AUTHOR’S PREFACE.
As soon as the microscope was applied to the investigation of the structure of plants, the great simplicity of their structure, as compared with that of animals, necessarily attracted attention. Whilst plants appeared to be composed entirely of cells, the elementary particles of animals exhibited the greatest variety, and for the most part presented nothing at all in common with cells. This, harmonised with the opinion long since current, that the growth of animals, whose tissues are furnished with vessels, differed essentially from that of vegetables. An independent vitality was ascribed to the elementary particles of vegetables growing without vessels, they were regarded to a certain extent as individuals, which composed the entire plant; whilst, on the other hand, no such a view was taken of the elementary parts of animals. An essential difference both in the mode and in the fundamental powers of growth was thus maintained.
It soon, however, appeared that animal tissues do also occur which grow without vessels; for instance, in the formation of the ovum, and the earlier stages of development of the embryo previous to the formation of the blood; and, secondly, certain tissues of the adult, the epidermis for example. With respect to the ovum, which manifested indubitable proofs of an actual vitality, all physiologists were agreed in ascribing to it a so-called plant-like growth. This resemblance to the plant had reference to a growth of the conspicuous parts of the ovum without vessels, and was in no way connected with the form and mode of growth of the elementary particles. No one, however, considered that the analogy of the ovum entitled him to infer the operation of a plant-like growth of the elementary particles in the non-vascular tissues of the matured animal; on the contrary, the opinion rather gained ground, that these tissues originated and grew by means of a secretion from the surface of the organised tissues. Such was supposed to be the case with the epithelium, the crystalline lens, &c. This
