118 



Cellular Structure and Activity 



size, and the amount of DNA per nucleus in- 

 creases by a factor of two with each endomi- 

 tosis. The doubling in nuclear size is fol- 

 lowed by an increase in cytoplasm. 



Endomitosis is a major factor in the 

 growth of many differentiated tissues. Ap- 

 parently it interferes less with the function- 

 ing of differentiated cells than does a com- 

 plete mitosis. Although the number of cells 

 remains the same, it accomplishes an in- 

 crease in the functional elements of the cell 

 from chromosomes and nucleolus to the vari- 

 ous cytoplasmic elements, and therefore aug- 

 ments the functional capacity of the tissue 

 as a whole. The relationship of endomitosis 

 and polysomaty (polyploidy of tissue cells 

 in a diploid organism) to differentiation has 

 been discussed in detail by Geitler ('41) and 

 by Huskins ('47). 



In mammalian tissues polyploid cells orig- 

 inate not only through endomitosis but also 

 through fusion of spindles during mitosis of 

 binucleate cells (Beams and King, '42; Fell 

 and Hughes, '49). Excellent discussions of 

 polysomaty in mammals in connection with 

 the problem of nuclear size classes have been 

 published by Teir ('44) and Helweg-Larsen 

 ('52). Mitosis may be modified also by a 

 change in the sequence of mitotic processes. 

 An interesting example was described by 

 Berger ('38) and by Grell ('46) in the mid- 

 gut of the mosquito. In these cells several 

 cycles of chromosome reproduction occur in 

 the larva and polytene chromosomes are 

 formed. During metamorphosis these cells 

 then divide repeatedly without chromosome 

 reproduction until the chromatids produced 

 by endomitosis in the larva are divided up 

 into the newly formed cells and the diploid 

 condition is restored. Chromosome reproduc- 

 tion thus takes place in the larva, chromo- 

 some separation and cell division in the 

 pupa. 



More commonly karyokinesis is separated 

 in time from the division of the cytoplasm. 

 Repeated nuclear divisions give rise to a 

 multinucleated cell which is subdivided later 

 simultaneously into the appropriate number 

 of uninucleated cells (insect cleavage; other 

 examples in Miihldorf, '51). 



The cytological literature is rich in de- 

 scriptions of interesting modifications in the 

 behavior of mitotic organelles. The elimina- 

 tion of certain chromosomes during cleavage 

 in some dipterans, for instance, is accom- 

 plished through failure of the chromosomal 

 fibers of the eliminated chromosomes to con- 

 tract during anaphase. These chromosomes 

 are therefore left behind and are not in- 



cluded in the daughter nuclei. Thus somatic 

 cells with a reduced chromosome number are 

 formed (DuBois, '33; Reitberger, '40; White, 

 '46). 



Then there is the puzzling behavior of 

 the chromosomes in the primary spermato- 

 cytes of Sciara (Metz, '33). The chromo- 

 somes having failed to synapse in prophase, 

 all form chromosomal fibers toward the sin- 

 gle aster (monocentric mitosis). No meta- 

 phase plate is formed. At anaphase the 

 maternal chromosomes move toward the 

 active center while the paternal chromo- 

 somes move away from it, and backward 

 too, since they are still attached to the 

 center by chromosomal fibers that appear 

 to be under tension and attenuate the chro- 

 mosomes. How can we account for this 

 unorthodox chromosome movement? I think 

 it is vmnecessary to introduce any special 

 mechanisms, since it can be understood on 

 the basis of slight modifications in the 

 behavior of the known mitotic organelles. 

 First, the chromosomal fibers contract only 

 in the maternal chromosomes, pulling them 

 to the poles. Secondly, the spindle elongates 

 and carries the paternal chromosomes along 

 passively away from the single aster. The 

 presence of a spindle is indicated by the 

 distribution of the mitochondria and spindle 

 stretching is suggested by the elongation 

 of the cell precisely in the direction in 

 which the paternal chromosomes move. 

 The observation that low temperature, 

 which is known to destroy the spindle, 

 inhibits the backward movements of the 

 chromosomes and also the elongation of the 

 cell supports our interpretation. 



The most general and mo<:t important 

 modifications of mitosis are found in the 

 meiotic divisions during gametogenesis. In 

 the first division the most significant modifi- 

 cation is the pairing of homologous chromo- 

 somes during prophase. So far there is no 

 satisfactory explanation of this phenomenon. 

 Theories involving changes in timing (the 

 precocity theory of Darlington, for instance) 

 have no factual basis since Swift and Klein- 

 feld ('53) have shown that chromosome re- 

 prodvxction (DNA doubling) takes place be- 

 fore prophase as in somatic mitoses. Since 

 the association of homologous chromosomes 

 continues into metaphase, either because of 

 chiasmata or owing to a localized or general 

 persistence of the "pairing force," homolo- 

 gous kinetochores are co-oriented in meta- 

 kinesis instead of kinetochores of sister chro- 

 matids. As a result this division segregates 

 homologovis kinetochores and, depending on 



