nuclear matrix become continuous; the cytoplasm separates into a clear and a granular zone, and the granules arrange themselves radially from the polar corpuscles (cytasters). The chromatic fibres sink to the equator of the spindle, and arrange themselves so as to project horizontally from it (see fig. 48 a and b).

5. Metakinesis. The sister threads separate, one going towards one pole, the other to the other pole of the spindle (fig. 49); one set of sister threads form one daughter nucleus, the other the other.

a.

b.

c.

FIG. 49.-Karyokinesis. Separation of sister threads; (Metakinesis) one set moving toward one pole of the spindle, the other towards the other. a, b, and c, show successive steps in the process. In e (1) the uniting filaments (v. Beneden) are seen; and the appearance from each pole is like that in fig. 48 b, except that the chromatic fibres are single not double. This stage (c) is called the Dyaster or daughter star stage.

6. Dyaster, or daughter star stage; this stage occurs when the two sets of sister threads are separated, as in fig. 49 c. The fibrils which still unite them are regarded by v. Beneden as different from the spindle, which gradually disappears; Strasburger believes these are the spindle fibres along which the chromatic filaments shift. Each daughter nucleus then goes backwards through the same series of changes; the dyaster is followed by the

7. Dispirem or daughter skein stage (upper part of fig. 50). The new nuclear membrane begins to form in this stage at the antipolar region, and the polar corpuscle disappears. The cell itself then divides; the cell membrane being formed in plants by thickenings or knots in the equatorial region of each spindle fibre; these thickenings coalesce. They are called dermatosomes, and are absent in animal cells.

8. The resting daughter nuclei; when the cytoplasm has divided, the remains of the spindle disappears, the chromatic fibres become more twisted, lose their equal calibre, and become connected by secondary fibres, as is shown in the lower nucleus (figure 50).

1 Also called aureola and helioma.

In the egg cells of certain animals when dividing (e.g. Ascaris megalocephalus, v. Beneden), the chromatin filaments are but little

FIG. 50.-Karyokinesis, final stages. The

place of division of the cell proto

plasm is seen. The upper nucleus still shows the remains of the spindle. The chromatic loops are now twisted

(daughter skein). The lower nucleus

is further advanced; the position of the spindle is marked by a depression

nucleus. The primary loops have

marked; the whole or nearly the whole of the granules of the cell protoplasm are arranged in a radial way round the extremities of the spindle. At each end of the spindle is a polar corpuscle, and a spherical mass of protoplasm which acts as an attraction sphere surrounds it.

Each attraction sphere consists of protoplasm arranged in two zones. The cytasters, as generally seen in karyokinesis, are probably due to a less highly developed condition of the same state of things. v. Beneden calls the polar corpuscle the central corpuscle, and he believes that it is in the central corpuscle and its surrounding attraction sphere that we must seek the cause of subdivision, not in the nucleus.

We must next proceed to examine or hilus, the polar area of the new the properties of the various chemical become connected by secondary substances found in the nucleus, many fibres, and a nucleolus has appeared (resting daughter nucleus). of which have been alluded to in the

Nuclein

Dr. Lauder Brunton 1 was the first to investigate the chemical composition of cell-nuclei. He separated the nuclei from the red corpuscles of birds, by shaking them with a mixture of ether and water; the undissolved nuclei floated at the junction of the two liquids. Brunton described the nuclei as consisting of a mucin-like substance. Plósz2, however, found on analysis that it was not inucin, as it contained a high percentage of phosphorus, an element absent from mucin. He considered it to be identical with the substance separated by Miescher3 from pus corpuscles, and termed by him nuclein. The method adopted by Miescher was to subject the pus to gastric digestion; the nuclein alone remained undissolved.

Later Miescher1 prepared a similar substance from the spermatozoa

1 Journ. of Anat. and Physiol. 2nd series, iii. 91.

2 Hoppe-Seyler's Med. Chem. Untersuchungen, Heft iv. (1871), 460.

3 Ibid.

P. 441.

4 Verhandl. der nat. Ges. Basel, vi. (1874), Heft i.

6

of different animals and from the yolk of hens' eggs, Hoppe-Seyler, Kossel, and Loew3 from yeast, Plósz1 from the liver, von Jaksch' and Geoghegan from the brain, Lubavin' from cow's milk, and WormMüller from yolk of egg. In fact, wherever nuclei are present, a substance is found which is rich in phosphorus, soluble in weak alkalis, insoluble in weak acids and in artificial gastric juice, and with the sticky character of mucin to a certain extent. A similar body is also found in the substances, like milk and yolk of egg, which form the food of the young animal. This substance is termed nuclein.

