376 LIFE : OUTLINES OF GENERAL BIOLOGY 



which are probably the seat of the photosynthetic process {q,v.). 

 Moreover, the essential nuclear material (chromatin) may occur in 

 the external cytoplasm, as granules called chromidia; indeed, some 

 of the lowest forms of life have no definite nucleus. The Nissl bodies 

 of ner\'e-cells arc of the same type, but in some cases it is doubtful 

 whether the definite granules visible are not formed by precipitation 

 on the death of the protoplasm. 



Constant but elusive constituents of cytoplasms are the mito- 

 chondria, which are destroyed by the more usual methods of 

 fixation, and hardly visible in the living cell except with the dark- 

 ground illumination; fortunately, they can be made visible by 

 staining the living cell with a particular dye (Janus green B). They 

 are rod-like or thread-like, semi-solid, constantly in motion, and 

 their function remains quite obscure. Some have held that they are 

 not truly part of the cell, but independent organisms — bacteria 

 (Altmann, Portier, Wallin), a view which has little to commend it; 

 while others have gone so far as to ascribe to the mitochondria an 

 important part in the handing-on of the inheritance from one 

 generation to the next, although the paramount importance of the 

 chromosomes in this respect is now very widely recognised. They 

 are probably foci of particular processes of metabolism. 



Somewhat similar in its properties, but less readily demonstrable 

 in living cells, is the "internal reticular apparatus" of Golgi. In 

 vertebrates this appears as a dense network, often as large as the 

 nucleus, and usually lying in a definite position in the cell; in 

 invertebrates it is more scattered; it occurs in Protozoa and in 

 plants. In gland cells the apparatus is always found between the 

 nucleus and the lumen or passage into which the secretion of the 

 cell is poured; while in Protozoa it may form a ring round the con- 

 tractile vacuole, which is usually regarded as a means of getting 

 rid of the waste-products of the cell's chemical activity. It seems 

 very likely that the apparatus has something to do with the accumu- 

 lation of materials, useful or useless, which are shortly to be dis- 

 cliargod from the cell — secretion or excretion. But our knowledge 

 of this, as of the mitochondria, chromidia, and other constituents 

 of the cytoplasm is still all too incomplete. 



Types of Cells. — Many of the Protozoa are so highly organised 

 that it seems clear to speak of them as non-cellular rather than as 

 single cells; but the Amceba at least is so simple that it may fairly 

 be regarded as the most generalised type of cell, displaying within 

 itself the five great functional properties, of contractility, irrita- 

 bility, secretion, growth, reproduction. That is, it lias the powers 

 of movement, of responding to stimulation, of forming new sub- 

 stances from the food materials with which it is supplied, either to 

 add to its own size or for special purposes, and eventually of dividing 



