that the mantle of other Ascidians, which is perforated by vessels, as in Phallusia, is at first structureless, and in this condition is not composed of cellulose, but that cells are formed in its substance which multiply and secrete the cellulose; at a latter period, however, themselves again disappearing. They also detected in the stomach and intestines of Phallusia, Clavellina, and Diazona, both the remains of Al gm as well as Closteria [in salt water?]. The cellulose, therefore, would seem to be introduced from without; in what way, however, it is separated from the blood, in order to be secreted in certain parts of the body, remains unexplained; an accurate analysis, therefore, of the blood of the Ascidians would be of great importance.
If, now, the occurrence of cellulose in the mantle of the Ascidians above described, be compared with the conditions under which the same element exists in the vegetable kingdom, the following very essential differences are apparent:—
In the vegetable kingdom the cellulose constitutes the so-called primary cell-membrane, and the thickening layers of the cell deposited upon it. The vegetable cell-wall, consisting of cellulose, is always separated from the wall of the neighbouring cells, by an interstitial substance (intercellular substance) which is soluble in chlorate of potass and nitric acid. On their being boiled, therefore, with caustic potass and by maceration [in chlorate of potass and nitric acid] these cells separate from each other; but in the mantle of Phallusia no such separation takes place, because there, the cellulose, although probably distinct from the nitrogenous membrane of the cells, itself constitutes the interstitial substance; the intercellular substance of the plant being entirely absent.
The vegetable cell is thickened by the laminated deposit of new cellulose in the previously existing layers of that substance; such a laminated structure, which is demonstrable by proper treatment in all thickened vegetable cells, is altogether absent in the cellulose of the mantle of the Ascidians.
In the vegetable kingdom the cellulose never occurs in the form of free fibres, as in the mantle of Cynthia, &c.; the band in the spiral vessels of plants, apparently composed of a fibre, arises in the unequal development of the thickening layers.
In the vegetable kingdom, the cellulose never appears as a homogeneous substance, either between the cells or nuclei of cells; as is the case in the mantle of the Ascidians.
These differences in the mode of occurrence of the cellulose are so essential, that it would seem to be impossible to confound an animal tissue containing cellulose with any vegetable tissue whatever. [The appearances exhibited in a section of the stem of Laminaria saccharina, when treated with iodine and sulphuric acid, are adduced and figured by the author, as a contrast to what takes place under the same re-agents in the mantle of Phallusia.
The chemical relation of the cellulose itself, however, in the Ascidians examined by me, is not essentially different from vegetable cellulose. Caustic potass has no effect upon either; sulphuric acid dissolves both; iodine and sulphuric acid colour both equally, blue; ioduretted chloride of zinc induces, it is true, in most vegetable tissues the same blue colour as that produced by iodine and sulphuric acid; there are, on the other hand, vegetable tissues (such as, in Fucus serratus, Chordaria scorpioides, the wood-cells of Pinus sylvestris, &c.) upon which the same re-agents produce no effect; the ioduretted chloride of zinc appears generally to be less energetic in its action than sulphuric acid. After they have been boiled with caustic potass both the cellulose-substance of the mantle of the Ascidians and the thickening substance of the so-termed plant-cell are coloured blue or-violet by the ioduretted chloride of zinc solution, the potass probably in both cases removing a material which prevented the action of the re-agent.
By maceration after Schultz’s method, the last-mentioned material, in the Ascidians above noticed, is as little dissolved as the cell-membrane in Phallusia, nor is it, in the vegetable kingdom, always removed by the same maceration; the thickening layers of the epidermis cells of several plants are not coloured blue by ioduretted chloride of zinc after maceration, whilst after boiling with potass that re-agent produces the characteristic colour. The substance, therefore, in the mantle of the Ascidians, soluble in caustic potass, appears to be closely allied in its properties to the so-termed incrusting substance of the vegetable tissue.
In the mantle of Phallusia, we have, as I have certainly proved, both a homogeneous, interstitial substance composed of cellulose, and also indications of fibres composed of the same element; besides which there are, in the interstitial substance and between the fibres, nuclei and cells, thus the same elements as those which occur in Cynthia and the new species from Chili; in the case of the Phallusia the cells are more abundant, in the latter the nuclei and fibres; which seem generally to accompany each other. In the fibrous part of the mantle of Phallusia we find only nuclei, and no cells; in the portion, again, which consists of cells, no fibres, and but few nuclei. As in this case we are without any history of the development of the tissue, no further conclusions can be drawn respecting it.
