The ova of the frog in some of their properties nearly resemble those of fish. They appear to be composed chiefly of an albuminous matter and oil-the former coagulable by the admission of water, and, after coagulation, redissoluble by admixture with common salt, or with any other of the neutral salts; and further, like the ova of fish, not coagulable at a temperature of 180° or even of 212° Fahr., if not immersed in water. Their loss of vitality is denoted by their becoming of an opaque white; thus too resembling the same ova, and from the same cause, the imbibition of water.

From the observations made by Mr Higginbottom-these in opposition to those of Dr Edwards-it would appear that neither the ova nor the tadpoles of the batrachians are influenced by light;* with which, the trials I have made on those of the frog agree, as they do also with his results on the latter, showing how materially they are affected in their progress by temperature, and food, and air-a low temperature and a scanty supply of food retarding their development, and vice versa. The influence of air, I may remark, is well shown by placing the spawn of the frog in a deep glass vessel full of water the ova nearest the surface will first be hatched, those lower next, and so on, many days intervening between those first and last appearing.

Mr Higginbottom found tadpoles kept in a deep cellar, the temperature of which varied from 50° to 54°, still in their larva state so late as the middle of November, or about eight months from the time of their leaving the egg; I have found some in this state so late as the 13th of February, or about eleven months from their birth. These were kept in the open air in an earthenware pan, were supplied with very little food, and with no water, except by rain. The last survivor was active when the temperature of the water was only 33°. On the 1st March it was found dead, and then its hind extremities were partially protruding. Dr Edwards, it would appear, has kept tadpoles equally long, and under the same circumstance of a very scanty supply of food.†

It is well known that the metamorphosis of the tadpole is * Philosophical Transactions for 1850, p. 401.

On the Influence of the Physical Agents on Life (English Trans.), p. 52.

attended with a diminution of bulk. The following results which I have obtained may be adduced in illustration, and for the further purpose of showing that, though there is a decrease of volume, there is a proportional increase of solid matter in the transition.

The ovum of a frog, carefully detached from the including jelly, weighed 0.1 gr.; dried over steam, it was reduced to 0.04 gr., or 4 per cent. solid matter.

A young tadpole, its branchial filaments and tail distinct, deprived of excess of moisture by blotting-paper, weighed 0-24 gr.; dried, was reduced to 0.06 gr., or 6 per cent. solid matter. It measured 0-40 inch in length, including its caudal fin, which was 0.24 inch long.

A tadpole, its hind feet formed, and protruding, its tail but little diminished, weighed 5.3 grs.; dried, was reduced to 0.4 gr., or 7.54 per cent.

A tadpole, its fore and hind feet formed, its tail but little diminished, weighed 2.8 gr. ; dried, it was reduced to 28, or 10 per cent. solid matter.

A young frog fully formed, its tail entirely absorbed, weighed 1-4 grs.; dried, it was reduced to 18 gr., or 12:35 per cent. solid matter.

This proportional increase of solid matter, I find, is associated with the formation of bone. This is well shown by the result of the incineration of the tadpole, and of the young frog. In the one instance, when the animal matter is entirely consumed, the residue appears without any regular form; in the other (however young, provided it be fully formed), a distinct skeleton is obtained. And, examined chemically, I have found it to be composed chiefly of phosphate, with a little carbonate of lime. In the residual ash obtained from the tadpole, a reddish spot was observable, owing its colour to peroxide of iron, and, it may be inferred, derived from blood in the heart. There was also seen a convoluted marking, attributable, it may be conjectured, to the intestine; and in confirmation, it may be remarked, when this portion of the ash was microscopically examined, some infusorial forms were to be seen in it, which might have constituted part of the food.

From seeing the reddish spot just alluded to, I was induced

to make trial of the blood corpuscles. They were incinerated on slides of glass fit for use under the microscope, exposed to heat in a crucible. The whole of the animal matter consumed, the minute residual ash, in a finely granular state, exhibited the forms of the corpuscles, with a partial reddish tinge just perceptible, some circular, some elliptical.

On the temperature which the tadpole and young frog are capable of bearing, I have made a few trials, the results of which are briefly the following:-A temperature gradually rising to 94° has been borne by tadpoles for a considerable time without loss of life, or sensible loss of activity. At about 96° or 97° their activity has diminished, and they have become torpid. In this torpid state they have sustained a temperature of 100°, not long continued; on its reduction they have become active. The less advanced in growth, the more tolerant they have proved of the influence of heat. Thus, when two tadpoles well advanced in size, and other two of smaller size, were kept in water of 100° for the space of thirty minutes, gradually cooling from this degree, the first were found motionless, and never recovered; the second, from a torpid state soon recovered their activity. At a temperature between 110° and 115°, even the youngest have been found to die, and that rapidly; in the instance of the larger, the abdomen has been seen to burst, and the intestines to protrude. The full-grown tadpole and the young frog are in a remarkably less degree tolerant of heat. A temperature so low as 88° has proved fatal to both.

