2 COMPARATIVE ANATOMY chap, i 



There are very simple creatures, small masses of protoplasm, in 

 which no nucleus has been found. Should the absence of nuclei in these 

 creatures be established they would rank lower in the scale than cells. 

 To such organisms the name " Cytods " is applied, and Haeckel has 

 united them, as the simplest of all organisms, under the group of 

 Monera. 



A frequent though not necessary portion of the cell is the cell 

 integument or cell membrane, a product of secretion, serving for 

 protection or support. Such a membrane can also arise by the 

 hardening and modification of the peripheral layers of the protoplasm 

 itself. 



A single cell (a unicellular organism, an egg cell) is in itself from 

 the first capable of all those activities and functions which are contained 

 in the conception of Life. These phenomena of life, though they may 

 not as yet be physically and chemically explained, are certainly not to 

 be referred to the working of any special vital force peculiar to 

 organisms. There is also no special fundamental substance, no life 

 substance, which can be found in organisms, and with which a special 

 vital force is connected. We have to do here with the same forces 

 and the same substances that we meet with elsewhere in nature. 



The life of the cell shows itself in the simplest cases in — 



1. Motion. — Protoplasm is contractile. The finest visible portions 

 can change their relative positions. The cell can change its form and 

 its position in space. 



2. Irritability. — The cell responds to external stimuli by such 

 movements. 



3. Metabolism. — By means of its life -activity some of the cell 

 substance is used up, decomposed. What has become useless is 

 excreted (excretion). By means of the ingestion of food foreign 

 substances are introduced. These are digestible if, when assimilated 

 by chemical action, they can be changed into ingredients of protoplasm 

 (digestion, assimilation). If, owing to their chemical properties, such 

 a conversion is impossible, they are indigestible, and are expelled out 

 of the body. 



4. Growth. — By nourishment more protoplasm can be produced 

 than was formerly present. The cell in consequence increases in size — 

 it grows. 



5. Reproduction. — It may be assumed that the size of an indi- 

 vidual cell is limited. If it exceeds these limits of individual size it 

 divides into two cells (reproduction by means of division). Each 

 of the two portions has the same physical and chemical properties as 

 the mother cell (simplest form of inheritance). The daughter cell by 

 growth attains the size of the mother cell. 



As the cell is the starting point both in the animal and in the vegetable 

 kingdom,?, it can easily be understood that no sharp line of demarcation 

 between the lower forms in these two kingdoms can be established. 

 Haeckel has therefore set up an intermediate kingdom, that of the 



