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782 the same thing follows at every subsequent division of the growing embryo, so that paternal and maternal chromatin is distributed in equal amounts to every cell of the body.

The ripe germ cell consequently possesses only one-half the num- ber of chromosomes which the ordinary body cell possesses, and therefore at some time in its history a reduction of chromosomes must take place. The older view was that this occurred at one of the ripening divisions in consequence of the spireme becoming segmented into half the usual number of pieces: each of these pieces then exhibited a transverse split which was regarded as an indication of the belated appearance of the full number: at the first maturation division these halves were, however, supposed to be distributed to different cells, so that each daughter cell received only half the original number of chromosomes this division was known as the " reduction division," or " meiotic division."

Since the chromosomes are usually invisible during the resting stage of the nucleus the question has been raised whether they re- tained their individuality throughout the whole growth cycle. Various considerations lead to the conclusion that their individuality is retained. In some cases where the number is very small, as in the cells of the nematode worm Ascaris, the chromosomes or at least their ends can be detected in the resting nucleus: moreover the chromosomes are not all alike, but differ in size and shape from one another, and to each paternal one there is a corresponding maternal one of similar size and shape, and it seems unlikely that, if they van- ished in the resting stage, they should reappear in exactly the same form at the subsequent mitosis. It has been surmised that this in- dividuality in form and size was an indication of a difference- in function in distributing the hereditary qualities and Boveri's ' discovery that, when an echinoderm egg was entered by two sperma- tozoa, one alone fused with the female pro-nucleus whilst the other acted as an independent nucleus, so that at the first division the egg was divided into four cells led to the same conclusion. For Boveri showed that under these circumstances an abnormal spindle was formed connecting all four daughter nuclei, and on this spindle the chromosomes were irregularly distributed: and that if the four resulting cells were separated and allowed to develop separately they developed abnormally; whereas Driesch 2 had proved that it was possible to rear any of the first four cells into which a normally fertilized egg divides into a perfect larva of diminished size. Since an unfertilized egg has also been induced by appropriate stimuli (see EMBRYOLOGY) to develop into a perfect larva, the conclusion is inevitable that one complete set of chromosomes (maternal or paternal) is essential to the normal development, and that cells receiving fewer chromosomes than these cannot grow into normal embryos; hence every kind of chromosome has its appropriate function to play in growth.

Progress since 1910: the Cytoplasm. If we now turn to the great advances in our knowledge of the cell which were made in the 15 or 20 years ending in 1921, we may direct our atten- tion first of all to the cytoplasm.

About 1899 Hardy published his first paper 3 in which he showed that the effect of the usual preservatives used in killing cells was to produce fibrous networks which had no counter- part in the living protoplasm, for exactly the same effect could be produced by the use of these same fluids on dead proteid: that in fact all colloid solutions, which he termed " sols," could be easily induced to pass into a semi-solid or " gel " phase- in which the molecules were arranged in strings. From such networks the intervening fluid could be easily forced out, but by gently heating colloid solutions a different form of " gel " was produced, from which a pressure of several atmospheres failed to force the fluid out. In the first case the fluid contents were called the continuous phase and the fibres the disperse phase but in the second case the fluid is locked up in tiny droplets in- side the semi-solid gelatine and then the fluid was the disperse phase and the gelatine the continuous phase.

This discovery led to great scepticism as to the existence in life of the various structures seen in stained protoplasm. Fresh attention was given to the study of protoplasm in the living state and a most ingenious instrument designed by Kite 4 was used with effect by Chambers 5 for this purpose. This was an

1 Th. Boveri, " Die Entwicklung dispermer Seeigeleier," Zel- lenstudien, No. 6 (Jena 1907).

2 H. Driesch, " Die isolirten Blastomeren des Echiniderkeimcs," Archiv f. Entwicklungsmechanik, vol. x. (1900).

3 W. A. Hardy, " Structure of Cell Protoplasm," Journal of Physiology, vol. xxiv. (1899).

4 G. L. Kite and R. Chambers, " Vital Staining of the Chro- mosomes and the Function and Structure of the Nucleus," Science, vol. xxxvi. (1912).