Nuclein is a compound of carbon, hydrogen, nitrogen, sulphur, phosphorus, and oxygen. Elementary analyses of nuclein from different sources yield very discordant results. The following examples may be quoted :—

The nuclein from spermatozoa differs from other nucleins in containing no sulphur. Miescher's formula for it is C29H9N9P3022From these results we must conclude, either that nuclein is not a chemical unit, but a mixture of organic phosphorus compounds with proteids or proteid-like substances (Worm-Müller), or more probably that several varieties of nuclein exist (Hoppe-Seyler).9 Miescher himself found that some nucleins were more insoluble in alkalis than others. Kossel 10 confirms Hoppe-Seyler's view of the case, for he finds that on heating yolk-nuclein and milk-nuclein with weak acids, no bases rich in nitrogen like guanine and hypoxanthine are formed, whereas such bases are obtainable from cell-nuclei. Yolk-nuclein and egg. nuclein contain iron, cell-nuclei do not. A compound of nuclein with iron, called hepatin, is also found in the liver (Zaleski),11

An intermediate product between nuclein and hypoxanthine is called adenine (C,H,N5+3H2O) by Kossel, 12 its discoverer. It crystallises in the rhombic system, forms compounds with bases, acids, and salts. On heating it with sulphuric acid, NH is replaced by O, and

2

hypoxanthine is thus formed (C2H5N5+H2O=C¿H1N ̧O+NH3). Both adenine and hypoxanthine contain a radicle C,H,N, called adenyl. (See also p. 90.)

Adenine is obtainable from both animal and vegetable tissues which are rich in cells; it cannot be obtained, or only in small quantities, from muscle. It appears that muscular fibres which have lost the morphological characteristics of cells to a great extent, have also lost some of the chemical distinctions of cells; but in those tissues the cells of which retain their original character, hypoxanthine and xanthine occur, not uncombined, but in union with other groups of atoms, especially with phosphoric acid and proteids as part of a still more complex union, nuclein. The word differentiation can thus be applied not only in a morphological, but also in a chemical sense to cells.

Artificial preparation of nuclein.-Liebermann' believes that nuclein is a compound of albumin with metaphosphoric acid. He finds that the composition and reactions of the precipitate obtained by adding this acid to a solution of 'albumin cannot be distinguished from those of nuclein. Pohl, however, has shown that although this substance resembles nuclein in its solubilities, it differs from true nuclein (i.e. the nuclein from nuclei) in the fact that substances of the uric acid group (xanthine and hypoxanthine) are not obtainable from it on decomposition.

Plastin

This substance we have seen is described as forming an outer shell to the nucleoli, and to the chromatic filaments; it is also present in the cytoplasm. Its existence rests to a large extent on micro-chemical evidence. It is very like nuclein, but is more insoluble. The fact that it is a different substance from nuclein supports the statement that has been already made as to the presence of several organic phosphorised compounds in the nucleus.

E. Zacharias thus describes the micro-chemical differences between nuclein and plastin. After artificial gastric digestion two substances in the cell remain undigested; one confined to the nucleus is characteristically bright and sharply defined. It has a special affinity for certain pigments, and is in fact chromatin or nuclein. The other substance, occurring both in the nucleus and the cytoplasm, and also in yolkspheres, appears after treatment with gastric juice ill-defined and swollen. Though swollen by acids and other reagents (solutions of

1 Berichte d. deutsch. chem. Ges. xxi. 598.

Zeit. physiol. Chem. xiii. 292.

5 Botan. Zeitung, 1887, p. 281.

sodium chloride, sodium hydrate, &c.), it is very insoluble. It is dissolved by concentrated hydrochloric acid; it is also much more insoluble in alkalis than nuclein, and is stated to withstand pancreatic digestion.

The term plastin was first applied to this substance by Reinke and Rodewald; it appears to be identical with Miescher's insoluble nuclein. Like nuclein, it contains phosphorus (Reinke). Löw3 by treating it with alkalis has separated a proteid from it. These facts all correspond with the hypothesis that plastin is a nucleo-albumin ; and there appear to be varieties of plastin, just as there are varieties of nuclein. Schwartz speaks of that in the cell protoplasm as cytoplastin, and that in chlorophyll grains as chloroplastin. He, however, uses the term plastin in a different sense from Zacharias, namely, as a name for the whole of the proteid matter of the protoplasm, and not for any special constituent of it. He regards the reticulum seen in the cell as an appearance due to the action of reagents, in fact, a kind of coagulation; in this opinion he differs from Carnoy (see p. 191).

Schwartz describes the plasmatic substratum of the chlorophyll grains as consisting of two proteids; one the chloroplastin mentioned above, which is not digestible by pepsin nor by trypsin; the other he calls metaxin. This is easily digested by both these ferments, and dissolves after swelling in very weak hydrochloric acid (1:1000).

Histon. --This was a proteid prepared from nuclei by Kossel1; it belongs to the group of proteids known as albumoses or propeptones. As Kossel extracted it from the nuclei by means of dilute hydrochloric acid, there is, however, but little doubt that it is an artificial product produced from the native proteids of the nucleus by means of the reagent employed.

FUNCTIONS OF CELLS

To the anatomist the single egg cell, or the unicellular organism, is an extremely simple object. To the physiologist on the other hand simplicity of structure means an increased difficulty in understanding function. In the higher animals certain cells are set apart specially to perform one function, certain other cells to perform another; some for instance are concerned in muscular contraction, others in elaborating secretions, others in reproduction, and so forth. But in such an animal as the amoeba all these functions-movement, secretion, digestion, excretion, and multiplication are performed by one cell. In the higher animals the various functions are unravelled from one another,

1 Unters. aus d. botan. Lab. Univ. Göttingen, 1881.
2 Ibid. 1883.

4 Zeit. physiol. Chem. viii. 511,

3 Bot. Zeit. 1884.