morphologically, of cells, and subsists, physiologically, by means of the ‘reciprocal action’ of the cells,”—this was the cell standpoint of Schleiden and Schwann, and it is no exaggeration to say that this same conception has dominated the cell-theory almost to the present day.[1] The frequently striking correlation between cell-division and cell-differentiation in development has caused this process to be regarded as dependent on cell-division, while a wholly exaggerated importance has been attached to the distinction between “unicellular” and “multicellular” organisms—between “intercellular” and “intracellular” organs. The influence of the “cells” upon one another, the subordination of the cell’s growth, division and differentiation, to the requirements of the whole organism—seen in normal growth, but nowhere more strikingly than in development and regeneration,—is, however, very difficult of explanation in terms of the cell-theory as this was, until quite recently, generally understood. The very elaborate regional differentiation of the protoplasm often seen in the Protozoa sufficiently indicate that multicellular structure is no essential condition for complex regional differentiation. That the regional differentiation of the protoplasm in the Metazoa should usually correspond with cell-limits is scarcely surprising. Nor is it to be wondered at that, with so convenient a mechanism for segregation to hand as cell-division, the progressive differentiation seen during development should often appear to go hand in hand with this process. In recent years, however, evidence has been steadily accumulating to show that this association between cell-division and regional differentiation of the protoplasm in development is a casual one—as casual, and as natural, as the correspondence between cell limits and regional differentiation in the formed tissues. The fact that the regional differentiation may be foreshadowed in the egg before cleavage begins,[2]—that as Driesch has shown, the mode of cleavage may be artificially altered without affecting the ultimate organization of the embryo,—and many other similar observations, tend to emphasize the importance of the “organism” standpoint (C. O. Whitman, 1903, p. 642) in contradistinction to the widely prevalent “cell” standpoint. The occurrence of syncytial organs and organisms, and the increasing frequency with which protoplasmic continuity is being demonstrated between all kinds of cells, are facts tending in the same direction. In the plant kingdom the growth of the mass has been recognized as the primary factor in development;[3] die Pflanze bildet Zellen, nicht die Zelle bildet Pflanzen (de Bary). For the animal kingdom this “Inadequacy of the Cell-Theory of Development” has been maintained amongst others by Whitman,[4] and by Adam Sedgwick.[5] The latter author, mainly as the result of work on the development of Peripatus and of Elasmobranch embryos, regards the developing embryo as a continuous protoplasmic reticulum, for the nuclei of which the limiting epithelial layers constitute as it were a breeding ground. Differentiation is a regional specialization of this nucleated meshwork, and is not to be regarded as the result of the proliferation and subsequent specialization of cells predestined by cleavage for this end.
It is possible to suggest a mechanico-physical explanation of multicellular structure which will deprive the cell of much of its assumed significance as a unit of organization. The fact that surface area becomes relatively less extensive as bulk increases would alone set a limit to the size of “unicellular” organisms; for not only is there a constant reaction between nucleus and cytoplasm through the nuclear membrane, but the surface of the cell serves both for the intake of food and the elimination of waste material. In addition to the limit thus imposed upon the cytoplasmic area which can be effectually controlled by the nucleus, and the necessity for a minimum surface area to the protoplasmic mass, the advantages of the more or less complete subdivision of the living substance into—as far as their metabolism is concerned—semi-autonomous units, is indicated by the mechanical support derived from the specialized cell walls and turgescent cells of the plant, and the intercellular secretions of the animal tissues. It is more than possible that these two conditions—i.e. surface area for diffusion, and mechanical support—are alone responsible for the origin of multicellular structure, and that the sharply defined character this now so generally possesses has been secondarily acquired as a result of the facilities it undoubtedly offers for regional specialization in the protoplasmic mass.
