Page:Proceedings of the Royal Society of London Vol 2.djvu/81

Rh not exceed 1 cwt. per week; but it is of very good quality, and is used extensively in Italy under the name of Sal Inglese.

From the smallness of tadpoles in this country, they have not attracted the notice of naturalists so much as their peculiarities deserve. But those of the Rana paradoxa of Surinam being of a much larger size, are fitter subjects for observation. The tadpole of this frog bears so strong a resemblance to a fish, that it is commonly sold as such for the use of the table. But as these are not to be had here in sufficient quantity for examination, the author had recourse to the common tadpole of this country.

This animal, as soon as it leaves the ovum, has ten filaments projecting from the neck on each side, which answer the purpose of gills. In the young shark, while yet in the egg, there are twenty-four similar filaments to answer the same purpose. In the common newt also is a similar apparatus, but the number is only three on each side. In each instance this structure is but temporary, and drops off when the permanent structure of lungs in the frog, and of gills in the shark, is completely formed.

During the growth of the tadpole its abdomen becomes distended, the intestine being then very capacious, and filled throughout its whole extent with a soft substance, that burns with a vegetable smell. Behind the intestine, along the posterior part of the abdomen, is accumulated a quantity of fat of a yellow colour, inclosed in long thin transparent membranous bags. During the conversion of the tadpole into the frog by development of the legs, lungs, and other organs before wanting, the whole of this fatty matter becomes absorbed, in the same manner as the yolk of the hen's egg is taken up during the progressive growth of the young chicken. So that although the egg of the frog differs from that of other animals of the same class in having no yolk, a substance corresponding to it appears to be necessary previous to the formation of bones, and other more solid parts of the perfect frog. For the production of this matter, it is observed that the tadpole is provided with a most uncommon length of intestine, which contracts to one of ordinary size as soon as the full supply of fat is generated.

The author adds the result of various chemical experiments, made by Mr. Hatchett and Mr. Brande on the spawn of the frog, from which it appears, that it is of a nature between gelatin and albumen; that it contains no concrete oil like that of the hen's egg; that the ova of the lizard and snake, and of cartilaginous fishes, have, on the contrary, yolks which do contain a concrete oil, that in its nutritive qualities corresponds to the butyraceous part of milk with which the young of viviparous animals are supported for a certain time after birth.

Rh