32 THE ELEMENTS OF STRUCTURE. 



also the notochord of Vertebrates : the middle layer of the 

 embryo (the mesoderm or mesoblast) forms all the rest, e.g., 

 skeleton, connective swathings, muscle, and (according to 

 most authorities) the vascular system. But we shall return 

 to this subject in the next chapter. 



It is important to adopt some order of description. It is 

 obviously prejudicial to the success of your work and to the 

 health of your brains, to describe an animal in any order 

 that occurs to you, to skip from, food-canal to kidney, or 

 from heart to reproductive organs. Therefore, in my de- 

 scriptions I shall follow, almost consistently, this order of 

 treatment : — mode of life, form, external appendages, skin, 

 skeleton, muscle, nervous system, sense-organs, food-canal, 

 body-cavity, vascular system, respiratory system, excretory 

 system, reproductive organs, development. 



III. Tissues. 



Zoological anatomists, of whom Cuvier may be taken as 

 type, analyse animals into their component organs, and dis- 

 cover the real resemblances or homologies between one 

 animal and another. But as early as 1801, Bichat had pub- 

 lished an Anatomie Genkrale in which he carried the analysis 

 further, showing that the organs were composed of tissues, 

 contractile, nervous; glandular, etc. In 1 838-9, Schwann and 

 Schleiden formulated the " cell theory," in which was stated 

 the result of yet deeper analysis — that all organisms have a 

 cellular structure and origin. The simplest animals (Protozoa) 

 are almost always single cells or unit masses of living matter ; 

 as such all animals begin ; and all, except the simplest, consist 

 of hundreds of these cells united into more or less homo- 

 geneous companies (tissues) which may be compacted, as 

 we have seen, into organs. If we think of the organism as 

 a great city of cells, the tissues represent streets (like some 

 of those in Leipzig) in each of which some one kind of 

 industry predominates, while subsidiary activities are also 

 retained. 



The study of the structure of tissues and cells is some- 

 times called microscopic or minute anatomy, or is dignified 

 by the special title of histology. Since Leydig gave a strong 