Bibliography.—The special literature of cytology has grown to large dimensions. The following are the more important text-books and papers of general interest: E. B. Wilson, The Cell in Development and Inheritance (2nd ed., 1900); A. Gurwitsch, Morphologie und Biologie der Zelle (Jena, 1904); O. Hertwig, Allgemeine Biologie (Jena, 1906); Korschelt and Heider, Lehrbuch der vergl. Entwicklungsgeschichte der wirbellosen Tiere, Allgem. Teil, “The Germ Cells and Experimental Embryology” (Jena, 1903); Whitman, “The Inadequacy of the Cell Theory of Development,” Journ. Morph. viii., 1893; Adam Sedgwick, “On the Inadequacy of the Cellular Theory of Development,” Quart. Journ. Micro. Science, xxxvii.; G. C. Bourne, “A Criticism of the Cell Theory” (an answer to Sedgwick’s paper), Quart. Journ. Micro. Science, xxxviii.; Th. Boveri, “Befruchtung,” Merkel-Bonnets Ergebnisse der Anat. u. Entwicklungsgesch. Bd. i. (1892), Das Problem der Befruchtung (Jena, 1902), Ergebnisse über die Konstitution der chromatischen Substanz des Zellkerns (Jena, 1904); J. Rückert, “Die Chromatinreduktion bei der Reifung der Sexualzellen,” Merkel-Bonnets Ergebnisse, Bd. iii. (1894); V. Häcker, “Die Reifungserscheinungen,” Ergebn. Anat. u. Entwicklungsgesch. Bd. viii. (1898); F. Meves, “Zellteilung,” Merkel-Bonnets Ergebnisse, Bd. viii. (1898, 1899); W. Waldeyer, “Die Geschlechtszellen,” in O. Hertwig’s Handbuch der vergleich. u. experiment. Entwicklungslehre d. Wirbeltiere (1901, 1903). (G. C. C.)
CYZICENUS, the architectural term given by Vitruvius to the large hall, used by the Greeks, which faced the north, with a prospect towards the gardens; the windows of this hall opened down to the ground, so that the green verdure could be seen by those lying on the couches.
CYZICUS, an ancient town of Mysia in Asia Minor, situated on the shoreward side of the present peninsula of Kapu-Dagh (Arctonnesus), which is said to have been originally an island in the Sea of Marmora, and to have been artificially connected with the mainland in historic times. It was, according to tradition, occupied by Thessalian settlers at the coming of the Argonauts, and in 756 B.C. the town was founded by Greeks from Miletus. Owing to its advantageous position it speedily acquired commercial importance, and the gold staters of Cyzicus were a staple currency in the ancient world till they were superseded by those of Philip of Macedon. During the Peloponnesian War (431–404 B.C.) Cyzicus was subject to the Athenians and Lacedaemonians alternately, and at the peace of Antalcidas (387 B.C.), like the other Greek cities in Asia, it was made over to Persia. The history of the town in Hellenistic times is closely connected with that of the dynasts of Pergamum, with whose extinction it came into direct relations with Rome. Cyzicus was held for the Romans against Mithradates in 74 B.C. till the siege was raised by Lucullus: the loyalty of the city was rewarded by an extension of territory and other privileges. Still a flourishing centre in Imperial times, the place appears to have been ruined by a series of earthquakes—the last in A.D. 1063—and the population was transferred to Artaki at least as early as the 13th century, when the peninsula was occupied by the Crusaders. The site is now known as Bal-Kiz (Παλαία Κύζικος?) and entirely uninhabited, though under cultivation. The principal extant ruins are:—the walls, which are traceable for nearly their whole extent, a picturesque amphitheatre intersected by a stream, and the substructures of the temple of Hadrian. Of this magnificent building, sometimes ranked among the seven wonders of the ancient world, thirty-one immense columns still stood erect in 1444. These have since been carried away piecemeal for building purposes by the Turks.
See J. Marquardt, Cyzicus (Berlin, 1830); G. Perrot, Exploration de la Galatie (Paris, 1862); F. W. Hasluck and A. E. Henderson in Journal of Hellenic Studies (1904), 135-143. (F. W. Ha.)
- ↑ Whitman, Jour. Morph., 1903.
- ↑ This “Precocious segregation” (Lankester, 1877) is well seen in the eggs of many Ctenophorae, Annelids, Gastropods and Nematodes. See the papers by Lillie (1901), Conklin (1902), &c., and especially Wilson on “Dentalium,” Journ. of Exp. Zool., No. 1, 1904.
- ↑ Hofmeister, de Bary, Sachs, &c.
- ↑ Loc. cit.
- ↑ Quart. Journ. Micro. Science, 1894, vol. xxxvii.
