advocate a belief in the so-called 'germ theory of disease,' or rely upon the exclusive doctrine of a 'contagium vivum,' seem to be absolutely broken down and refuted. We may give that attention to the appearance and development of independent organisms in association with morbid processes which the importance of their presence demands, but we must regard them as concomitant products, and not at all, or except to any extremely limited extent, as causes of those local and general diseases with which they are inseparably linked."

Hydroida of the Gulf Stream.-Dr. Allman's Report on the Pourtalès Collection of Hydroids from the Gulf Stream describes a very large number of new and interesting forms, and is one of the most important contributions to their natural history that has appeared of late years. It is illustrated by thirty-four plates.

A Water-lens Microscope.-Mr. G. M. Hopkins, in the Scientific American,' describes a microscope which, it is claimed, "renders a drop of water available as a microscope lens by confining it in a cell, thus obviating the tremor of the early water microscopes, a defect which rendered them almost if not quite worthless." The cell consists of a brass tube inch long and to inch internal diameter, blackened, with a thin piece of glass cemented to the lower end, and having in one side a screw for displacing the water, to render the lens more or less convex. Several bushings may be fitted to the upper end of the cell to reduce the diameter of the drop, and thus increase the magnifying power of the lens.

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The Structure and Development of Sponges. In Siebold and Kölliker's 'Zeitschrift f. wiss. Zoologie,' vol. xxx. part 4, is an article "On the Structure of Reniera semitubulosa: a Contribution to the Anatomy of the Siliceous Sponges," by Dr. E. Keller. Mr. E. Ray Lankester, in a Biological Note' contributed by him to Nature' of July 18, gives the following summary of the article, with additional observations of his own:

The sponges are at present attracting a very large amount of attention from zoologists and are undergoing investigation in the fresh condition, so that their living soft tissues are subjected to the refined methods of modern histology. Professor Franz Eilhard Schulze, of Gratz, is foremost in this study, the way in which was led by Ernst Haeckel in his monograph of the Calcispongia. Dr. Keller, of Zürich, who has previously published on the development of certain calcareous sponges, has now given attention to Reniera semitubulosa, O. Schm., a representative of the commoner marine fibrous sponges. Schulze, by the use of silver nitrate, discovered a differentiated epithelial covering to the body surface, which was previously denied by Keller, who now admits Schulze's observation to be correct, and adds a similar observation of his own on Reniera. Keller describes the syncytium of Reniera, denies the existence of muscular cells, and recognizes certain "nutritive wander-cells" in the body-wall of the sponge. His observations on "starch-containing cells are of special importance. He was led to attach a high functional importance to the nutritive wander-cells which pass inwards from the flagellate endo

