At first, these globules are grouped towards the periphery of the cavity of the primitive (Fig. 3) ovum, and, in this way, form a membrane which we shall study under the name of epithelium. As in the perfected organism, an epithelium rests upon a fibrous or undetermined tissue, so also does the ovular epithelium rest upon the membrana pellucida (Fig. 3, A). We see then even at this stage (and great importance must be attached to these forms) the organism represented successively by a cell, and secondly by an epithelium; this latter might be called epithelium of the zona pellucida (Fig. 3 B); and as this serves as the germ of all the other portions it has been Diagram of the Blastoderm. called the germ-membrane, or, more generally, blastodermic membrane, or blastoderm. Fig. 3. C This change of position of the globules, whence a globular membrane results, is soon followed by a change of form, whence there occurs a separation into distinct layers in this membrane; take for example one of the meridians of the blastoderm where the globules become multiplied more than in any other place; here the blastoderm, as with every epithelium which becomes hypertrophied in a certain portion, is obliged, as will be seen later in the formation of glands and papillæ, to swell out and form a sort of pouch on which may be lodged the new globules that are formed. This pouch or villosity (Fig. 3 C) is the first rudiment of the embryonic body. Without now going into the details, it is necessary simply to mention that at this point the globules become separated into three layers or folds, viz., the external, internal, and intermediate folds. The external fold, called the corneal, maintains its globular condition, and from this is formed our epiderm, our external cuticle,' and such of the organs as may be derived A comparison of the two kingdoms demonstrates the fact that both animals and vegetables have an external envelope, composed of analogous cells; so that we can apply to each the name cuticle. Yet in the vegetable the cuticle is very simple, and almost everywhere the same, but in the animal it is complex, and, accord *A, Vitelline membrane. B, Simple form of Blastoderm. C, Point where the Blastoderm is already composed of three layers of cells, three folds. therefrom (we shall see farther on whether the nerve globules are derived from the external or the intermediate fold of the blastoderm). The internal fold will give, by means of the envelopment which forms the internal cavity of the embryo, the internal cuticle, or the epithelium of the future intestinal canal of the embryo, and also the numerous adjuncts of this canal, most of the glands, and also the lungs. The globules of the intermediate folds undergo transformations which are much more complicated; some are transformed, by the mechanism already mentioned when treating of the globules in general, into muscular, nervous (perhaps also into nerve cells) elastic, and connective fibres, and other forms of the connective tissue; others remain in the condition of globules, though changing their form; and again others become fused with the fibrous elements of the connective tissue (embryonic globules, cells of cartilage, of bone and of tendons) and others bathe in a liquid (blood globules); thus, in short, the intermediate gives origin to two globular forms, viz., the embryonic cell and the blood globule (and possibly the nerve-cell.) 1 The elements of the external cuticle, and those of the internal cuticle or internal epithelium being then united in the single term epithelial (or lining) globules, since they line the surfaces, we have but four kinds of globules to study, viz., the epithelial, the nerve, the blood, and the embryonic globule. 1. Epithelial globules, placed upon fibrous membranes, destined only for their support, form the sole living portions ing to whether it covers all the superficial parts of the body or the cavities communicating with the exterior, it is either internal or external. 1 This distinction of the blastodermic cells may at first seem surprising, though a similar phenomenon is continually passing under the observation of every surgeon. In a fresh superficial wound, there appears first a mass of globules, primarily alike, which separate themselves so as to become either epiderm-cells, connective fibre, etc., before the cicatrix is formed, and exactly in the same way as in the folds of the epiderm. 2 In fact, the word "epithelium" was primarily used to designate the epiderm of the nipple, and afterwards to designate the epiderm of the mucous membrane, to which there is now a tendency to limit its application. Astruc says: "La peau fine et délicate qui recouvre le mamelon, et qu'on appelle Epithelion" (eπi, Oŋλý; upon, the nipple). of these membranes, and, according to their functional activity, they present differing forms; if they are situated in a region where their functions are not very active they are few in number and in order to occupy completely the surface given up to them, they are flattened out, forming a sort of pavement, and hence are called pavement (or tessellated) epithelium (Fig. 4, A). If, on the contrary, as, for example, on the more important mucous membranes, their vital functions A B C are very active, they become ...... multiplied, accumulate in large numbers upon the same place, and make room for each other, by being compressed sideways; and so instead of being round they become cylindrical, hence they are called cylinder (or columnar) epithelium (Fig. 4, B). Finally, if a simple layer is insufficient, the globules are superposed, and hence are called stratified epithelium (Fig. 4, C). Fig. 4. Moreover, for the purpose of offering a large surface without occupying too much space, these epithelium cells overlap each other, an instance of which may have been remarked in the blastoderm, and according to whether the overlapping is on the superficial surface or on the side near the deeper tissues, these make up papillæ or glands; more particular mention will be made of this subject when we have occasion to speak of the formation of the epithelium of the mucous membrane of the mouth. The functions are of far greater importance than the form of the epitheliums; these may be divided into three classes. Certain of the globules present an obstructing surface to the passage of fluids, &c., and are impermeable, as, for instance, in the epithelium of the bladder and of the serous membranes in general. These might be called neutral globules. Another class, on the contrary, absorb very actively the substances (gas or liquid) brought in contact with them, and transport these to the more distant and deeper portions of tissues, as is done for instance by the blood globule. These may be called absorption globules. * A, Pavement epithelium. B, Columnar epithelium. C, Stratified epithelium. A third class possess a faculty of drawing to them certain substances contained in the neighboring tissues or fluids, and thus free the organism from which these are detached. In this way the scaly portions of the epiderm, before passing into this condition and falling off, attract certain calcareous salts, and more especially the phosphates which are contained in the organism. This is also an example of the functions of secretion, and these are called secretion globules. These globules, more than any of the others, are characterized by ephemeral existence, and form the largest portion of glands; the mammary gland is nothing but a membrane of canaliculæ, covered with globules which possess at certain times an excessively active life; then they become very rapidly transformed, and their remains constitute the milk. 2. The nerve globules (or cells) are not fixed upon surfaces under the form of membranes, they are hidden in the deeper parts, constituting what has been called the gray nerve-tissue. By direct experiment it is impossible to judge of their life. Yet, like the others, these globules seem to live, and are nourished, and though we cannot judge de visu of their transformations, at least by comparison upon the dead body, these are found differing in appearance and age, some being smaller and transparent, others greater, pale, or filled with granulations, thus indicating a commencement of their decay. Influenced by their metamorphoses, these globules, as well as the nerves with which they enter into communication, are electro-motor. Indeed it is these prolongations or nerve tubes by which the nerve globules are characterized, and which give them their stellate form. 3. The blood globules, whose existence is best known and the most accessible to our senses, form in blood, and consequently in the body about one-twelfth of our whole weight. They differ from the preceding globules in the fact, that instead of having a fixed place they course through the whole organism; their discoid shapes render their transit easier, and during their course they are continually being transformed, certain of them perishing in order to give room to others. During this nomadic state, the blood globule is still characterized by the phenomena of repulsion and attraction, changes of form and composition, loading itself at certain places with chemical products, which seem destined for deposition in other places. 4. The embryonic cells are so called, because they are the same in the adult that they were in the embryonic stage; distributed in the midst of the tissues, they continue to serve for the production (the periosteal cell continually forming bone) or for the reparation of breaches which may have accidentally made a rent in or destroyed the tissues; hence also their name plasmatic cells. Some of these, incertæ sedis, help sometimes by means of the circulation to nourish the tissues where they are distributed, and then are seen in star-shaped form with anastomoses of their prolongations; the cornea offers a beautiful illustration of this distribution (Fig. 5). At other times the plasmatic cells undergo a sort of decay, by accumulating fat in their interior, and thus afford adipose tissue; in this condition they are no longer susceptible of undergoing transformations; they are so to speak dead. But most, though changing form and becoming almost mummified (stellate plasmatic cells), preserve, in their latent con Fig. 5. dition, all their vital characteristics, ready to wake up if the excitation is sufficiently strong; in this way they can furnish new forms, as, for instance, cancer, different tumors, and, in general, purulent abscess globules. In this way the embryonic cells become pathological. Supposing now that we are familiar with the different kinds of globules, excepting the embryonic globule, we can represent them in a diagram, grouping together the functions. of the three classes of globules. We can represent the organism as a homogeneous mass, more liquid than solid, on the surface of which is a layer of Section of cornea cut parallel to the surface. Stellate corpuscles, with their anastomotic prolongations (His). |