12 GENERAL PART. 



Tlie organism commencing as a simple cell, the egg or germ, 

 develops by a gradual process of differentiation and change of its 

 parts up to a definite point at which it has the power of reproducing 

 itself ; finally it dies, and breaks up into its elements. The greater 

 part of the substance composing organised bodies is more or less 

 semifluid and liable to osmotic action, a condition which appears 

 to be necessary both for the carrying on of chemical changes (corpora 

 non agunt nisi soluta), and for the modification of the entire form of 

 the organism ; it is not however homogeneous and uniform, but is 

 composed of solid, semifluid, and fluid parts which exist as com- 

 binations of elements of a peculiar form. Crystals do not possess 

 heterogeneous units subordinated to one another, which, like the 

 organs of living bodies, serve as instruments for the performance of 

 different functions, but are composed of molecules of similar atomic 

 constitution ; the absence of uniformity in their structure in differ- 

 ent directions (planes of cleavage) being due to the arrangement of 

 the molecules, and not to any difference in the molecules themselves. 

 Organs again prove, on examination of their finer structure, to be 



built up of different parts 

 or tissues (organs of a 

 lower order), and these 

 a " : -i?,; . %r/ ^ again are composed of the 



FIG. 1. a, young ova of a Medusa; J, mother-cells ultimate unit of cell, the 



of spermatozoa of a Vertebrate; one of them pre- ^ The cell, last of all, 

 sented amoeboid movement. 



is to be traced back to 

 the germ cell (ovum, spermoblast) (fig 1.) 



The cell by its properties stands in direct contrast to the crystal, 

 and potentially possesses the properties of the living organism. It 

 consists of a small lump of a semifluid albuminous substance (proto- 

 plasm}, containing, as a rule, a dense or vesicular structure, tlie 

 nucleus, and is frequently surrounded by a peripheral structureless 

 membrane. If the latter is not developed, the presence of life is 

 indicated by a more or less pronounced amoeboid movement, the 

 fluid protoplasm sending out and drawing in processes of a continually 

 changing form. 



In this organised fundamental structure, from which all tissues 

 and organs of animals and plants are developed, lie latent all the 

 characters of the organism. The cell is, therefore, in a certain sense 

 the first form of the organism, and indeed the simplest organism. 

 While its origin points to the pre-existence of cells of a similar kind, 

 its maintenance is rendered possible by metabolism. The cell has Its 



