contain only half the usual number of chromosomes, consequently the male pronucleus, which is formed from the nucleus of the spermatoblast (head of the spermatozoon), also is formed by one half the usual number. The same is the case with the female pronucleus which has parted with one half its total number of chromosomes in extruding the second directive corpuscle, so that the blending of the male and female pronuclei restores to the ovum its full number of chromosomes, and since in the division of this and in all subsequent processes of division, the chromosomes split and pass half into the one and half into the other daughternucleus, the number of chromosomes is constant for all the resulting cells of the organism.

The two daughter-cells are each at first smaller than the mother-cell, but they soon grow, and the process may recommence and be repeated in them, and in this way cell-multiplication may be exceedingly rapid. The cells, as is most generally the case, may become entirely separated, but in some cases they remain in partial

a, a large cell the nucleus of which appears to be partly divided into three by constrictions; b, a cell the enlarged nucleus of which shows an appearance of being constricted into a number of smaller nuclei; c, a so-called giant-cell with many nuclei; d, a smaller cell with three nuclei; e-i, other cells of the marrow.

conjunction, united by filaments of protoplasm which vary in number, length, and thickness.

Sometimes, a multiplication of nuclei within a cell occurs without immediate separation into new cells, as in the large cells which are found in the medullary cavities of bone (fig. 221).

Instances have been observed out of the body in which the amoeboid movements of the protoplasm seem to have produced cell-division (Klein, Ranvier), but this occurrence is seldom, and most likely abnormal; indeed, it is found, as a general rule, that whilst cell-division is proceeding, the external manifestations of activity of cell-protoplasm cease almost entirely.

The following are the more important phases of change of the chromoplasmic filaments put in tabular form :

Double Skein OR DISPIREM. 8. Open skein in daughter nuclei. {3. Close skein in daughter nuclei.

NETWORK OR RETICULUM

...

10. Resting condition of daughter nuclei.

Historical. The existence of utricles or saccules enclosed by a membrane was recognized in the tissues of plants as long ago as the latter half of the 17th century (by Hooke, Malpighi, Grew, and Leeuwenhoek), and a nucleus was noticed and described by Fontana about a century later. At the beginning of the present century, the cellular constitution of plants was further studied and described by Mirbel and Turpin ; but it was not until the third decade of the century that the improvements which had taken place in the microscope led to the general recognition of the fact that, amongst plants at least, the higher organisms are entirely composed of cells, each of which is essentially formed of a membrane enclosing cellcontents and contains a nucleus (R. Brown, Schleiden, 1831-1838). This generalization was extended to the animal tissues by Schwann, in a remarkable work published in German in 1839 (Microscopical Researches into the Accordance in the Structure and Growth of Animals and Plants. Sydenham Society's Translation, 1847), in which he further showed that in all probability every cell is derived from a pre-existing cell.

The researches of Schwann were so widely extended and the evidence he adduced was so conclusive that his ideas, under the name of the "cell-theory" still remain as the accepted doctrine of the constitution of plant and animal organisms. The term protoplasm was applied by Purkinje to the substance of animal cells in 1840, but first came into extensive use after its employment by v. Mohl, in 1846, who applied it to the living substance of the plant-cell. The material itself, with all its most prominent characteristics as displayed in Infusoria, was however described, in 1835, under the name of "sarcode," by Dujardin, the accuracy of whose description has, it will be seen, left but little for subsequent observers to add "I propose to name sarcode that which other observers have termed a living jelly, a substance glutinous, diaphanous, homogeneous, refracting light a little more than water, but much less than oil, extensible and ropy like mucus, elastic and contractile, susceptible of spontaneously forming within itself spherical cavities or vacuoles which become occupied by the surrounding liquid Sarcode is insoluble in water, but is eventually decomposed by it, leaving a granular residuum. Potash does not dissolve it suddenly like mucus or albumen, and seems simply to hasten its decomposition by water; nitric acid and alcohol immediately coagulate it and render it white and opaque. . . . . . The most simple animals, such as ambæ and monads, are entirely composed, at least to all appearance, of this living jelly. In higher infusoria it is enclosed in a loose integument which looks like a network on its surface Sarcode is found in ova, zoophytes, worms, and in other animals; but it is here capable of assuming with age a degree of organization more complex than in animals at the bottom of the scale . . . . Sarcode is without visible organs and has no appearance of cellularity; but it is nevertheless organized, for it emits various prolongations along which granules pass and which are alternately extended and retracted in one word, it possesses 'life.'

