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changing anything but its size. In living matter, however, nature presents an exception to the universality of this law; for in living bodies the molecules, which are far smaller than the smallest visible particles, are united to form masses of limited size, which represent so many units of the body, just as we might say the bricks represent so many units of a wall. These units of life, as I may call them at the risk of being misunderstood, are the cells before mentioned. The great material difference, therefore, between living and unorganized bodies was at once demonstrated by the discoveries of Schleiden and Schwann, and it is on this account that naturalists attribute such importance to the work of these two men. In truth Schwann's investigations caused as great a change in the direction pursued by zoölogists in their researches as the reform either of Linnæus or Cuvier. But Schwann unlike his great predecessors did not continually make further discoveries, and has not, as far as I am aware, participated in the work of original research, which has been in progress during his life time, so that to most of us perhaps he already seems a person of the distant past.
Yet during Schwann's lifetime one of the principal labors of zoologists has been the working out in detail the applications of his generalization, and determining the variations and modifications which cells undergo in the different tissues and species of animals, until finally the subject has assumed an importance even more vast than could at first have been foreseen. It is in fact hardly an exaggeration to say that all our knowledge of animals groups itself about the doctrine of cells, as the central factor upon which all others depend; and whether we labor as physiologists, embryologists or anatomists, we are alike forced ultimately to base all our conclusions, and demonstrate all our theorems by the character and property of cells. In brief, what a knowledge of waves is to the student of sound, a knowledge of cells is to the student of life.
It would be a pleasant task to expatiate at length upon the various bearings of the cell doctrine, for we should deal with many of the fundamental problems of zoology, and with some of the most interesting additions to our knowledge in this department of science which have ever been made. This course would take us away from the real subject of this article, the object of which is to give an example of how much may be learned by
the study of the cellular anatomy of even the commonest animals, therefore I must resign for the present the wider and more attractive field, and descend to details, in order to show by a concrete example some of the modifications which cells present, and to describe the appearance of some of them when prepared for microscopical examination.
My illustrations are all taken from the common locust, and are selected from the results of a recent original investigation on the histological structure of that abundant pest. The work was undertaken at the desire of Dr. A. S. Packard, Jr., in connection with the more directly practical labors of the U.S. Entomological Commission, and it is to the kindness of Dr. Packard that I owe the opportunity of utilizing my observations for this article.
I will merely remind the reader that the anatomy of the locust may be most readily understood by saying that its body is formed by an outer wall, including the external crust and the underlying muscles, and an internal tube, the digestive canal, the diameter and course of which are very irregular, as is shown in Fig. I of the accompanying plate. Between the body walls and the alimentary canal there is a large cavity in which various internal organs, notably those of circulation, respiration, and reproduction are situated.
Now, all these parts are composed of minute cells, and the examination of almost any one of them will suffice to show cells that are very characteristic. Let us take for instance a male grasshopper. The sexes may be readily distinguished by the position of the claspers at the end of the abdomen, which is straight in the female, while in the male it is curled upwards, so that the end of the abdomen appears club-shaped and the claspers seem placed on the back.
Opening the insect along its back, and spreading out the sides so as to expose the internal organs, almost the first things that strike the eye are the numberless glistening silvery threads, the ramifications of the tracheal tubes. These must be torn asunder in order to lay bare the reproductive organs, which form a large mass overlying the stomach in the anterior part of the abdomen; trace these organs downwards, following them around the sides of the intestine to the ventral and posterior part of the abdomen, and there will be found numerous long white tubes; these are the
vesicula seminales, which open into the long ducts of the spermaries and end blindly. If one of these be isolated, and then colored with hæmatoxiline or carmine, and examined in a drop of glycerine with the microscope, its walls will exhibit a great many minute colored dots of oval shape; these are the so-called nuclei of the cells; it is evident that they form but a single layer, for in no part of the wall of the tube do they lie over one another. By looking carefully it is possible to distinguish a faint polygonal outline around each nucleus; these outlines correspond to the surfaces by which the cells abut against one another. Thus we learn at once that a cell is a very minute body with granular contents, and a distinctly differentiated central portion, the nucleus, and moreover that the cells are laid close against one another, and cemented together by thin intervening layers known as the intercellular substance. But every vesicula consists of a wider upper portion, which is usually found filled with spermatozoa in the mature animal, and the walls of which are composed almost entirely of the layer of cells just described; and a narrower portion enclosed in a sheath of muscular fibres. This difference can be most plainly recognized by preparing transverse sections, which may be made with a razor from tubes that have been hardened in alcohol. The operation has already been described in the NATURALIST for July, 1877, and to that the reader is referred. A section through the upper part is represented in Fig. 1. The
single cells, each with its darkly stained and coarsely granular nucleus, lies close against its fellows. They are all of about the same height, and form a single continuous layer. Every layer of this kind that lines any cavity whatsover is called
an epithelium. A section through FIG. 1.—Section of upper part of the lower segment of the vesicula Vesicula seminalis.
