serve that if the Indicator be placed upon the stage and accurately centred, with its guide line C, D, parallel to the front edge of the stage, and a slide be then placed upon the Indicator so that its horizontal guide line shall coincide with C, D, and the right hand vertical guide line stand at 70, (i. e. in the position which would be recorded as ',) or its left hand guide line at ; then a motion of the stage itself bearing with it the Indicator and slide, or an equal motion of the slide upon the Indicator and fixed stage will bring the same point of the slide to the centre of the field of view.

40

Therefore by attaching to the stage in any convenient manner graduations corresponding to those of the Indicator, and by having lines corresponding to and ruled upon the stage, it will only be necessary to place the slide directly on the stage at these numbers, the stage itself being set either ator of its graduations. By turning the milled heads of the screws which give the vertical and horizontal motions of the stage, the object can be brought into the field of view and recorded or found again by means of the numbers attached to the stage; while the record may be used for any other Indicator as if made in the usual manner. If the distance between the guide lines upon the slide agrees accurately with that between 40 and 70 of the Indicator, the slide when placed upon the moveable stage at either or will need no displacement for the whole series of numbers; but if this distance does not agree, the slide must be put with its left hand vertical coinciding with the left hand vertical of the stage for all numbers from 0 to 50 of the horizontal series; while from 60 to 110 of the same series the slide must be set so that its right hand vertical coincides with the right hand vertical of the stage; in each case the horizontal lines of the stage and slide being adjusted to coincide. By observing this rule the necessity of perfect accuracy in the position of the guide lines upon the slides is done away with.

There are some objections, but not insuperable ones, to the moveable stage Indicator as above described. In the first place the stage as usually made has its motion too limited to correspond to the whole range of the Indicator, and secondly the guide lines ruled upon the stage for one object-glass may not answer for other powers on account of slight inaccuracies of mounting.

The stages can doubtless be constructed to give as wide a range for motion as required, which will do away with the first mentioned objection. The second may be removed by placing an Indicator upon the upper plate of the stage when the latter stands at and adjusting it so that when well centred for the power employed the line C F shall be parallel to the front edge of the stage. The slide being then placed upon the Indicator with its guide lines at or the remaining motions may be made with

the screws in the usual manner and the numbers may be read off from the stage scales instead of the Indicator.

The above mentioned modifications are excellent for individual convenience; but for the general purposes of science, the comparable, transferable, reproducible Indicator, in its simplest form, must be preserved; and it is only in that form that it deserves the name, suggested by a friend, of the Universal Indi

cator.

As a proof of the utility and accuracy of the Indicator, and of its convenience as a means of scientific exchange, I may state that numerous mounted slides of minute recent and fossil Diatoms have been exchanged through the Post Office by Judge A. S. Johnson of Albany, and myself, and that each has found by the ordinary as well as modified forms of the Indicator all the shells however minute, fragmentary or previously unknown, which the other had recorded. Some of these objects were less than ath of an inch in diameter, and yet they were found without difficulty by means of the Indicator.

To determine whether different impressions of the Indicator when made on the same kind of paper were comparable, a set of objects was registered successively by seven different impressions made on enamelled cards some of which were arranged with the ordinary paper centre piece, and others with the central guide lines ruled upon glass. The numbers being recorded for the objects when well centred upon one of these Indicators, the slide was then transferred to each of the other Indicators and each object being brought into the field by its recorded numbers, the position was carefully adjusted so that the object should be well centred, and a record for each copy of the Indicator was thus made. On comparing the different numbers it was found that the coincidence was almost perfect, the difference never exceeding of one of the divisions of the Indicator, an amount which might be quadrupled before an object would be thrown out of the field of view of my inch objective.

The Indicator having been put to so many and such severe tests I feel no hesitation in recommending it as a means of scientific intercourse among observers, and as a means by which collections of microscopic objects may be registered, arranged, and catalogued; and an index to the whole so made that any particular specimen may be found at will either by the original observer or any one into whose hands the slides and accompanying register may at any time come.

The copy of the Indicator which accompanies this paper is not given for use with the microscope, as the kind of paper upon which it is printed is different from that used for the standard Indicator, and therefore in consequence of unequal shrinkage a slight deviation is produced. The Indicator for use with the microscope

is printed upon enamelled cards, and the different impressions have been found to agree so closely with each other as well as with the original plate that no appreciable error is perceived.

I can not close this paper without expressing my warm thanks to Judge A. S. Johnson of the New York Court of Appeals, for his cordial sympathy and aid in testing the merits of the Indicator and for some excellent suggestions as to its best form for general use. I should also express my obligations to the engraver, J. E. Gavit, Esq., of Albany, who has spared no pains in making the steel plate from which the Indicator is printed as accurate as possible.

ART. VII.-On the Composition of Eggs in the animal series; by VALENCIENNES and FRÉMY.-Part III.*

Eggs of Reptiles.

In the two preceding memoirs, we have given the results of our observations on the eggs of Fishes. We have pointed out in these eggs the presence of the proximate principles, ichthin in Rays and Sharks, ichthulin in the species of the numerous families of osseous fishes, and have directed attention to the important fact, that this principle is gradually modified in proportion as the egg approaches its maturity, and that it ends by disappearing in the egg when it is ready for fecundation, the ichthulin being then replaced by albumen.

We now continue the account of our researches on the eggs of other oviparous animals.

