May 19, 1 



NATURE 



67 



destinations, and whilst they are gradually forming into the 

 (laughter-nuclei, a curious change usually occurs at the equator 

 of the spindle in the fibres which still stretch across the 

 intervening space between the two poles. The threads become 

 more numerous and present over the area mentioned a thicken- 

 ing of their substance, and by the fusion of the swellings a 

 cell wall, dividing the original cell into two halves, may be 

 formed. And whilst this is happening, there is evidence to show 

 that the fibres themselves, which become strongly arched, are 

 in a state of compression and thus the young wall is stretched 

 > its utmost extent. The plane of equilibrium within the 

 indie depends on the shape of the cell; and thus at first, 

 id whilst still plastic, one can predict what position it will 



take up as regards the existing boundaries of the cell. Indeed 

 the resemblance of such a nascent wall to a soap film has struck 

 more than one investigator, and has been worked out in some 

 detail by Wildemann. 



The general relation of cell division to mechanical conditions 

 is well illustrated during the development of pollen cells. In 

 the monocotjledons the original pollen-mother-cell gives rise to 

 the pollen grains by two succeeding divisions with an interval of 

 rest between them. The first karyokinesis is followed by a par- 

 titioning of the cell, which is thus divided into two symmetrical 

 halves, often hemispheres. When the latter finally divide, they 

 also are symmetrically partitioned, though this, of course, can (and 

 usually does) happen by means of walls which are not similarly 

 orientated in both of the two first formed cells. In Dicotyledons, 

 on the other hand, in which also there are two successive biparti- 

 tions of the nucleus, the appearance of the cell walls is deferred 

 until the full number (four) of nuclei has been produced. And, 

 just as might have been expected, the way in which the actual 

 partitioning takes place is consequently modified. If spherical, 

 as is commonly the case, the quadrinucleated cell is simultane- 

 ously divided into four tetrahedral cells by walls converging to 

 the centre at an angle of 60°. 



It would be difficult to find an example which more strongly 

 witnesses to the influence of the form of the cell as governing 



the disposition of the walls which partition it, than is furnished 

 by the spore formation of a common liverwort, Fe^alella conica. 

 And as it also illustrates some other points touched upon in the 

 preceding pages, this paper may be fitly brought to a close by a 

 description of the more salient peculiarities attendant on the 

 process. The spore-mother-cell, which ultimately gives rise to 

 four spores, is shaped like an oval box flattened above and below. 

 When its nucleus divides (Fig. 5), the two daughter-nuclei lie 

 in the line of its major axis, and a rudimentary cell wall begins 

 to be formed at the equator of the spindle (Fig. 6), after the 

 fashion already indicated above. But, unlike most structures 



NO. 1490, VOL. 58] 



of this kind, it does not extend to the peripheral walls owing, 

 apparently, to the relatively small size of the spindle. As the 

 two daughter-nuclei pass into the resting condition, the spindle 

 fibres die away, and an interesting change comes over the 

 character of the uncompleted division-wall. It ceases to be 

 stretched out, and becomes somewhat crumpled and obviously 

 thicker, whilst its area correspondingly diminishes (Fig. 7). 



Then, after a lime, the two daughter-nuclei again divide (Fig. 8), 

 and after this division, resulting in the production of four nuclei, 

 preparations for the real partitioning of the cell l)egins. What- 

 ever position they may have previously occupied, the nuclei now 

 take up that shown in F"igs. 9 and 1 1 ; and they are apparently 

 compelled to do so by the action of the radiations, which 

 extend from each one of them into the surrounding cytoplasm. 

 Whilst they are settling down to their final positions, the 

 original cell-plate, above spoken of, is caused to rotate through 

 an angle of 90°, so that it now is parallel with, instead of at 

 right angles to, the major axis of the elliptical cell. Its nxMion 

 is clearly seen to be the result of a directive action on the part 

 of the highly developed systems of radiating fibres, and wherk 

 it has turned round it is seen to have lost its thick crumpled 

 appearance, and to have become thin and tense. As soon as it 

 has ceased to cut across the hne of protoplasm between the 

 nuclei belonging to opposite pairs, the radiations are seen to 

 arrange themselves into a spindle form, just like that formed 

 between the daughter-nuclei of each pair, a fact of considerable 

 theoretical importance in the elucidation of the genesis of spindle 

 structures generally. In the equators of these two newly 

 differentiated spindles, as in each of the two normal ones, cell- 

 plates are formed, four in all, and they become attached in pairs 

 to the ends of the primary plate, now lying longitudinally in 

 the cell (see Figs. 9 and 10), and thus the partition ol the 

 space is completed (Fig. 11). A point of special interest in 



T-"^'^ 



^^ \l 



this case of Fe^atella lies in the remarkable fact that we here 

 meet with two perfectly different conditions of cell division, 

 nnd that the transition from the one to the other can be fol- 

 lowed in every stage. Theoretical requirements are here 

 demonstrably satisfied in a manner such as we can seldom hope 

 to equal in our atteinpts to solve the many problems with which 

 cytology has to deal. J. B. Farmer. 



VASCO DA GAMA> 



WE are assembled this evening to commemorate one of the 

 greatest events in the history of the world— the discovery 

 of the ocean route to India by the Portuguese. Vasco da Gama 

 completed the mighty enterprise on the day when the ghats of 

 India were sighted from the deck of his ship just four hundred 

 years ago to-morrow. The credit of this discovery is due to the 

 Portuguese people, to their constancy and heroic perseverance, 

 even more than to the skill and ability of their leaders ; and I 

 think that many of the illustrious navigators of Portugal are 

 equal in merit, and should be equal in renown. We contem- 

 plate the perseverance of this people and the continuity of their 

 work during a century and a half of jnighty effort, rather thaT> 

 a single stroke of genius. Yet it is right that Vasco da Gama, 

 who forged the last link, should have the first place which 

 Camoens has assigned to him, prim us inter pares. 



Prince Henry gave the first impetus, and during a quarter of 

 a century he created a school of seamen who rounded Cape 

 Bojador in 1435, Cape Blanco in 1443- Cape Verde in 1445, and 



1 Addres.s delivered before the Rcyal Geogiaph ca Society, on May 16, 

 by the President. 