' R. Chambers, " Microdissection Studies: II. The Cell Aster," Jour. Exp. Zool., vol. xxiii. (1917).

excessively fine needle point of hard glass, bent at right angles to the glass tube from which it was drawn, fixed so that the point projected into a glass cell from the roof of which in a hang- ing drop was suspended the living cell or cells it was desired to explore. The needle could be manipulated by screws and the glass cell was mounted on the stage of a microscope. It was discovered that, generally speaking, the cytoplasm of a cell was a sol which was sometimes very thick and viscid and some- times more fluid, but that the outer layer next the cell wall was a gel, of which indeed the cell wall might be regarded as an intensification. The various inclusions contained in the central cytoplasm, such as coloured granules, oil drops, etc. could be freely pushed about by the needle. When, however, the nucleus of the cell approached mitosis, a change took place, and the astral rays were found to be strings of semi-solid material, as were also the mantle fibres of the mitotic spindle: the astral rays became connected with the peripheral gel surrounding the cell. On the other hand no centrosome could be detected in the living cytoplasm, but the sphere surrounding the centro- some was found to consist of fluid material which Chambers sup- posed to be squeezed out from the cytoplasm during the process of gclatinization of the astral rays, and from it proceeded fluid rays visible as clear streaks in the living cell which alter- nated with the astral rays. When the spindle divided, the mantle fibres passed again in the centre into the sol state, and this change propagated itself towards the poles as the two daughter cells separated from one another.

Examined in the same way, the cross-striped myonemc's characteristic of the muscle cells of arthropods and vertebrates turned out to be composed of alternate discs of gels of different consistencies 6 ; but no trace could be made out of neuro fibrillae in the living nerve fibre and the nerve cell, i.e. the body of the neuron containing the nucleus when examined in the living condition exhibited Brownian movement i.e. the granules pulsated under the impact of freely rolling molecules^a cir- cumstance which proves it to be in the sol condition. 7 On the other hand it should be recorded that Chambers 8 found that the cytoplasm of the ganglion cells from the central nerve- cord of the lobster was a very viscid substance which could be pulled out into long threads without undergoing essential change. When pulled away from the nucleus a clear empty space appeared on the side of the nucleus in the direction of the pull, which was only slowly filled by inflow of the plasma from the two sides.

It will be observed that in living protoplasm the change from the sol to the gel condition is reversible and very frequently takes place, and many of the phenomena exhibited by living cells will find their explanation in this circumstance. Dr. Gates, professor of botany in King's College, London, has described to the present writer a beautiful demonstration once shown him by Chambers. It consisted of living spermatogorria (im- mature male germ cells) from the testis of an insect ; these when stimulated by the needle could be induced to undergo mitotic division; the chromosomes could be seen like bunches of grapes of a slightly more granular consistency than the rest of the cytoplasm moving along the mantle fibres.

The Brownian movement affords a criterion of whether protoplasm is in the condition of a sol or a gel, although not an absolute one, for Chambers has shown that a sol may be so thick as to prevent this movement and yet it may be possible to move particles in it freely by means of the glass needle. Bayliss 9 has shown that the actively moving pseudopodia of amoeba show a vigorous Brownian movement, but that when

6 G. L. Kite, " Studies on the Physical Properties of Protoplasm," American Jour. Physiology, vol. xxxii., No. 2 (1913).

7 F. W. Mott, " The Bio-physics and Bio-chemistry of the Neu- rone," Brit. Med. Jour., Sept. 1912.

8 R. Chaml>ers, " Report on Results obtained from the Micro- dissection of Certain Cells," Trans. Roy. Soc. (Canada), vol. xii., Series 3, 1918.

"W. Bayliss, "The Properties of Colloidal Systems: IV. Re- versible Gelation in Living Protoplasm," Proc. Roy. Soc. (London), Series B, vol. xci. (1920).