I li 



NA TURE 



[June 2, 1898 



In materials not of the very finest grain {e.g. prussian blue), 

 the forms shown by the dried powder consist usually of two sets 

 of elements, coarser and finer. The former tend to develop as 

 branching stems, which are either bent and winding, forming a 

 kind of maze, or somewhat rectilinear with terminal knobs. 

 The finer material forms a feathery or fan-shaped pattern, 

 generally at the margin of the deposit, and in the intervals 

 between the coarser stems. It consists of successive curving 

 lines, or of radial streaks and furrows. The different structures 

 are combined in various patterns, of which one illustration is 

 reproduced (Fig. i). In addition, vesicular structures, cracks, 

 and joints are developed in the dried mud, some of the cracks 

 bearing even a certain resemblance to the appearance in 

 frozen gelatin described by Prof. Sollas ( Trans. Koy. Jr. Acad. , 

 1890). The mode of formation was studied by watching the 

 films in the process of drying, and by comparing different 

 examples. As the edge of the film gradually retreats, coarser 

 stems begin to form, while fine material, remaining for a time in 

 moist condition, afterwards dries as the fine pattern. 



These various forms seem to illustrate, more or less closely, 

 structures which occur in nature. Thus, dendritic deposits 

 along joint-planes, or on other surfaces in rocks, although un- 

 doubtedly they often are the effects of crystallisation, may be 

 sometimes formed by mechanical drying, or both conditions may 

 co-operate. It is possible that some of the " pseudorganic " 

 structures described in rocks, might really be the casts or 

 replacements of dried streaks. Similar principles to those 

 shown in these films may govern the formation of structures in 



Fig. I.— This shows ne.ar one edge a fine pattern with concentric lines and 

 radial furrows ; then coarser bent stems, which become smaller and 

 reduced towards the further margin, while the fine material forms 

 rather feathery tufts. Prussian blue. (Natural size.) 



the mass of a rock, as, for example, the development oi 

 Landscape Marble (as explained by Mr. B. Thompson), or the 

 growth of some agates and chalcedonic deposits' Even the 

 solidification of certain igneous rocks, or the processes of 

 secondary silicification, may be somewhat analogous. Further, 

 the similarity shown in cracks and vesicles to those in some 

 pyromerides seems to give support to the hypothesis that these 

 nodules at an early stage were often in a semi-solidified 

 condition with an external crust. 



In conclusion, the forms resulting from the processes of 

 crystallisation, which have been described in numerous papers by 

 different authors, may be shortly compared ; and some ad- 

 ditional experiments have been made, especially as to the effects 

 produced by the admixture of material in suspension (or of a 

 colloid) with the solution of a crystallising salt. Spherulitic 

 and dendritic forms, as described by Lehmann and other 

 observers, may be developed, or even an imitation of micro- 

 pegmatitic structure. Certain interesting forms of ice crystals 

 (shortly described in Nature, 1892, by Prof. Meldola, Prof. 

 Bonney, and others) are shown, in a note by Prof. Bonney, to 

 have been developed under somewhat similar conditions. 



" The Relations between the Hybrid and Parent Forms of 

 Echinoid Larvae." By H. M. Vernon, M.A., M.B. Com- 

 municated by Prof. Ray Lankester, F.R.S. 



The object of this research was to determine systematically, 

 during a period of several months' duration, the exact relation- 

 ships of structure and size existing between certain hybrid and 



NO. 1492, VOL. 58] 



parent Echinoid larval forms. The method of procedure was 

 similar to that described in a former paper {Phil. Trans. 

 , B., 1895, p. 577). 



Upon the cross .Sphccrechimis 9 -Strongylocenlrotus S , twenty- 

 two experiments were made. As a rule only 10 per cent, of the 

 ova were fertilised, and 'only i per cent, of them reached the 

 eight days pluteus stage. The hybrids were most easily 

 obtained in the summer months, few or none of the ova being 

 cross-fertilised in the winter. The hybrids obtained in May, 

 June, and July were of an almost pure SphcErechinus type, only 

 a third or less of them being of an intermediate or Strongylocen- 

 trolus type. In November, on the other hand, only about a 

 sixth were of the maternal, and five-sixths of a semi-paternal 

 type. Finally, in December and January all the hybrid larva; 

 were of this paternal type. 



