GENERAL HISTOLOGY. 71 



tlie separate pliases of which follow one another 

 a definite law. The plane of division is per- 

 pendicular to the long axis imiting the two 

 poles of the spindle. Ikit the interrelation of 

 cytoplasm and nucleus is by no means an unchange- 

 able and indissoluble one, for very often nuclear 

 division takes place without participation of the 

 cytoplasm. If this process be repeated several 

 times, there results a mass of protoplasm with many 

 nuclei (fig. 23), which now on its part may become 

 many cells, if subsequently the protoiilasm divides 

 according to the number of nuclei. Hence multi- "^oeh with many 

 nucleated protoplasmic masses are transitional stages nif^iei. 

 between the simple mononucleated 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 or syncytia. In the following pages a 

 multinucleated mass of protoplasm will be considered as a single 

 cell, because the essential feature of the cell is that it constitutes a 

 vital unit, it has a physiological individuality, and in this respect 

 a multinucleated mass of protoplasm behaves 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 particles of protoplasm are 

 separated from one another, and many vital units are formed, i.e.^ 

 when in place of multinuclearity a true multicellularity appears. 



II. The Tissues of the Animal Body. 



Definition of Tissue In the formation of tissues two processes 



are operative: (1) the multiplication of cells by means of division 

 into cell-complexes, and (2) the histological differentiation of cells. 

 A tissue, therefore, can be defined as a complex of differentiated 

 cells histologically similar. 



Nature of Histological Differentiation. — The histological differ- 

 entiation consists chiefly in that the cells have definite form and 

 definite position relative to the neighboring cells; in addition, 

 there almost always occurs, as a second and more important 

 feature, the histological modification of the cell. The fact has 

 already been mentioned that the cell uses its food-material, not 

 only for its own growth, for increase of its protoplasm, but also, 

 in another manner, for forming substances, protoplasmic products, 



