294 SOME PROBLEMS OF CELL-ORGAXIZATION 



est visible granules first come into existence when they first come 

 within view of the microscope. The ** homogeneous " substance must 

 itself contain or consist of granules still smaller. The real question 

 is not whether such ultra-microscopical bodies exist, but whether they 

 are permanent organize dho^i^?. possessing besides the power of growth 

 also the power of division. This question can be only indirectly ap- 

 proached ; and we shall find it convenient to do so by beginning at 

 the opposite end of the series, through a reconsideration of the 

 phenomena of nuclear division. 



C. Morphological Composition of the Nucleus 



I. TJie Chromatin 



(a) HypotJiesis of the Individuality of the CJiromosomes. — It may 

 now be taken as a well-estabhshed fact that the nucleus is never 

 formed de novo, but always arises by the division of a preexisting 

 nucleus. In the typical mode of division by mitosis the chromatic 

 substance is resolved into a group of chromosomes, always the same 

 in form and number in a given species of cell, and having the power 

 of assimilation, growth, and division, as if they were morphological 

 individuals of a lower order than the nucleus. That they are such 

 individuals or units has been maintained as a definite hypothesis, es- 

 pecially by Rabl and Boveri. As a result of careful study of mitosis 

 in epithelial cells of the salamander, Rabl ('85) concluded that the 

 cJironiosomes do not lose their individuality at the close of division, but 

 persist in the chromatic reticnhim of the resting nnclens. The reticu- 

 lum arises through a transformation of the chromosomes, which give 

 off anastomosing branches, and thus give rise to the appearance of a 

 network. Their loss of identity is, however, only apparent. They 

 come into view again at the ensuing division, at the beginning of 

 which "the chromatic substance flows back, through predetermined 

 paths, into the primary chromosome-bodies " (Kernfaden), which re- 

 appear in the ensuing spireme-stage in nearly or quite the same posi- 

 tion they occupied before. Even in the resting nucleus, Rabl believed 

 that he could discover traces of the chromosomes in the configuration 

 of the network, and he described the nucleus as showing a distinct 

 polarity having a ''pole" corresponding with the point toward which 

 the apices of the chromosomes converge {i.e. toward the centrosome), 

 and an '' antipole" (Gegenpol) at the opposite point {i.e. toward the 

 equator of the spindle) (Fig. 22). Rabl's hypothesis was precisely 

 formulated and ardently advocated by Boveri in 1887 and 1888, and 

 again in 1891, on the ground of his own studies and those of Van 

 Beneden on the early stages of Ascaris. The hypothesis was supported 



