(1) Prophases.—In the eytopla=m the (pit tnisome teem es double. if not so already. and the two centre: move apart. A set of radiations now make their appearance in the cytoplasm— the aster—having the centrosomes at their cen tres. Between the centrosonws the asters pas: over into each other. making a spindle-shaped figure composed of lines—the ka•yokinetic spindle. At the sante time change: are occurring in the lindens. The chromatin, which has pre viously consisted of scattered particle:, becomes condensed into a deeply staining thread. which is coiled as a twisted or spiral thread within the nuclear membrane (skein or spireme stige I. Eventually this thick thread, from which the mitotic process take: its name (Gk. pfro; , a thread ). breaks into a number of deeply stain ing rids (eentrosonies). The nUnlber of eentro :min, is to be constant in each speeies throughout the whittle series of cell-divisions in the individual, and is always even. In the threadworm I Ascaris) there are 2 or 4 chromo some:: in certain liverworts. ti: in certain in sect:. 12. I R, 20. etc.: in the frog and mouse. 24; in the crustaeean Artemia, 14;S; in man, prob ably Id. The nucleoli are either east out into the cytoplasm or are gradually dissolved in place.
(2) th.taphase.—In this phase the spindle has to lie in the equator of the nucleus, and the nuebbar membrane ha: disappeared. Eaeli 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 ch roma t ic substance of the mother nucleus.
(3) linapham.s.—The separated parts of the (b•omosomes move to the I we pole, of the spin dle, and these group themselves closely together. For a time the spindle fibres still persist as fine threads connecting the chromosomes, and in the middle of their course a plate of fine granule: laten appears lying across the fibres. The asters fade away and the process of division is accomplished.
(4) Te/ophages.—The whole cell now divides, the division plane passing through the plate of granules. which plate helps form the new eellwall. The ehromatophores seem to al), PH) water, swell up, press against each other, and form spherical nuclei. Alongside each is found the centrosome of the new cell. are the purpose and the meehanisin of mitosis? The purpose is quite certainly the exact division of the chromatic material. Concerning th-2 mech anism there is still mueh difference of 4/pillion. It seems probable that currents in the plasma films convey the chromosomes from each other and toward the opposite pole:. The division of the ectoplasmic body may result from a centrip etal flowing toward the centres of the two asters. But we are ignorant of the causes whieh deter mine the direction of Clow.
Amitotic Dirision.—This consists of a. con striction of a nucleus without any formation of chromosomes. After 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 rell Thcory.—The first in vestigations into the finer structure of were made by and by 1-;row, at the end of the Sixteenth Century, plants. They discovered in them small, fluid-filled spaces with firm walls. But it was not until the early part of the Nineteenth Con tury that the general no tion that the whole body of the higher organisms w:ts composed of a mass of cell, was gained. This generalization became established by Sehleid en and by `4•hwann in The importance of the Pell-contents was not at first appreciated. but when they were found in constant motion in the live plant-cell (Corti. 1772. aid Troviramis, 1417). dab idea that they were essential liv ing substance came to prevail. The name of protopla,m was first assigned to the cell-con tents by Mohl (1S4(1). Gradually. as cells with out walls were discovered, the idea of cell took on this form—a mass of protoplasm possessing a single nucleus.
The eh( mica 1 composition of protoplasm throws little light on vital action. although vital action is a chemical process. The reason is that the form of the molecule: rather than the quantita tive analysis or the enumeration of the elements is at the basis of life Carbon, hydrogen, oxygen, nitrogen. are always present, and sulphur, phos phorus, potassium, aml certain other alkalis and metals are usually found in small quantities. The dead protoplasm consists chiefly of allmini including undein, 34 per vent.; lecithin and fats, 1I per emit.: various allm minoids. 14 per (put.: and, for the rest, numer tms other substances.
yell .tetirity.—Cells, apart from the process of division, show active processes in their proto plasm, el'el"ilY their capacity for n"'ve nient and response to stimulus. Cell-movement takes on diverse forms according to circum stances. In naked, free cells, like the Annetta. the most evident IllOVP111011tti are the throwing out of blunt, linger-like processes (pseudopodia ). If one of these takes the lead it determines the of locomotion of the whole eell, for the rest of the body flows into the pseudopod. Often the pseudopodia are numerous and ex tremely fine; under these circumstances minute particles may be seen streaming, in the current of the protoplasm which moves along the thread. In the ease of cells with a firm wall, the proto plasm often streams in fine threads through the Zrell-space, or in other eases a ma• velous rotatory movement is seen. The causes of protoplasmic movement are not altogether clear. Itevently. however. attention has been called to the fact that other foamy masses—especially a drop of oil tilled with vesicles of water—will send out psendopoditun•ike processes. So that the con stant mov(ment of the protoplasm may be the physiological •omornitant of its unstable strut- tore. Other forms of cell-movement are the lash ing of whips (flagella) or little hairs (cilia) covering the free swimming cell ( and Protophyta).