Gradually, both in plants as well as in the lower animals, it came to be generally recognized that the sarcode of Dujardin and the protoplasm of v. Mohl are endowed with similar attributes, and a cell was defined as composed of structureless protoplasm, endowed with irritability and contractility, containing a nucleus and enclosed by a cell-membrane. That a cell-membrane is, however, not an essential character and is often absent, especially in animal cells, was shown by Leydig (in 1856), and was especially emphasized by M. Schultze and by Brücke (in 1861). Even at the present time the definition of a cell proposed by Leydig still holds good-" a mass of protoplasm furnished with a nucleus."

Up to 1865 protoplasm was universally held to be homogeneous and structureless. Attention was drawn by Frommann, however, to a fibrillar structure in the protoplasm of many cells, and such structure was regarded by him as of universal occurrence. This view was somewhat later expanded by Heitzmann, Klein, and others, who described a reticular structure as occurring in all protoplasm, but it was by no means certain that the structure described might not have been produced by the reagents which were employed to exhibit it; for it must be borne in mind that precisely such a reticulum as is exhibited in protoplasm which has been treated with alcohol or chromic acid can be equally well produced in solutions containing albumen or mucus. Nevertheless, it is the opinion of many histologists at the present day who have given special attention to the subject (Leydig, Kupffer, Flemming, Carnoy) that protoplasm invariably contains a reticulum, although others, and probably a smaller number, still regard the reticular or spongy structure as non-essential to its constitution (Kollmann, Strasburger, Schwartz). The reticular structure of the nucleus, although it had been previously described by various observers, was for the most part regarded as merely a localized and specialized part of the general cell-reticulum. The existence and

The following account is mainly derived from the "Histoire de la Cellule," given by Carnoy (Biologie Cellulaire, Fascicule 1, 1884).

independence of the nuclear network, and the distinction of the nuclear substance with chromatic and achromatic material was demonstrated by Flemming in 1879. The changes which are characteristic of karyokinesis have since been studied and described by Strasburger and others in plants, by Flemming, Klein, Arnold, Rabl, Carnoy, v. Beneden, Boveri, and many others in animal cells.1

RECENT LITERATURE.

Altmann, Studien über die Zelle, Leipzig, 1886, 1890; Die Structur des Zell-kerns, Arch. f. Anat. u. Phys., Anat. Abth., 1889; Die Elementarorganismen, Leipzig, 1890.

Arnold, J., Beobachtungen über Kerne und Kerntheilungen in den Zellen des Knochenmarkes, Arch. f. pathol. Anat., Bd. 93; Weitere Beobachtungen über die Theilungsvorgänge an den Knochenmarkzellen und weissen Blutkörperchen, Ibid. Bd. 97; Ueber Kerntheilung und vielkernige Zellen, Ibid., Bd. 98; Ueber Theilungsvorgänge an den Wanderzellen, ihre progressiven und regressiven Metamorphosen, Arch. f. mikrosk. Anatomie, 1887; Weitere Mittheilungen über Kern- und Zelltheilungen in der Milz; zugleich ein Beitrag zur Kenntniss der von der typischen Mitose abweichenden Kerntheilungsvorgänge, Ibid., 1888.

Auerbach, L., Ueber zweierlei chromatophile Kernsulstanzen, Sitzungsb. d. Berliner Akademie,

1890.

Balbiani, E. G., Sur la structure du noyau des cellules salivaires chez les larves de Chironomus, Zoolog. Anz., 1881; Merotomie des infusoires, Recueil Zoolog. Suisse, ix., 1888; Sur la structure du noyau du Toxophyllum meleagris, Zool. Anz. 1890.

Beneden, E. van, et A. Neyt, Nouvelles recherches sur la fécondation et la division mitotique chez l'ascaride mégalocéphale, Bulletins de l'acad. royale de Belgique, 1887.

Boveri, Th., Ueber Differenzirung der Zellkerne während der Furchung des Eies von Ascaris megalocephala, Anatom. Anzeiger, 1887; Zellen-studien, Jena. Zeitschrift, 1887, 1888, 1890. Brass, A., Die chromatische Substanz in der thierischen Zelle, Zool. Anz., 1883.

