x AUTHOR’S PREFACE. 
As soon as the microscope was applied to the investigation 
of the structure of plants, the great simplicity of their struc- 
ture, 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 forma- 
tion 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 
