JfONF/rOLOG/CAL COMPOSITION OF yilE NUCLEUS 215 



C. Morphological Composition of the Nucleus 



I. The CJironiatin 



(a) Hypothesis of the Individuality of the Chromosomes. — It 

 may now be taken as a well-established fact that the nucleus is 

 never formed dc novo, but always arises by the division of a pre- 

 existing 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, especially by Rabl and Boveri. As a result of a careful 

 study of mitosis in epithelial cells of the salamander, Rabl ('85) 

 concluded that the ehromosomes do not lose their individuality at the 

 close of division, but persist in the chromatic reticulum of the resting 

 nucleus. The reticulum arises through a transformation of the 

 chromosomes, which give off anastomizing 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 reappear in the ensuing spireme-stage in 

 nearly or quite the same position 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 de- 

 scribed the nucleus as showing a distinct polarity having a " pole " 

 corresponding with the point towards which the apices of the chro- 

 mosomes converge {i.e. towards the centrosome), and an " anti- 

 pole " (Gegenpol) at the opposite point {i.e. towards the equator 

 of the spindle) (Fig. 17). 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 by extremely strong evidence, derived especially from 

 a study of abnormal variations in the early development of Ascaris, 

 the force of which has, I think, been underestimated by the critics 

 of the hypothesis. Some of this evidence may here be briefly 

 reviewed. In some cases, through a miscarriage of the mitotic 

 mechanism, one or both of the chromosomes destined for the second 

 polar body are accidentally left in the egg. These chromosomes 

 give rise in tlie egg to a reticular nucleus, indistinguishable from 