Bütschli, O., Ueber die Structur des Protoplasmas, Heidelberg Verhandl., Bd. iv., 1889; see also Quart. Journ. of Micr. Science, 1890, and Biol. Centralbl., 1890.

Carnoy, J. B., La cytodiérèse chez les arthropodes, La Cellule, t. i. 1884; La cytodiérèse de l'œuf; La vesicule germinative et les globules polaires de l'ascaris mégalocéphale, Ibid., ii.; La cytodiérèse de l'œuf chez quelques nematodes, La Cellule, t. iii., 1886; Les globules polaires de l'ascaris clavata, La Cellule, t. iii., 1887; also "Biologie Cellulaire," 1884.

Clark, J., Protoplasmic movements and their relation to oxygen pressure, Proceedings of the Royal Society, xlvi., 1889.

Cornil, Sur un procédé de division indirecte des cellules par trois dans les tumeurs, Archives de Physiologie, 1886 et 1887.

Denys, J., La cytodiérèse des cellules géantes et des petites cellules incolores de la moëlle des os, La Cellule, t. ii., 1886; Quelques remarques à propos du dernier travail d'Arnold sur la fragmentation indirecte, Ibid., t. v., 1889.

Flemming, W., Zellsubstanz, Kern u. Zelltheilung, Leipz., 1882; Zur Orientirung über die Lezeichnung der verschiedenen Formen von Zell- und Kerntheilung, Zool. Anzeiger, 1886; Neue Beiträge zur Kenntniss der Zelle, Arch. f. mikr. Anat., Bd. 29, 1887; Amitotische Kerntheilung im Blasenepithel des Salamanders, Arch. f. mikr. Anat., Bd. 34, 1889; Attraktionssphären u. Centralkörper in Gewebszellen u. Wanderzellen, Anat. Anzeiger, 3, 1891.

Frommann, C., Untersuchungen über Structur, Lebenserscheinungen und Reactionen thierischer und pflanzlicher Zellen, Jenaische Ztschr. f. Naturwiss., Bd. xvii., 1884; Beiträge zur Kenntniss der Lebensvorgänge in thierischen Zellen, Ibid., Bd. xxiii., 1889; Ueber neuere Erklärungsversuche der Protoplasma-strömungen, &c. Anat. Anzeiger, 1890.

Gehuchten, A. van, L'axe organique du noyau, La Cellule, t. v., 1889.

Guignard, Recherches sur la structure et la division du noyau cellulaire, Ann. des sciences nat.,

t. xvii.

Gulland, G. L., The nature and varieties of leucocytes, Laboratory reports of R. Coll. of Physicians of Edinburgh, vol. iii., 1891.

Hanstein, J., Das Protoplasma, Heidelberg, 1880.

Hermann, F., Die Entstehung der karyokinetischen Spindelfigur, Münchener med. Wochenschr.,

1890,

Hertwig, O., Vergleich der Ei- und Samenbildung bei Nemadoten, eine Grundlage für celluläre Streitfragen, Arch. f. mikr. Anat. 36, 1890.

Hertwig, O. u. R., Ueber den Befruchtungs- und Theilungs-Vorgang des thierischen Eics, Jena,

1887.

Hertwig, R., Ue. Kernstructur u. ihre Bedeutung f. Zelltheilung u. Befruchtung, Sitzungsb. d. Gesellsch. f. Morph. u. Physiol. in München. iv. 1888.

Hofer, B., Experimentelle Untersuchungen ü. den Einfluss des Kernes auf das Protoplasma, Jena. Zeitschr. f. Naturw., xxiv., 1889.

Howell, W. H., Observations upon the occurrence, structure and functions of the great cells of the marrow, Journal of Morphology, 1890.

Kölliker, A. von, Das Aequivalent des Attractionsphären bei Siredon, Anat. Anzeiger, iv.,

1889.

1 For a full account of the history of the more recent researches on the structure of cells and nuclei at rest and during division, the student is referred to the paper by Waldeyer before mentioned.

Korschelt, E., Beiträge zur Morphologie u. Physiol. des Zellkerns, Zool. Jahrb., Bd. iv., 1890. Kossel, A., Ueber einen peptonartigen Bestandtheil des Zellkerns, Zeitschr. für physiol. Chemie, Bd. viii., 1884; Beiträge zur Chemie des Zellkerns, Zeitschr. f. physiolog. Chemie, Bd. vii., 1883; Weitere Beiträge zur Chemie des Zellkerns, Ibid., Bd. x., 1886.

