LIVING SUBSTANCE 



71 



On the other hand, in the large majority of cases it remains 

 continually, even during the resting-stage of the cell, outside the 

 nucleus. Hence, as Heidenhain ('94) and Boveri ('95) remark, there 

 is ground for considering the centrosome a constituent neither of 

 the nucleus nor of the protoplasm ; it must rather be regarded 

 as an independent constituent of the cell. Since, however, many 

 forms of cells, especially among unicellular organisms, are known 

 in which thus far it has not been possible to discover a centrosome, 

 it is not justifiable at present to regard it as a general cell-con- 

 stituent. Among the vital phenomena of the cell, it is known thus 

 far to share only in reproduction and fertilisation. 



In accordance with these considerations the protoplasm in its 

 entirety and the nucleus with its differentiations can be contrasted 

 as the sole general cell-constituent, in distinction from all special 

 constituents, such as the cell-membrane, starch-grains, pigment- 

 grains, oil-droplets, chlorophyll-bodies, centrosomes, etc. 



3. Multinucleate Cells and Syncytia 



Five stages of individuality have been distinguished sharply 

 from one another in organic nature; it must be remembered, 

 however, that no sharp limits are to be found in the living world. 

 Cells were distinguished as elementary organisms from the next 

 higher stage of individuality, tissues ; and it might seem as if no 

 sharper boundary exists than that between the single cell and the 

 tissue, which consists of a number of similar cells, and as if it 

 would be very easy to distinguish the two stages of individuality 

 from one another. In 

 reality this is not so. 

 There are individual or- 

 ganisms in which a distinc- 

 tion, as to whether they are 

 elementary organisms or 

 tissues, does not readily 

 appear; and such cases, 

 like many others in which 

 boundaries are to be estab- 

 lished in nature, show that 

 the fixing of sharp limits 

 and definitions must con- 

 tain finally a more or less 

 arbitrary element, that, in- 

 deed, all limits and definitions are only psychological helps toward 

 knowledge. 



The transition-forms between typical cells and genuine tissues 

 are numerous. They consist of a unitary protoplasmic mass con- 

 taining more than the one nucleus that is characteristic of the 

 cell-type. Double-nucleated cells are found in many tissues, such 



FIG. 12. Cartilage ; the cells contain two nuclei. 



