Jan. 27, 1887] 



NA TURE 



303 



nuclei have fvised, two centres appear in the egg, each 

 with radii — the required bi-polarity is established. The 

 exchanges and movements in the protoplasm are then 

 followed ; the result is that certain constituents accumu- 

 late to excess in the equator between the two radiating 

 centres, or " suns." The chief points are illustrated by 

 diagrams. The two '' suns " are the centres of the future 

 daughter-cells ; the still single nucleus lies between them 

 in a bridge of the same protoplasm as the " suns " (these 

 " suns," by the bye, are the Attractions-Kugeln of Van 

 Beneden, and the Pol-Kiigehi of others) are embedded in : 

 the more peripheral protoplasm of the cell (ovum) has 

 accumulated chiefly around the nucleus— /.«. in the equa- 

 torial plane. This equatorial protoplasm then begins to 

 cut in two the nucleus, which has assumed the " karyo- 

 l<inetic " condition. Passing over many details, we may 

 sum up the explanation shortly. The superficial shells of 

 protoplasm are assumed to put forth pseudopodia between 

 the " suns " — i.e. the author regards it as fundamentally a 

 wetting process, due to changes at the surfaces. The 

 processes are essentially of the same nature in vegetable 

 cells, though it is impossible in a short space to sum- 

 marise Berthold's discussion as to the relative importance 

 of the numerous details which occur in different cases. 

 Obviously the stumbling-block which is best worth further 

 attack is the origin of bi-polarity in a spherical mass : that 

 Berthold's suggestions do not satisfy the requirements 

 will probably occur to everyone. The explanation offered 

 to account for the complex karyokinesis cannot be re- 

 garded as fully satisfactory. At the same time some 

 advantage may accrue from the new lights in which he 

 puts the central figures of cell-division. We are here 

 only half through the book however, and must proceed, 

 confining our remarks still more closely. 



Chapter VII. treats of the cell-network of plants, and 

 the directions of cell-divisions, &c. It is in great measure 

 a criticism of Sachs's celebrated view of the structure of 

 the higher plants, and deals at some length with several 

 of his positions. Of course, Berthold assumes primarily 

 that the plant is to be regarded as chambered — cut up 

 into cells, not built up of them. Two main principles are 

 then employed, (i) The cell- divisions are, as a rule (at 

 least in growing-points, &c.), halvings — i.e. each daughter- 

 cell has the same cubic contents. This leads to a dis- 

 cussion of very many cases. Of course the shape of a 

 segment does not forthwith enable us to judge of its 

 relative contents, and difficulty occurs sometimes on this 

 account : it is impossible to summarise the remarks, and 

 especially since reference to the figures is necessary. 

 (2) The second fundamental principle is that which re- 

 gulates the position of fluid lamelte elsewhere — the prin- 

 ciple of least areas. The rule is that the new cell-wall 

 takes such a direction that its area is the smallest possible. 

 There are exceptions, e.g. cambium cells ; but at least 

 one feature appears to indicate a tendency to follow the 

 principle — cell-walls never abut in the angles of cells. 

 Sachs's law of rectangular division is comprehended as a 

 particular case of Berthold's more general law : it fails 

 where simultaneous divisions result in the formation of 

 polygonal cells— £\tf. in the embryo-sac — with walls 

 inclined at angles greater than the right-angle. 



The eighth chapter deals with the sculpturing on the 

 interior of cell-walls, and allied phenomena ; while 

 Chapter IX. (the last) is devoted to "free cell-forma- 

 tion." 



Enough has been said to show the wide scope of the 

 book, though full criticism of it will only be possible after 

 some of Berthold's test-cases have been worked over. Of 

 course, from the nature of the work, it is open to the 

 charge of being transcendental ; but at the same time it 

 must be allowed that we are getting into serious diffi- 

 culties with protoplasm, and good, bold, shaking criticism 

 is beneficial. In any case, several investigators will, no 

 doubt, have something to say to Berthold's statements, 



for there is no lack of observations, old and new, as well 

 as hypothesis, in the book we dismiss with this short 

 review. H. Marshall Ward 



ON THE EXPLOSION OF METEORITES 



WE have received from M. Him a tirage a part of a 

 communication to L'Astronoinie, in which he dis- 

 cusses the various phenomena accompanying the ex- 

 plosion of meteorites, with a view to explaining their 

 causes. 



M. Daubrde, a long time ago, pointed out how very 

 striking and difficult of explanation the noises are which 

 are often heard in connection with the passage of meteor- 

 ites, and called in question the explanation which had 

 been given of their being really due to a veritable 

 explosion. 



M. Hirn, in his paper, begins by con-idering the causes 

 which are at work in the production of the thunder which 

 accompanies electric discharges, and of this he writes as 

 follows : — " The sound, which we call thunder, is due, as 

 everybody knows, to the fact that the air traversed by an 

 electric spark, that is, a flash of lightning, is suddenly 

 raised to a very high temperature, and has its volume more- 

 over considerably increased. The column of gas thus 

 suddenly heated and expanded is sometimes several miles 

 long ; as the duration of the flash is not even a millionth 

 of a second, it follows that the noise bursts forth at once 

 from the whole column ; but for an observer in any one 

 place it commences where the lightning is at the least 

 distance. In precise terms, the beginning of the thunder- 

 clap gives us the minimum distance of the lightning ; and 

 the length of the thunder-clap gives us the length of the 

 column. It must be remarked that when a flash of light- 

 ning strikes the ground, it is not necessarily from the 

 place struck that the first noise is heard " M. Hirn then 

 gives an interesting case which proves the truth of this 

 remark. He next points out that a bullet whistles in tra- 

 versing the air, so that we can to a certain extent follow 

 its flight ; the same thing happens with a falling meteorite 

 just before striking the earth. The noise actually heard 

 has been compared to the flight of wild geese or the sound 

 produced when one tears linen : it is due to the fact that 

 the air rapidly pushed on one side in front of thj pro- 

 jectile, whether bullet or meteorite, quickly rushes back 

 to fill the gap left in the rear. 



The most rapid cannon-shots scarcely attain a velocity 

 of 600 metres a second, while meteorites penetrate the 

 air with a velocity of 40,000 or even 60,000 metres per 

 second ; and this increased velocity gives rise to pheno- 

 mena, which, although insignificant where cannon-shots- 

 are in question, become very intense and important when 

 we consider the case of the meteorite. With that velocity 

 the air is at once raised to a temperature of from 4000' to 

 6000' C. The matter on the surface of the meteorite will 

 be torn away by the violence of the gaseous friction pro- 

 duced, and will be vaporised at the sanre time by the 

 heat. This is undoubtedly the origin of the smoke which 

 meteorites leave trailing behind them. 



We have, then, precisely as in the case of lightning, a, 

 long narrow column of air, which is expanded, not so in- 

 stantaneously certainly as by lightning, but at all events 

 in an extremely short time and through a great length. 

 Under these circumstances we should have an explosicn 

 in one case as in the other : a clap of thunder followed 

 by a rolling noise more or less prolonged If a cannon- 

 ball could have imparted to it a velocity of 100,000 metres 

 per second, it would no longer whistle, it would thunder, 

 and at the same time it would produce a flash, as of 

 lightning, and would be instantly burnt up. M. Hirn 

 depends upon this line of reasoning to show that meteoric 

 thunder need not necessarily have anything to do with an 

 actual explosion. He then points out that the intensity 

 of the noise produced in every point of its trajectory 