Kühn, H., Ue. vitale Reaction der Zell-granula, &c., Archiv f. Anat. u. Physiol., Anat. Abth., 1890.

Leclercq, E., Contrib. à l'étude du Nebenkern, &c., Bull. de l'acad. roy. de Belgique, 1890. Leydig, Fr., Zelle und Gewebe, Bonn, 1885.

Löwit, Ueber Amitose, Centralbl. für allgem. Pathologie., Bd. i., 1890.

McKendrick, J. G., The Modern Cell-theory, Proc. of the Glasgow Phil. Soc., 1888. Martin, Zur Kenntniss der indirecten Kerntheilung, Arch. f. pathol. Anat., Bd. 86, 1881. Nussbaum, M., Ueber die Theilbarkeit der lebendigen Materie, Arch. f. mikr. Anat., Bd. 26. Ogata, M., Die Veränderungen der Pankreaszellen bei der Sekretion, Archiv f. Anat. und Physiol., Physiologische Abtheilung, 1883.

Pfeffer, W., Zur Kenntniss der Plasmahaut u. der Vacuolen, &c., Abhandlungen der K. Sächs. Gesellsch., xvi., 1890.

Pfitzner, W., Ueber den feineren Bau der bei der Zelltheilung auftretenden fadenförmigen Diffe renzirung des Zellkerns, Morphol. Jahrb., Bd. vii., 1881; Beiträge zur Lehre vom Baue des Zellkerns und denen Theilungserscheinungen, Arch. f. mikr. Anat., Bd. 22, 1883.

Platner, G., Ueber die Entstehung des Nebenkerns und seine Beziehung zur Kerntheilung, Arch. f. mikr. Anat., Bd. 28; Beiträge zur Kenntniss der Zelle u. ihrer Theilung, Ibid., Bd. 33; Die Karyokinese bei den Lepidopteren als Grundlage für eine Theorie der Zelltheilung, Internation. Monatsschrift für Anat. u. Histologie, Bd. iii., 1886.

Quincke, Ueber periodische Ausbreitung an Flüssigkeitoberflächen und dadurch hervorgerufene Bewegungserscheinungen, Wiedemann's Annalen f. Physik u. Chemie, 1888.

Rabl, K., Ueber Zelltheilung, Morph. Jahrbuch, Bd. x., 1885; and in Anat. Anzeiger, vi., 1889. Reinke, J., und Rodewald, H., Studien über das Protoplasma, Untersuchungen aus dem botanischen Laboratorium der Universität Göttingen, 1881.

Rüffer, A., On the phagocytes of the alimentary canal, Quart. Journ. Micr. Sci., 1890.

Sanderson, J. S. Burdon, Address to the Biological Section of the British Association, Report for 1889.

Sattler, E., Die Verwendung des Lapisstiftes z. Untersuchung der Epithelien, Arch. f. mikr. Anat., Bd. 21, 1882.

Schäfer, E. A., On the structure of amaboid protoplasm, &c., Proceedings of the Royal Society, Vol. xlix., 1891.

Schottländer, J., Ueber Kern- u. Zelltheilungsvorgänge in dem Endothel der entzündeten Hornhaut, Arch. f. mikrosk. Anat., Bd. 31, 1888.

Schultze, O., Ueber Zelltheilung, Ber. der Würzburger phys.-med. Gesellsch., 1890.

Schwarz, Frank, Die morphologische und chemische Zusammensetzung des Protoplasmas, Breslau, 1887.

Strasburger, Ed., Ueber Kern- und Zelltheilung im Pflanzenreiche, nebst einem Anhänge über Befruchtung, Jena, 1888; Ueber den Theilungsvorgang der Zellkerne und das Verhältniss der Kerntheilung zur Zelltheilung, Arch. f. mikr. Anat., 1882, Bd. 21; Neue Untersuchungen ü. d. Befruchtungsvorgang bei den Phanerogamen als Grundlage für eine Theorie der Zeugung, 1884; Die Contoversen der indirecten Kerntheilung, Arch. f. mikrosk. Anat., Bd. 23, 1884.

Stricker, S., Photogramm eines farblosen Blutkörperchens, Arbeiten, a. d. path. Institut zu Wien, 1890.

Tangl, F., Ueber das Verhältniss zwischen Zellkörper und Kern während der mitotischen Theilung, Arch. f. mikr. Anat., Bd. 30, 1887.