Eggs of Tortoises -We have examined the eggs of two species of Chelonian reptiles belonging to two distinct genera. Some were hatched by a land Tortoise from Algiers, which M. Dumèril has called Testudo Mauritanica. This female has lived for many years in France, at the house of a resident of Grandville. The second is the fresh-water Tortoise of Europe, the mud Tortoise (la bourbeuse) of Daubenton and Lacépède, which M. Dunèril calls Cistudo Europea. Although these two species of Chelonians inhabit different countries and climates, the resemblance in the constitution and composition of the liquids of their eggs is well worth remarking; but there is a further similarity which we hardly expected; it is that they have a great analogy to those of cartilaginous fishes. They are formed of a whitish gelatine, not very abundant, hardly albuminous, shut up in the cells of large transparent membranes. The yellow very rich in albumen, contains besides a considerable quantity of phosphuretted oil, and also peculiar grains constituting an entirely new principle. These eggs do not contain a material precipitated by water analogous * From the Journal de Pharmacie for August, 1854: translated by Dr. Rosengarten. For Parts I and II, see last volume, p. 38, 238.

SECOND SERIES, Vol. XX, No. 58.-July, 1855.

9

to ichthulin. On treating their yolk with water, the yellowish oil of the egg is seen swimming on the surface of the liquid, and is precipitated in little white grains. The water retains in solution albumen and the salts. The little grains can be easily purified by washing in water, alcohol and ether. By these processes a material is obtained which presents, when in great purity, a certain analogy to ichthin, but which differs by characteristics. that prevent its being confounded with the substances from the eggs of fishes. We have given to this new substance the name

of Emydin.

Emydin.-The grains of this substance are rounded or a little ovoid, and covered with little roughnesses. They are white, transparent, harder and denser than the grains of ichthin. The largest grains of emydin taken from the eggs of the Mauritanian Tortoise (Testudo Mauritanica) are 10 hundredths of a millimeter. They appear enveloped in a very thin membrane, which has the appearance of tearing when compressed under the microscope, or which shows, by its transparency, a sort of nucleus frequently of a hexagonal form. This nucleus increases gradually, its angles disappear, and it ends by becoming spherical. Other granules break as they expand or swell out. They are of all sizes, from one hundredth of a millimeter to very large dimensions. The grains of the Cistudo Europea Dum., are smaller than those of the land Tortoise, for the largest did not exceed 6 hundredths of a millimeter, and they appear in general, to be more spherical than those of the other species. Although resisting under the pestle, we have broken them, and found that they break into spherical fragments, from the circumference to the centre. Potash, well weakened, immediately dissolves emydin, while it acts quite feebly on ichthin. Acetic acid, which, as we know, dissolves ichthin with the greatest ease, simply swells the grains of emydin, but does not dissolve them. These characteristics seem to establish very clear differences between ichthin and emydin. Its grains are dissolved in boiling hydrochloric acid, without giving to the liquid any violet color; this action proves that emydiu is not the vitellin of bird's eggs.

Emydin submitted to analysis, shows the following composition:

Proportion of azote.

Solid matter, 0.313 Solid matter, 0-370 Carbon,

Per-centages.

49.4

This substance appears to us to be isomeric with ichthin. The grains of emydin leave, on burning, a residue of calcareous salts which never exceed 1 per cent. We have also established that the eggs of our Testudo Mauritanica from the same laying present remarkable differences. The yellow of certain eggs did

not contain grains of emydin; but in that case, the white, in place of being transparent and colorless, had a slightly yellowish tinge. There were visible in it, under the microscope, little grains of irregular form which seemed to have some analogy to the grains of emydin of the vitellus.

Lizard's Eggs.-According to our observations on the eggs of the Lacerta "vert piquetè," and on those of the Lacerta stirpium (Lezard des souches), their vitellus presents a certain resemblance in composition to that of the yolk of birds. We found neither grains of ichthin nor emydin.

Adder's Eggs. We have examined eggs of the Ringed adder (Conleuvre à collier), and those of the Esculapian adder. Our analyses confirm those of MM. Martin-Saint-Ange and Baudrimont. We found the vitellus of these eggs scarcely surrounded by a very thin layer of albumen. The yellow is formed of albumen and of phosphuretted fat, and it appears to throw down vitellin when washed with water.

Viper's Eggs.-The eggs of the common viper differ in several ways from those of the Adder. They contain a white, like that of the Ray, of a gelatinous consistency; it is formed by elastic hyaloid membranes, with a slightly albuminous fluid. The yellow of the Viper's egg, presents itself, like that of the adder, in the shape of a very albuminous liquid, holding suspended in it a considerable quantity of fat; when this yellow is put into contact with water, it produces a phenomenon that we have remarked in no other species of eggs. The liquid, at first quite fluid, thickens gradually, and soon becomes completely gelatinous. This change of state in the liquor is due to a body like vitellin, found at first in solution in the albumen, which becomes insoluble under the slow action of the water, and finishes by making it harden. Is this vitellin, or something different? We were not able last year to resolve this interesting question; we hope to do it, if we are fortunate enough to get a sufficient quantity of Viper's eggs. We shall also during the present season continue to study the eggs of the Sauria and the Ophidia.

The Eggs of Batrachia.-The numerous observations made upon the eggs of Batrachia have not yet established the nature of the substances which constitute them. The gelatinous matter of the eggs of the Frog is formed of a tissue of hyaloid membranes, containing water absorbed after hatching. It is modified by cooking, becoming slightly opaque, and the vitellus hardens, which proves the existence of albumen. The vitellus blackened by a black pigment peculiar to these eggs and coloring the membrane, which is excessively thin but may be recognised by its dryness, contains in its liquid, a very small quantity of fatty matter united in transparent drops, a matter which looks under the microscope like a black punctation of extreme fineness, and then an·