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 



