20 COMPARATIVE ANATOMY. 



a number of cells ; that character first makes its appearance in the 

 course of further processes of differentiation. 



These processes of differentiation consist in the more or less 

 similar morphological elements (cells) which represent the organism, 

 acquiring, in larger or smaller groups, distinct characters : in their 

 being differentiated, and forming the rudiments (first stages) of 

 organs, by taking a definite order and arrangement. These organs 

 then are made up of cells, which form their tissues. We thus 

 arrive at the essence of the architecture of organisms ; we have 

 tissues, which make up organs, and are themselves composed of 

 form-elements — the cells. 



Origin of the Tissues. 



§ 17. 



The cell, then, in those organisms which we regard as animals, 

 constitutes the whole of the organism only for a time ; that is, so 

 long as it is an egg-cell. By division a multitude of cells is formed 

 out of the egg-cell, and these form the rudiments of the animal. In 

 later stages a part only of the material formed from the ovum 

 retains the primitive character of the cells ; the form and substance of 

 most of the cells are altered, and therefore their physiological 

 properties are altered ; that is, new relations are established. The 

 new cell-complexes formed from aggregates of similarly altered 

 cells, and their derivatives, are the tissues. The process which leads 

 to the formation of these tissues is essentially a differentiation. 

 This, again, affords us an example of division of labour, for each 

 differentiated aggregate of cells has to perform a definite function 

 for the organism, which function was not the duty of a definite set 

 of cells when the cells were indifferent, and indeed were performed 

 in common with all others by one cell only (the egg-cell), in the 

 earliest condition of the individual organism. 



In all cases histological differentiation commences in the proto- 

 plasm of the primitive cell ; the nucleus is less strikingly affected, 

 but numerous changes may be seen to occur in it. When the chief 

 part is played by a substance differentiated from the protoplasm, the 

 nucleus becomes of but slight importance. According to the 

 characters of their form-elemeuts the tissues are divided into several 

 large groups: these I call Epithelial tissues, tissues of the 

 Connective Substance, Muscular and Nervous tissue. The 

 first two form a lower group, which, as Vegetative tissues we may 

 distinguish from the other two, which are the Animal tissues. 

 The difference between the two groups lies in the quality of their 

 differentiation ; the products of the differentiation of the former 

 having a more passive relation to the organism, while the products 

 of the differentiation of the latter exhibit an independent activity in 

 the carrying on of the life of the organism. The vegetative group, 

 or tissues analogous to them, are, moreover, most widely distributed in 