presents quite another appearance, as is shown in Fig. 2. The epithelium, Ep, still lines the cavity, but the cells are very much smaller than in the upper part, though they form but a continuation of the same layer. From this we learn that cells vary greatly in size, but the limits are much further apart than is here indicated. Outside the epithelium is a very thick and powerful coat of muscular fibres, Mu, which
encircle the canal. Among the fibres occur elongated nuclei. Each muscular fibre in fact is a greatly elongated and peculiarly modified cell. But into this matter I Mu cannot enter here; but I wish to point out that the lining membrane of the canals and ducts of animal bodies is generally if not always an epithelium, and that we frequently find the epithelium surrounded by a muscular coat. Thus may fundamental facts be observed on a single organ. In the body of the locust there
FIG. 2.-Section of lower part are long tubes, often pigmented, of Vesicula seminalis. and opening into the digestive canal in the hind end of the stomach—they are the Malpighian vessels, so named after their illustrious discoverer. They make very beautiful preparations, if merely picked out, colored with carmine and mounted in glycerine, and are interesting to us because they have an epithelium which is very different from that above described. An optical section of part of one of them is represented on Plate II, Fig. 2. There is a very delicate external membrane which is hardly noticeable, though it forms a continuous external coating. Inside the epithelium is very distinct, but the cells which compose it instead of being high in proportion to their breadth are compressed; the nucleus is rounder, and the cell itself different from those of the seminal vesicle. The mass of matter which surrounds the nucleus is termed the protoplasm. Now, in the epithelium under examination the protoplasm of the cells is charged with coarse spherical granules. We naturally regard these peculiarities as somehow connected with the special function of these tubes, but in the majority of cases we are still unable to trace the relations of histological appearance to the physiological functions of organs. We have now made the acquaintance of a second kind of epithelium, and have learned to recognize cells by the presence of the nuclei, which, as far as we know, always have the property of being more darkly stained by various dyes than any other part of the cell. Moreover, each nucleus corresponds to a single cell, and there are never two nuclei in one cell. There are, however, some exceptions; thus the nervous cells (ganglia) of the sympathetic ganglia of vertebrates and of the
ureters of mammals (Englemann) frequently have two nuclei, and Dr. E. L. Mark, in his very valuable memoir on the Coccide, states that in the malpighian vessels of those insects he has likewise found cells with two nuclei.
The height of epithelial cells may be still further diminished, so that in some cases it may be said to have nearly disappeared, the cells assuming the form of a thin lamella. This is the case upon the air tubes. If one of these be colored and mounted in the usual way, the flattened epithelial cells may be easily recog. nized by their oval nuclei, Fig. 3 b. Each nucleus contains one or
sometimes two minute spherical dots, eccentrically placed; these are the nucleoli. We have now
seen the three constituent parts, a
which probably always enter into FIG. 3.-Small air tube from the the composition of every cell; abdomen.
these are the protoplasm, the nucleus and the nucleolus. In addition we often find that the outside layer of the protoplasm becomes hardened and more resistent, and it is then called the membrane.
In every epithelium we distinguish two kinds of surfaces on each cell, those which lie against other cells, and those which are free, facing the cavity. On the free surfaces the membrane is often considerably thickened, and the thickened portions are then so joined together that they form a continuous lamella, which is called a cuticula. Now the flat epithelium of the air tubes forms a very curious cuticula, which lines all the tracheæ, and is remarkable for being thickened in some places more than in others, thus developing a spiral thread, which can be seen in Fig. 3, underneath the nuclei. The spiral filament was observed very long ago, but its real nature was only recently discovered. For a more detailed account the reader is referred to the NATURALIST for July, 1877.
It is hoped that these illustrations will suffice to exemplify the more important features of epitheliums, tissues which are found in all animals except the protozoa, and represent one of the simplest and most frequent modes in which cells are aggregated. I propose to add a brief account of the structure of the digestive canal, in order to show some of the further modifications which epitheliums may undergo.