derm-cells, carrying with them assimilated matter necessary for the nutrition of the syncytium, which forms a thick wall beyond. His conception of their importance was confirmed by the discovery that many of them contain starch. Keller has made an extensive search for starch in the cell-elements of sponges, and has found it, or rather we should say has obtained the blue reaction with iodine, in cells from the following sponges :-(1) Spongilla lacustris, (2) Reniera litoralis, nov. spec., (3) Myxilla fasciculata, (4) Geodia gigas, (5) Tethya lyncurium, (6) Suberites massa, (7) Suberites flavus. The substance, whatever it may be, which gives the blue reaction, is not in a granular condition, but fluid, and in those cells in which it occurs occupies a large vacuole comparable to a fat-vacuole. Neither ordinary nor absolute alcohol, nor cold water, dissolve the contents of this vacuole. Keller could not find this starch-like substance in Halisarca nor Chondrosia, nor in any Calcispongiæ. It seems desirable in this connection to refer to the strictly granular condition in which chlorophyll appears in the case of Spongilla, the granules having the form of concavo-convex disks. In colourless (etiolated) specimens of Spongilla, the same granules are present of a little different form, and as in Neottia and other similar plants, these granules turn green (develop into chlorophyll ?) on the addition of strong sulphuric acid (see Quarterly Journal of Microscopical Science,' 1874, vol. xiv. p. 400, where I have recorded these facts, and also that of the occurrence of starch in Spongilla, though I have not yet been able to find the authority for the latter observation, which was made many years previously to Keller's investigation). With regard to the question of the formation of a gastrula in sponges, and as to the development of the endoderm of that gastrula into the endoderm of the adult sponge, and therefore the continuity of the archenteric cavity of the gastrula with the digestive cavity and canals of the sponge, Keller has some remarks to offer which do not, in point of fact, amount to very much. Like Franz Eilhard Schulze, Keller fell into a complete error in his earlier publication on the development of calcareous sponges. Haeckel, in his monograph, stated that the sponge embryo was at first a hollow one-cell-layered sac, on the inner wall of which a second cell layer formed, by delamination, whilst subsequently a mouth broke through. This was vehemently denied and ridiculed by Metchnikoff; it was also denied by Oscar Schmidt, and by F. E. Schulze, who published a beautiful set of drawings showing that after the embryo sponge had acquired some thirty or forty cells, one hemisphere of cells became granular and enlarged, and then invaginated-sunk into the other hemispherethus forming a gastrula with endoderm and archenteron by invagination. This account was at first accepted as the true one, but it was strongly insisted upon by Keller in his former memoir, that the orifice of invagination closes up, as in fact the blastopore so usually does throughout the animal kingdom, and that the young sponge is then a mouthless closed sac with two layers of cells. It was in this condition that Haeckel saw it and described the further stage in which the true mouth breaks through. There is, however, still a great difficulty about the development of the gastrula of sponges; for no one can

doubt, who will examine a common calcareous sponge, or who looks at Barrois' valuable memoir on the subject, that F. E. Schulze was—as he himself has admitted-so far misled in his account of the development of Sycandra ræphanus as to transpose two very important stages of the development. In fact, the concavo-convex stage of the embryo sponge, with one set of cells (endodermic) tucked into the narrower, clearer, longer, ciliate cells, actually precedes that in which the same cells form respectively a hemisphere of clear ciliate cells and a hemisphere of large swollen cells, not tucked into the former at all, but so arranged that a small central cavity is closed in by the two groups. How we pass from this stage to the young sponge, or even to the twocell-layered sac, is still a complete mystery. One thing, however, is obvious. Haeckel could hardly have been led to the generalization known as the gastrea theory, which, on the whole, is a truthful and productive generalization, by erroneous observation. We must, therefore, respect his positive statements of fact.

The Eyes of Insects. In a paper by Mr. B. T. Lowne, on "The Modifications of the Simple and Compound Eyes of Insects," read before the Royal Society, the author states that in his opinion the extent and curvature of the cornea and the size and curvature of the facets afford the most important indications as to the manner in which vision is accomplished. In the true compound eye, he thinks the structure indicates that J. Müller's theory of vision is the most probable; this is also Dr. Grenacher's view, and it is supported by the curvature of the cornea and the size of the corneal facets in different insects, as well as in different parts of the same eye.

The semi-compound eye introduces no new difficulty in this theory. In order to determine the effect of the long, fine, highly refractive threads of the eyes of insects upon the light, he made some experiments on the transmission of light through fine threads of glass.

He took a capillary tube of glass of an inch in thickness, about of an inch in diameter, and an inch in length, placed it upright in a small trough under the microscope and examined it with an inch objective. He found that no light passed through the lumen of the tube, but that the section of the wall of the tube appeared brightly illuminated. He next placed a few fine glass threads, drawn from a glass rod, in the interior of the tube; these were as nearly as possible the same length as the tube and measured of an inch in diameter. The upper end of each of these rods appeared as a brightly illuminated disk in the dark field; when the focus of the microscope was altered, the disk was enlarged, showing that the rays left the rod in a divergent direction; in some cases when the ends of the rods lay beyond the focus of the microscope, the disks of light exhibited grey rings, the result of interference.

When the lower ends of these rods were lenticular, or fused into a drop, or drawn into a core, the phenomena were the same, and in all cases the action of an oblique pencil, even when the obliquity was very slight, was feeble as compared with that of a pencil having the direction of the axis of the rod.