Although, in the present state of science, the occurrence of cellulose does not suffice for a distinction between plants and animals, yet the previously established law that the animal cell-membrane always contains nitrogen retains its force. The animal cell is in all cases, as far as I know, entirely different from the vegetable cell. The intercellular substance is always wanting in tissues composed of animal cells; the animal cell itself corresponds with the primordial sac of the plant-cell, which also does not consist of cellulose, but is probably nitrogenous, like the animal cell-membrane. Whilst the plant-cell is thickened by the secretion of cellulose around the primordial sac, and thus obtains the true cell-wall; the animal cell also secretes a material—in the mantle of the Ascidians the same cellulose—but this material does not form a special envelope around the previously existing nitrogenous cell, the secretions of the individual cells, owing to the absence of any intercellular material, coalescing into one substance. In this way probably is formed the interstitial substance composed of cellulose in the mantle of Phallusia; and in like manner the stroma of the cartilaginous tissue, which is not composed of cellulose, and the interstitial substance, impregnated with calcareous salts of the osseous tissue. The want, therefore, of an intercellular substance constitutes the principal distinction between the animal and vegetable cellular tissues. Owing to this, the animal cells, even in cases where cellulose occurs, never have a wall composed of that substance, which is characteristic of the vegetable cell. It is to be regretted that this diagnostic character is wanting in the lowest unicellular animals and plants.
The existence of the intercellular substance has, it is true, been very recently disputed, with respect to the plant-cell, by Wigand (Intercellular Substance and Cuticula. Braunschw. 1850). That author has termed the true intercellular substance, which, so far as my most recent investigations extend, is always present, the primary cell-membrane, whilst the latter is not to be distinguished, either optically or chemically, from the thickening layers of the vegetable cells consisting of cellulose.
The resumé of my reseaches therefore may be thus given:—
In the mantle of the Ascidians there is a substance insoluble in caustic potass, but soluble in sulphuric acid, which is turned a beautiful blue by iodine and sulphuric acid, and which consequently consists entirely of cellulose. This substance constitutes the interstitial substance of the cells; in the mantle of Phallusia it is homogeneous, but in Cynthia, &c., exists for the most part in a fibrous form.
The mantle of the Ascidians contains, besides this cellulose, another material soluble in caustic potass, but insoluble in sulphuric acid, and not coloured blue. by iodine and sulphuric acid, and which consequently is not cellulose; in the mantle of Phallusia it is only sparingly present, but in Cynthia and the new Chilian Ascidian it is much more abundant, and alone constitutes the corneous epidermis of their mantle.
The membrane of the cells in the mantle of Phallusia does not consist of cellulose; it is coloured brown by iodine and sulphuric acid; is soluble in caustic potass, and behaves exactly like an animal membrane, as do the nuclei and vessels.
In the mantle of Phallusia cells abound in a homogeneous, interstitial substance composed of cellulose; it is only at the inner margin of the mantle that fibres composed of cellulose, with nuclei amongst them, make their appearance. In Cynthia, &c., there are scarcely any traces of cells, whilst the nuclei and cellulose fibres abound.
A tesselated epithelinm, containing no cellulose, covers the inner surface of the mantle of the three Ascidians examined by me; the outer surface of the mantle of Phallusia appears to possess a similar epithelium.
There are two essential points of difference between the modes in which cellulose occurs in the Ascidians and in the vegetable kingdom—1. In Phallusia the cellulose constitutes the intercellular substance, but does not, as in plants, form an integral part of the cell-wall itself; 2. In Cynthia and other species the cellulose forms free fibres, a form in which it is never observed in the vegetable kingdom.
The substance of the mantle in the Ascidians is not disintegrated by boiling with caustic potass or by maceration with chlorate of potass and nitric acid, like the vegetable cellular tissue into its elementary parts; there is in it none of the intercellular substance universally present in vegetable tissues, and by which the cells are connected, but which intercellular material is never composed of cellulose, as it resists sulphuric acid, but is soluble in caustic potass, as well as by maceration.
