THE CELL. 15 



sation of which fresh organs again arise. Thus the principle of the 

 division of labour is the cause of very great variation in the organi- 

 sation, and all morphological phenomena are more or less closely 

 connected with it, and with the differentiation which is due to it. 



First Stage of the Animal Organism. 



The Cell. 

 § 13. 



Living matter appears in its simplest form as an albuminous sub- 

 stance, known as Plasma, or Protoplasm, which, by the aid of our 

 present optical instruments, seems to be homogeneous throughout. 

 This substance occurs in the form of small lumps, in which condition 

 we find the simplest organisms. In those simplest forms, where the 

 protoplasm is homogeneous, and in which only a few granules at 

 most are present as heterogeneous elements, there is no limitation of 

 the lump to the exterior by distinct enveloping structures ; but in 

 organisms of little higher grade we find an envelope produced by a 

 chemico-physical change in the most external layer of the proto- 

 plasm. Thus the protoplasm, which is endowed with all the 

 phamomena of life, and even of movement, is enclosed by a more or 

 less firm envelope, which forbids alterations in form, and is the 

 cause of a definite shape being maintained. Such structures may 

 be combined to form complex organisms, as is the case in many of 

 the lower plants. This kind of form-element, or morphological 

 unit, is known as the cy tod, and is rightly distinguished from another 

 more highly-developed form. 



In this higher form there arises in the protoplasm a sharply 

 marked- off dense structure, which is called the nucleus. It is the 

 product of the first process of differentiation of the protoplasm, which 

 no longer alone represents the living substance. In the nucleus a 

 small body, the nucleolus, generally appears. The nucleus, unlike the 

 protoplasm, is not contractile, or at any rate has not a large share of 

 contractility; but it not only takes a part in most of the vital phaeno- 

 mena of the surrounding protoplasm, but frequently gives evidence 

 of being their regulator. Such corpuscles of protoplasm as are pro- 

 vided with a nucleus are called cells (cellulee). These structures, 

 like the cytods, may form independent organisms, which are then 

 called " unicellular/" When the cells form a complex by multi- 

 plication, we have a multicellular organism. The smallest parts of 

 multicellular organisms, no longer separable into constituent pieces 

 like to one another, are cells; and are therefore the form-elements 

 of these organisms. The same remark applies to the cytods, or the 

 simpler condition. While these, however, are rarely present, we find 

 cells widely distributed in the Vegetable Kingdom, and as the sole 

 form-elements in the Animal Kingdom. 



