slightly curved,* perhaps about inch in diameter, and inch at thickest. They differed from the frost ferns,' in a much greater simplicity of structure, being more like little bunches of grass arranged along a stem than those exquisite and intricate fabrics of the ice-world. They reminded me sometimes of the groups of actinolite crystals in certain crystalline schists, as for instance at the St. Gothard. These actinolites, on examination under the microscope, prove to be not pure crystals, but much interspersed with granules of pre-existing minerals. It occurred to me that the probable reason for the formation of these ice structures was the impediment offered by the small grains in the mud to the growth of tiny crystals. Only here and there, where circumstances were exceptionally favourable, a crystal larger than usual would be developed, which, as it advanced, gathered to itself other crystalline molecules. Thus would be started a bunch of coarse crystals, springing from a common centre, growing more easily in the direction of the axis of the rhombohedron, just as in the case of frost ferns, but they would be simpler, coarser, and arrested by the impediments, instead of delicately flexured by the almost imperceptible inequalities of the glass. "I obtained on a later morning, strong confirmation of this view. Heavy rain fell all through the preceding day, followed by a clear night, with a ground-frost. In many places the pavements bad been washed very clean. On the clean paving stones I saw, not unfrequently, early the next morning, fairly delicate frost ferns. (though not equal to those on glass). Where the surface had been a little dirty, the forms were less intricate and coarser; in some rather dirty places the old types could be seen. Postscript.-What is written here was my impression at the time, but I have since doubted whether the crystals were not muddier' than the part around. It was not possible to examine the pavement very closely; also I noticed on a later occasion that the trampling of the film of mud (prior to freezing), which produced a kind of concentration in an irregular network of wavy lines or bands, appeared to have something to do with the formation of the flowers." This note suggests that analogous processes may be traced in schists, like those of the St. Gothard, and the mode of formation of certain crystals in schists has been described in papers already published. On this point, however, it is needless to enlarge, as we may hope to receive more results, since Professor Bonney has communicated a paper dealing with this subject to the Geological Society. * Similar forms were noticed on certain days in the winter, 1896–7. "On a Secondary Development of Biotite and of Hornblende in Crystalline Schists from the Binnenthal," by Professor T. G. Bonney, 'Quart. Journ. Geol. Soc.,' 1893, vol. 49, p. 104. Also "On some Schistose Greenstones, &c.," by the same author, ibid., p. 94. EXPLANATION OF FIGURES (PLATE 2). The results shown in figs 1-4 were obtained on glass plates, about 6 inches by 3 inches, with cover-glasses about 3 inches by 2 inches. The whole pattern is represented in figs. 1-3, but fig. 4 shows a small part only of one slide (magnified). In each experiment a drop of the mud was placed on the slide, and then covered, so that the mud spread out somewhat irregularly. In other examples similar results were obtained, after running in material or keeping up a current for a short time to carry the mud along. FIG. 1. Prussian blue in water. This was placed on the glass as a large drop, and a very small one was accidentally deposited near by, in which the pattern at a has been developed. The mud in a few minutes began to show indication of the future pattern. Very shortly the film retreated from two edges (a and b), where the mud was denser," and where it dried to a coarse maze. Afterwards the fine maze formed, and was completely developed in two or three days. FIG. 2. Chalk mass, somewhat thin; tilted so that the cloudy chalk gradually flowed down. In this pattern the rods tend to become straighter, connected by curved transverse bars. (A pattern from denser chalk in several cases, not figured, was that of a coarse maze.) FIG. 3. Prussian blue. The fine pattern near one edge (a) with concentric or wavelines, and some clearer radial furrows was first developed. Then coarser bent stems were deposited, and towards the further margin these became smaller, interrupted, and finally reduced to isolated spots, while the fine material formed rather feathery tufts (less distinct in the photograph). Dried in about two days. FIG. 4 (x 8 diameters). From a large slide of vandyke brown, which formed straight axes and a fine "wave" pattern (an indication of this is shown at one side of the figure). Part of the edge "caked" in drying, and within it, bubbles developed, often elongated transverse to the margin, and stellate cracks or series of cracks. Fine material, somewhat faintly marked in the figure, has formed roughly oval-shaped patches in the vesicles or a central streak in the cracks. Note.-Certain sharp lines, partly overlapping the pattern in figs. 3 and 4, mark the edge of a film of canada balsam, by which the cover was sealed down. FIG. 5 (× 25 diameters). Mixture of a solution of calcium sulphate and vermilion dried on an ordinary microscope slide beneath a cover-glass. The figure represents parts of two long skeleton crystals of calcium sulphate (a and b) in which a micropegmatitic structure has formed. A small part of a third crystal (c) is shown, the substance of which extends more continuously. The vermilion partly borders the edges of the large skeleton crystals, and is black and opaque in the figure. Other black patches in it are composed of aggregated crystallites of calcium sulphate with some vermilion. The fine scattered lines or radial tufts are similar crystalline needles of the salt. "The Relations between the Hybrid and Parent Forms of Echinoid Larvæ." By H. M. VERNON, M.A., M.B. Communicated by Professor RAY LANKESTER, F.R.S. Received March 29,-Read May 5, 1898. (From the Zoological Station, Naples.) * The object of this research was to determine systematically during a period of several months' duration, the exact relationship of structure and size existing between certain hybrid and parent Echinoid larval forms. Eight different species of Echinoids were worked with, but the larger number of observations were confined to three of them, viz., Strongylocentrotus lividus, Sphærechinus granularis, and Echinus microtuberculatus. The method of procedure was similar to that described in a former paper. It consisted in shaking pieces of the ovaries and testes of the various Echinoids in small jars of water, and then mixing portions of the contents. In the cross-fertilisations, precautions were of course taken to prevent any accidental direct fertilisation of the ova. After standing an hour, the now fertilised ova were transferred to large jars of water, holding, as a rule, 2 to 3 litres. Here they were allowed to develop for eight days, when the plutei formed were killed, preserved, and examined under the microscope. The structure of the hybrids, in relation to that of the parent forms, was studied; and all the larvæ, both pure and hybrid, were measured by means of a micrometer eye-piece in respect of their body length and anal arm length. Fifty larvæ were, as a rule, measured in each case. In addition, the ova were examined under the microscope twenty-four hours after fertilisation, and the numbers of blastulæ and unfertilised ova in a given volume of water counted. Again, after eight days, the number of plutei surviving was similarly estimated. Strongylocentrotus 8, twenty-two Upon the cross Sphærechinus experiments were made. As a rule only about 10 per cent. of the ova were fertilised, and only 1 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, unless the aids to fertilisation made use of by O. and R. Hertwig,† and by Born, were adopted. Thus the former observers showed that if the ova were shaken in water and kept some hours, so that their vitality became diminished, they underwent cross-fertilisation * 'Phil. Trans.,' B (1895), p. 577. 'Jenaische Zeitschrift f. Medicin,' vol. 19, p. 121 (1886). |