84 GENERAL PRINCIPLES OF ZOOLOGY. 



is found a mass of protoplasm with many nuclei (Fig. 22),. 

 which now again on its part may become many cells, if sub- 

 sequently the protoplasm divides according to the number 

 of nuclei. Multinucleated protoplasmic masses are conse- 

 quently transitional stages between the simple mononu- 

 cleated cell and a collection of several mononucleated cells, 

 and in consequence of this are sometimes regarded as the 

 equivalent of one cell, sometimes as equivalent to many 

 cells, and are called sometimes multinucleated giant-cells, 

 sometimes cell-complexes, Syncytia. In the following' 

 pages a multinucleated mass of protoplasm will be considered 

 as a single cell, for therein is given the essential feature of 

 the character of the cell, that it constitutes a vital unit, it 

 has a physiological individuality. But in this respect a 

 multinucleated mass of protoplasm behaves almost like a 

 mononucleated ; as the tissue cells and the protozoa show, 

 the plane of organization is not raised in the least by the 

 multinuclearity. A change only begins at the moment 

 when many small lumps of protoplasm are marked off from 

 one another, and thereby are formed many vital units; i.e., 

 when in place of multinuclearity a true multicellularity 

 comes about. 



Direct Nuclear Division. Before closing the consid- 

 eration of nuclear division we must briefly recognize the 

 simplification which obtains in the above-mentioned pro- 

 cesses. There are instances where the nucleus, without 

 essential changes of structure, stretches out in case of cell 

 division, and separates into two parts by an equatorial con- 

 striction (Figs. 21, 140, 145). We then speak of a direct 

 nuclear division, in distinction from the previously-consid- 

 ered process, the indirect nuclear division, or karyokinesis. 

 Between these there are many intermediate forms, thus 

 precluding the idea that these processes may differ in prin- 

 ciple. 



Probable Significance of Direct and of Indirect 

 Nuclear Division. For correctly estimating the differences 

 which occur, a valuable hint is given us by the fact that the 

 direct nuclear division obtains chiefly in the nuclei of the 



