62 GENERAL PRINCIPLES OF ZOOLOGY 



Connected with this great functional importance of the chromosomes as the 

 bearers of characteristics are two much disputed problems, (i) The 'individual- 

 ity of the chromosomes.' This sees the persistent organization of the cell in 

 the chromosomes, which persist between two cell divisions, but are not recog- 

 nizable as such because their substance is vacuolated and distributed through 

 greater space. Of course this view does not conflict with the fact that they, 

 like all living substance, undergo a gradual renewal, in which effete parts are 

 replaced by new and there is an increase of its substance without which a repro- 

 duction of chromosomes by division would be impossible. (2) The theory of 

 the functional diversity of the chromosomes. If the chromosomes carry the 

 characteristics, it is more probable that each one does not contain the germs of 

 all the peculiarities of the organism; rather there is a division of labor by which 

 the separate peculiarities are distributed among the different chromosomes. 

 This view is supported by the fact that there is, in numerous instances, a morpho- 

 logical differentiation between them (differences in size, shape, staining qualities). 

 That in the last analysis each category of characters consists of at least two 

 lines (male and female) is shown by the fact that, as is ex- 

 plained in the section on fertilization, half of each nucleus is 

 derived from the father, half from the mother. 



Nuclear Fragmentation is to be distinguished from 

 direct division; by it the nucleus becomes broken up into a 

 few or numerous parts. Such nuclear fragmentation is not 

 rare in the Infusoria but it occurs occasionally in Metazoa 

 (giant cells of bone marrow fig. 24 osteoblasts, certain 

 stages of the genital cells). It is explained as follows. There 

 normally exists a certain size relation between nuclear mass 

 and protoplasmic mass. With greater cell activity, as with 

 Infusoria which have long been well fed, the nucleus grows 



24 Giant- at * ne ex P ense of the protoplasm until it reaches a size which 



cell with many makes further assimilation and increase impossible. This 

 nuclei. kind of animal (or cell) can return to the normal vital 



activities if the nuclear mass be reduced. This is begun by 

 the fragmentation and is completed by the resorption of the nuclear substance. 

 Many cases of nuclear fragmentation, formerly regarded as amitoses are really 

 functional conditions of actively functioning cells and have erroneously led to 

 the view that amitosis is beginning cell degeneration. 



Multinuclearity, Multicellularity. Nuclear division and cell division 

 commonly constitute a well-arranged mechanical process, the separate 

 phases of which follow one another according to a definite law. The 

 plane of division is perpendicular to the long axis uniting the two poles of 

 the spindle; usually also each phase of division of the nucleus corresponds to 

 a certain phase of the protoplasmic division. But the interrelation of 

 cytoplasm and nucleus is by no means an unchangeable 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. 24), which now may become 

 many cells, if subsequently the protoplasm divide according to the number 

 of nuclei. Hence multinucleated protoplasmic masses are transitional 

 stages between the simple mononucleated cell and a collection of several 



