Page:The New International Encyclopædia 1st ed. v. 04.djvu/467

CELL. 3tt9 CELL. tiliiis of protoplasm by whioli the adjaoent cells are orjianiially connected. In plants where a {;reat weight has to be borne and great rigidity {;iven, the cell-wall often pains a great thick- ness luid constitutes the wood of the dead plant. The nucleus is essential to the processes of assimilation and growth, and probably controls in these operations. Wliile its general form is siitispherical. it may become greatly elongated, lobed, branched, form a scries like a string of beads, or ajipcar as two bodies or even as a mass of chromatin, scattered throughout the cell. In the chromatin network lie the chromatic par- ticles and certain larger bodies called nucleoli. There may be many nucleoli, or only one large one. The typical nucleolus is regarded as an excretion product which is eventually cast out of the nucleus. The plasm films separating the nuclevts from the cytoplasm are usually very evident, and, taken together, constitute the so- called nuclear membrane. It is of course a liv- ing membrane. Outside of the nuclear mem- brane is often found a minute particle, the centrosome. Ccll-Diiision. — The method by which the nuil- ticellular body is produced from the unicellular egg was long misunderstood. It was thouglit tliat cells crjstallized out of a homogeneous matrix, or that new cells were formed inside of the preexisting cells. It is now known that there is one process of cell-multipliealiim and one only — namely, division of a cell into two equal parts. This division involves all parts of the cell — cytoplasm, nucleus, and cen- irosome. The method of cell-division varies in diiferent eases: two main tyiies may, however, be distinguished — -mitotic division or 'karyo- kinesis,' and amitotic division. The mitotic division seems to be the more usual type, so it may be first considered. For purposes of descrip- tion, four series of stages or phases may be recognized: (1) The prophases, or preparatory changes : ( 2 ) the metaphase, or acme of the di- vision process; (3) the anaphases, or aggrega- tion of nuclear material at the centres: (4) the telophases, or those in which the cytoplasm di- vides and the two new nuclei are established. ( 1 ) Prophases. — In the cytoplasm the cen- trosome becomes double, if not so already, and the two centres move apart. A set of radiations now make their appearance in the cytoplasm — the asters — having the centrosomes at their cen- tres. Between the centrosomes the asters pass over into each other, making a spindle-shaped figure composed of lines — the karyokinetie spindle. At the same time changes are occurring in the nucleus. The chromatin, which has pre- viously consisted of scattered particles, becomes condensed into a deeply staining thread, which is coiled as a twisted or spiral thread within the nuclear membrane (skein or spireme stage). Kventually this thick thread, from which the mitotic process takes its name (Gk. /ilm^, a tliread). breaks into a number of deeply stain- ing rods (centrosomes). The number of centro- somes is believed to be constant in each species tlirouglifput the whole series of cell-divisions in the individual, and is always even. In the threadworm (Ascaris) there are 2 or 4 chromo- somes: in certain liverworts, 8: in certain in- sects, 12. 16, 20, etc.; in the frog and mouse. 24; in the crustacean Artemia, 108; in man, prob- ably 10. The nucleoli are either cast out into the cytoplasm or are gradually dissolved in place. {2) Metaphase. — In this phase the spindle has come to lie in the equator of the nucleus, and the nuclear membrane has disa])peared. Each chromosome splits lengthwise in equivalent parts, one-half of each going toward each pole. Con- sequently, each of the daughter nuclei receives exactly equivalent portions of the chromatic substance of the mother nucleus. (.3) Anaphases. — The separated parts of the cliromosomes move to the two poles of the spin- dle, and these group themselves closely together. I'or a time the spindle fibres still persist as fine tlireads connecting the chromosomes, and in the middle of their couse a plate of fine granules often appears lying across the fibres. The asters fade away and the process of nuclear division is accomplished. (4) ?'c/o;>/i«.st's. — The whole cell now divides, the division plane passing through the plate of granules, which plate lidps form the new cell- wall. The chroma topliores seem to absorb water, swell up, ])rcss against each other, and fonu spherical nuclei. Alongside each nucleus i.-, found the centrosome of the new cell. What are the purpose and the mechanism of mitosis? The purpose is quite certainly the exact division of ihe chromatic material. Concerning th3 mech- anism there is still nuich difference of opinion. It seems probable that currents in the plasma films convey the chromosomes from each other and toward the opposite poles. The division of the cytoplasmic body may result from a centrip- etal flowing toward the centres of the two asters. But we are ignorant of the causes which deter- mine the direction of flow. Amitotic Division. — This consists of a con- striction of a nucleus without any formation of chromosomes. -Vfter two nuclei are formed the cytoplasmic body may divide. The significance of amitosis is very uncertain. It is especially common among cells that are about to perish; it seems to be induced by peculiar conditions of the cytoplasm. The History of the Cell Theory. — The first in- vestigations into the finer structure of organisms were made by ^falpighi and by Grew, at the end of the Sixteenth Century, iipon plants. They discovered in them small, lluid-filled spaces with firm walls. But it was not until the early part of the ineteenth Century that the general no- tion that the whole body of the higher organisms was composed of a mass of cells was gained. This generalization became established by Schleid- en and by Schwann in 1838. The importance of the cell-contents was not at first appreciated, but when they were found in constant nioti(m in the live plant-cell (Corti, 1772, and Trcviranus, 1807), the idea that they were the essential liv- ing substance came to prevail. The name of protoplasm was first assigned to the cell-con- tents by Mohl (1840). Gradually, as cells with- out walls were discovered, the idea of cell took im this form — a mass of protoplasm possessing a single nucleus. The chemical composition of protoplasm throws little light on vital action, although vital action is a chemical process. The reason is that the form of the molecules rather than the quantita- tive analysis or the enumeration of the elements