ATYPICAL MITOSIS AND OTHER NUCLEAR PHENOMENA 191 



here tends to be somewhat irregular; in related types it disappears in the 

 prophase, as in higher plants.'^ 



Such a range of structural conditions as that reviewed in the foregoing 

 paragraphs suggests that karyosome nuclei differ from the ordinary nuclei 

 of higher organisms chiefly in the amount of nucleolar matter present and 

 in its tendency to be associated with other elements, notably centrosomes. 

 In higher plants the nucleolar matter, which has some structural and 

 functional relationship with the chromosomes (p. 118), ordinarily dis- 

 appears before the metaphase, but, if any remains, it may sometimes 

 undergo division in the mitotic figure (p. 164). In such forms as Clado- 

 phora the latter mode of behavior occurs with some regularity. In still 



.«-' 



Fig. 112. — Mitosis 

 in Cladophora glomerata. 

 (After T'Serclaes, 1922.) 



Fig. 113. — Stages of cell-division 

 in Anaboena circinalis. {After Haupt, 

 1923.) 



other forms centrosomal elements are present also. How such conditions 

 have come about is entirely a matter of conjecture. It is not to be 

 assumed, however, that all the cytological differentiations in Protista 

 can be fully explained by reference to those in higher organisms. 



With regard to their chromosomal mechanism, it becomes increasingly 

 evident as critical researches multiply that many of the Protista are in 

 certain essential features like higher forms. Both Metcalf (1915) and 

 Kofoid (1915, 1923) have emphasized the fundamental similarity of 

 protozoan and metazoan nuclei. In some representatives of all the main 

 groups of Protozoa, elongated chromosomes, which split and show evi- 

 dence of being made up like those of Metazoa, have been found. Kofoid 

 states that the chromosomes are constant in number and differ in size 

 and shape in certain flagellates. There are genetic data which indicate 

 that, so far as their life cycles show agreement. Protozoa and Metazoa 

 exhibit similar modes of inheritance (see Jennings, 1920). In many 

 Protista, on the contrary, this high degree of nuclear differentiation seems 



28 Nemec (19106) on the variety simplicior; Carter (1919c) on Rhizoclonium. 