The influence of a certain degree of heat, in rendering the tadpole and young frog torpid, brings to recollection what we read of the effect of tropical heat with drought on the alligator, which is said to remain in the passive condition just mentioned so long as the hot and dry season lasts.

I have also made trial of the action of salt-water on the tadpole. In a solution of common salt, of sp. gr. 10425, some lived about half an hour; in a very much weaker solution, viz. of sp. gr. 10130, the activity of others was not diminished; this after half an hour; after two hours they became languid; half an hour later they were found dead. In a still weaker solution, one of sp. gr. 10065, they lived about forty hours.

May not this effect of salt-water, and so dilute as that used in the two last trials, inferior to that of the sea, be considered as one of the causes tending to narrow the habitats of the species ?

2. On the Albumen of the Newly-laid Egg.

The albumen of the egg of the common fowl, newly laid, has properties differing in some particulars from those of the albumen of the stale egg. One of these, and that which is best known, is the milkiness which it exhibits when dressed for the table, provided the egg be not put into water of too high a temperature, and kept there unduly long. Another is seen in the manner of coagulating.

Though the differences alluded to may be well known, I am not aware that they have been elucidated by experiment. This I mention as a reason for entering upon the inquiry,-if I may use that term in reference to a subject which to many may appear too trivial to deserve the name.

1st, Of the Milkiness.—I may premise, that if the albumen of two eggs-one newly laid, one kept for a week or more-be compared before being boiled, each will be found to be equally transparent, commonly little or no milkiness will be appreciable in either of them, proving that the milkiness which appears in the newly laid one, after being boiled, is the effect of heat. The temperature required is about 150° Fahr. After immersion in water of this degree of heat for about two minutes, the white of the newly-laid egg will be found on examination to have very much the appearance of milk, being liquid, with little or no viscidity, and mixing with water, and imparting to it, and to a large quantity, the same quality, and for a considerable time. Seen under the microscope with a one-eighth inch object-glass, the milky albumen is found to abound in granules of great minuteness, so minute, indeed, as to be only just distinctly visible, with which are intermixed a few particles of a larger size, as if aggregates of granules.

These granules, which are the cause of the milkiness, appear to be a modification of albumen. This is inferred from not being able, by treatment with ether, to extract from the milky fluid any oily or fatty matter, and from the circum

stance that the granules disappear when acted on by a solution of potash.

2d, Of Coagulability.-The difference as to this property seen in comparing the white of the newly-laid egg with that of an egg kept some time, though only small, is pretty well marked. It is shown by the following trial. To two similar glass tubes, equal portions of the white of an egg laid the same day, October 26, which may be called No. 1, and of one laid on the 23d April, No. 2, were introduced, and were subjected to different degrees of temperature, by immersion in heated water, for the space of two minutes.

At 140° no change was seen in either.

At 150° falling to 145°, No. 1 had become in great part opaque, i.e. of milky opacity; No. 2 only at the surface, and very slightly.

At 162° falling to 159°, No. 1 showed a milky opacity throughout-had lost little of its fluidity, as shown by being very easily poured out; No. 2 was in part so coaguluted, as to admit of inversion ; a gelatinous coagulum adhered to the inner surface of the tube.

The results of another trial were the following:

At 155° falling to 145°, about half of No. 1 acquired a milky opacity-its upper portion; No. 2 had become so to less than the extent of one quarter.

At 166° falling to 160°, No. 1 had become opaque throughout, retaining its fluidity. No. 2 had also become opaque; it had lost its fluidity-at least it did not flow when the tube was inverted.

At 175° falling to 168°, No. 1 was still fluid.

At 195° falling to 185°, No. 1 was coagulated throughout; the coagulum moderately soft. These results seem to show that the white of the newly-laid egg is more readily affected by heat of a certain temperature than that of an egg exposed some time to the air, as indicated by the appearance of milkiness it exhibits, and yet that, within a certain range of temperature, the amount of coagulation or the degree of firmness is less. Many other trials which I have made have given similar results. I shall confine myself to the mention of one. A portion of the white of a newly-laid egg was poured into