On the reciprocal cross of Strongylocenirotiis 9 and Sphizr- 

 echiniis i eighteen experiments were made. During April, May, 

 and June a fair number of the ova were cross-fertilised, but no 

 plutei were obtained. In July and August some 47 per cent, of 

 the ova were fertilised, and 29 per cent, of them survived to the 

 eight days' pluteus stage. In November and December, on the 

 other hand, not only were no plutei obtained, but as a rule not a 

 single ovum was cross-fertilised. The hybrid larvae themselves 

 were of the pure Strongylocentrotiis type. 



These extraordinary variations in the capacity for cross- 

 fertilisation seem to be due to the variations in maturity which 

 the sexual products undergo with change of season. Thus in 

 July and August most of the Strongylocentrotiis individuals 

 contain but very small quantities of ripe sexual products, or 

 none at all ; and the larvae obtained may be as much as 30 per 

 cent, smaller than those obtained in the winter and spring. At 

 intermediate times of the year the larvje are of intermediate size. 

 It appears, therefore, that the Strongylocentrotiis 9 -Spfuer. 

 echinus i hybrid is only formed at the time when the Strongy- 

 locentrotiis ova have reached their minimum of maturity ; whilst 

 in the case of the reciprocal hybrid, it follows that as the 

 maturity of the Strongylocentrotiis sperm increases, it is able to 

 transform first a portion and then the whole of the hybrid larva- 

 from the Sphcerechiniis to its own type. In other words, the 

 characteristics of the hybrid offspring depend directly on the 

 relative degrees of maturity of the sexual products. 



As a result of the ten experiments made on the cross 

 Echinus 9 -Strongylocentrotus 6 , it was found that the hybrid 

 larvae were on an average about 8 per cent, larger than the pure 

 parental larval forms, and, moreover, that even more of the 

 cross-fertilised ova developed to plutei than of the directly 

 fertilised ones. In the reciprocal cross, only about i per cent, of 

 the ova reached the pluteus stage, and these plutei were about 

 13 per cent, smaller than the pure maternal larvae. 



Various crosses, in several instances reciprocal ones, were also 

 effected between Strongylocentrotus, Sphcerechinus, Echinus 

 niicrotuberculatus. Echinus acutus, Arbacia, Echinocardiuni 

 cordatum, Echinocardiuni niediterraneu/n and Dorocidaris. 



On performing cross-fertilisations with the colour varieties 

 of Sphcerechiniis, there was found to be a distinct diminution of 

 fertility. Series of experiments were made in June, July, 

 November and December, the differential fertility seeming to 

 gradually diminish with the progress of the season. Nevertheless, 

 it was always most distinctly present. There was little or no 

 infertility between the less definitely marked colour varieties of 

 Strongylocentrotus. 



April 28. — "A Compensated Interference Dilatometer. " 

 By A. E. Tutton, Assoc. R.C.S. Communicated by Captain 

 Abney, C.B., F.R.S. 



The author describes a form of Fizeau interference dilatometer 

 which he considers combines the best features of the apparatus 

 described by Benoit, and belonging to the Bureau International 

 des Poids et Mesures, in Paris, and that described by Pulfrich, 

 constructed according to the modifications introduced into the 

 method by Abbe. Moreover, besides other improvements, a 

 new principle, that of compensation for the expansion of the 

 screws of the Fizeau tripod which supports the object, is intro- 

 duced, which enhances the sensitiveness of the method so highly 

 as to render it applicable to the determination of the expansion 

 of crystals in general, including those of chemical preparations. 

 Hitherto the application of the Fizeau method has been con- 

 fined to such crystals as could be obtained large enough to 

 furnish a homogeneous block at least a centimetre thick. A 

 block only 5 mm. thick is ample for use with the author's 

 compensated dilatometer. The principle of the compensation 