Verson, E., Zur Biologie der Zellen, Zool. Anz., 1890.

Waldeyer, W., Karyokinesis and its Relation to the Process of Fertilization, Quarterly Journal of Micr. Science, vol. xxx., 1889 (Translated from the Arch. f. mikr. Anat.), containing a complete bibliography up to 1888.

Woodhead, G. S., and Wood, G. E. C., The physiology of the cell considered in relation to its pathology, Edinburgh Med. Journal, 1890.

Zacharias. E., Ueber die chemische Beschaffenheit des Zellkerns, Botan. Zeitg., 1881-1882; Ueber den Nucleolus, Ibid., 1885; Ueber Entstehung u. Wachsthum der Zellhaut, Tageblatt der 61 Versammlung deutscher Aerzte, 1889.

THE EPITHELIAL TISSUES.

An epithelium is a tissue which is composed entirely of cells with a comparatively small amount of intercellular substance. It usually takes the form of a membrane covering the free surfaces of the body. Thus it is well known, that when the skin is blistered, a thin, and nearly transparent membrane, named the cuticle or epidermis, is raised from its surface. In like manner a transparent film, similar in nature to the epidermis, may be raised from the lining membrane of the lips and mouth, in which situation it first received the name of " epithelium ;" and under the latter appellation, a coating of the same kind exists on nearly all free surfaces and membranes of the body.1

The following are the most important situations in which a covering or lining of epithelial tissue is found: viz., 1. On the surface of the skin. 2. On mucous membranes; a class of membranes to be afterwards described, which line those internal cavities and passages of the body that open exteriorly,-viz., the alimentary canal, the lachrymal, nasal, tympanic, respiratory, urinary, and genital passages; as well as the various glandular recesses and ducts of glands, which open into these passages or upon the surface of the skin.2 3. Lining the ventricles of the brain and the central canal of the spinal cord. 4. In the organs of special sense where the cells are often greatly modified and receive the endings of the nerves of special sense. 5. On the inner or free surface of serous membranes, which line the walls of closed cavities in the chest, abdomen, and other parts, and on the inner surface of the heart, blood-vessels and lymphatics. In these situations the epithelial lining has received the name of endothelium (His).

In many parts of the connective tissue the cells of that tissue are flattened out and arranged close together, edge to edge, in such a manner as to form a membrane of cells, which so far would come under the definition of the term epithelium. But the cells in question exhibit every transition to the other cells of the connective tissue, so that their enumeration under epithelium would create an artificial separation between cells of the same elementary tissue. They may, however, be conveniently described as epithelium-like (epithelioid). Many histologists are of opinion that a similar distinction should be made for the epithelium of the serous membranes and of the vessels, because these are developed within the mesoblast, and it is the following up of this idea which has led to the adoption of the word " endothelium" to express an epithelium so derived. But, if every epithelium which originates in the mesoblast is to be so designated, we shall be compelled to separate from the other epithelial tissues, with which they are in every way closely allied, the epithelia of the renal, and of the generative organs, since these appear to be derived from the same layer of cells. And indeed, since it has been shown in several instances amongst both invertebrates and vertebrates, that the epithelium of the serous cavities, and even that of the heart, is directly derived from an undoubtedly epithelial layer-the entoderm-it is probable that this is the original and typical mode of origin of all the so-called endothelia, although it has become obscured in the development of higher vertebrates.

Structure of epithelial tissue in general.-Every epithelial tissue is formed entirely of cells united together by cohesive matter, often in too small quantity to be apparent without the employment of nitrate of silver staining. The layer or layers thus formed take the shape of the surface to which they are applied, following accurately all its eminences and depressions. As a rule no blood-vessels

The term "epithelia," which has passed into "epithelium," was introduced by Ruysch to designate the cuticular covering on the red part of the lips. The word "epidermis" he considered inappropriate, as the subjacent surface is not skin (derma); but, as it is beset with papillæ, he named the covering layer "epi-thelia," from eɩ and Oŋλn, a nipple or papilla. The use of the term has, by a not unusual license been extended so as to signify the same kind of coating when it spreads over nonpapillary surfaces.

2 The hairs and nails and the enamel of the teeth, as the study of their development shows, are also undoubtedly of epithelial origin, but in the case of the enamel and in a lesser degree of the hairs, they have become so specialized that their epithelial structure is scarcely longer recognizable.