These results are such as would be predicted on theory; all the

light passing into the rod, except very oblique rays, would be totally reflected, without any change of phase in the undulations, at the surface of the glass, whilst all except the axial rays would be very much enfeebled by numerous reflexions and interference from the different lengths of the paths of the rays. In his opinion, threads of a highly refractive character immersed in a medium of a less refractive index, when less than of an inch in diameter, would destroy the effect of rays of only very small obliquity by interference.

In order to determine the effect of the pigment, he covered the exterior of some glass rods of of an inch in diameter with black varnish, and then found it impossible to transmit any rays of even the slightest obliquity through half an inch of such a rod.

From these facts he thinks it may be concluded that it is probable that the highly refractive structures may be regarded in the light of luminous points, which serve as stimuli in exciting the recipient protoplasm in which their ends are imbedded.

The focus of the facet when this is lenticular, in all the insects examined, is situated considerably deeper than the outer end of the rhabdion and below the surface of the rod cells in the microrhabdic eye, so that even for objects as close as of an inch to the cornea, we have to deal with convergent rays, and not with a focal point. This indicates some mode of nerve stimulation other than the union of homocentric pencils, in a point beneath the compound cornea in relation with the recipient elements. Considering the small size of the parts, it is quite possible that we must look to the phenomena of interference for the explanation; at least, they must play an important part in the phenomenon.

Whatever may be the manner in which vision is accomplished, the size of the corneal facets and the general curvature of the cornea render the theory of J. Müller highly probable. It is true that Claparède has expressed the reverse opinion, but he has done so on insufficient data. According to his calculation, a bee should be unable to distinguish objects of less than eight inches in diameter at a distance of twenty feet from it. This calculation is based on the idea that the acuity of vision in this insect is the same in all parts of the field of vision, and that the general surface of the common cornea is approximately a segment of a sphere. This is not the case, for the angles subtended by the adjacent facets in the centre of the cornea, which is considerably flattened, is not more than half a degree at the most; so that on J. Müller's theory, supposing each facet to give rise to only a single luminous impression, the bee should be able to distinguish objects of about two inches in diameter at a distance of twenty feet, an acuity of vision quite equal to account for all the phenomena of vision in bees.

The curvature of the cornea of a number of insects was measured, with a view to determining the angles made by the lines of vision drawn from the centre of adjacent facets. This is done in the following manner :-A magnified image of the cornea is thrown on a sheet of white paper, by means of a microscope and camera lucida, and the curve of its profile drawn; in this way the principal meridians

were found. These curves approach more or less closely to an epicycloid.

It is easy with such curves and the size of the corneal facets to determine the angles made by adjacent facets. The angles vary inversely as the radius of curvature, and, therefore, the acuity of vision varies directly as the radius of curvature when the diameter of the facets remains the same, and inversely as the diameter of the facets when these vary in size. In many insects, as Tabanus, the peripheral facets of the cornea are twice or three times the diameter of those in the centre, and the radius of curvature is very short at the extreme periphery.

In most insects the acuity of vision determined in this manner diminishes very rapidly at the periphery of the field. In the centre of the field it enables them to perceive, as distinct, objects which subtend one degree. In Eschna grandis the sharpness of vision is much greater, as the adjacent facets make an angle of only eight minutes with each other. This was the least angle measured in any insect; but there is no doubt, from the nature of the curve forming the meridians of the eye in the great dragon-flies, that a small part of the centre of the field has a much greater acuity of vision than this; in the wasp the angle subtended by the smallest visual perceptions is twice as great as in Eschna; and in the bee it is half a degree.

The size of the corneal facets varies in different insects from gooo to of an inch in diameter. Their size, except in a few insects, is dependent on the size of the insect, the largest insects having the largest, and the smallest the smallest corneal facets. From this it follows that the vision of large insects is more perfect than that of small ones, except where the curvature of the cornea is very flat. This corresponds with the manner in which the insects fly, the small Diptera flying round in small circles, whilst the larger species take long flights when disturbed or in search of food. The experiments of Müller and others have shown that the direction and length of flight of insects depend largely on their visual powers.*

*Proc. Roy. Soc., vol. xxvii. p. 261.