Cell Division 



109 



gest that nuclear material somehow en- 

 hances gelation in the aster forming material 

 and in the cortex. The whole question of 

 what is released by the germinal vesicle 

 into the cytoplasm needs further investiga- 

 tion. A particulate fraction that facilitates 

 cleavage has been demonstrated in the egg 

 of the Dendraster (Moore, '38), in the sea 

 lu-chin (Harvey, '36), and in the ascidian 

 egg (Reverberi, '40). 



These are the major factors that determine 

 cytokinesis. The development of the furrow 

 is a property of the cortical gel, influenced 

 by mitotic elongation, aster rays and per- 

 haps some nuclear substance. The site of the 

 furrow is mainly determined by the dividing 

 nucleus. If the spindle is in the center and 

 the asters of equal size, cytokinesis is equal. 

 Where the asters are of different size (Conk- 

 lin, '17) or the spindle is placed asymmetric- 

 ally in the cell, cytokinesis is unequal. What 

 the detailed mechanisms are by which the 

 cell insures that both davighter cells receive 

 a nucleus whether the spindle is in the 

 middle of the cell or near the surface to 

 one side, we hardly can surmise today. 



Differential Mitosis. The significance of 

 mitosis is often sought in the formation of 

 two equivalent cells. Yet the problem 

 whether mitosis can produce two funda- 

 mentallv different daughter cells may be of 

 equal if not greater interest to the student 

 of development. Is cell division a mechanism 

 for cellular differentiation? Many examples 

 are known where the two offspring of a 

 mitosis have an entirely different and dis- 

 tinctive fate. Often morphogenesis and dif- 

 ferentiation are associated with a specific 

 and constant number of cell divisions which 

 give rise to a determined number of cells, 

 each with its own specific fate. This is well 

 illustrated in the development of the various 

 cells of the lepidopteran wing (Henke, '47; 

 Henke and Pohley, '52). Other examples are 

 found in the determinate cleavage of an- 

 nelids and molluscs. Such cell divisions as- 

 sociated with differentiation are often called 

 differential mitoses. However only in few 

 cases has it been demonstrated that differen- 

 tiation took place at cell division and not 

 through some environmental factors after- 

 wards. A cell division should be called dif- 

 ferential only if it can be shown that it 

 leads to a qualitatii:>ely unequal distribution 

 of nuclear or cytoplasmic elements and that 

 this is responsible for the different develop- 

 ment of the daughter cells. Differential divi- 

 sion of the nucleus in somatic cells occurs 

 as an accident (e.g., nondisjunction) or a 



special adaptation (chroma tin-elimination in 

 Ascaris and some Diptera) but has been dis- 

 counted as a general mechanism of differen- 

 tiation. During meiosis the chromosomes are 

 of course segregated differentially, generally 

 in a random fashion. (For a recent discussion 

 of preferential segregation, see Rhoades, '52.) 

 Differential distribution of cytoplasmic ma- 

 terial, on the other hand, has been described 

 in a number of cases, for instance in the 

 determinative cleavage of annelids, molluscs 

 and ascidians. The segregation of cytoplasmic 

 constitvients must, however, occur before di- 

 vision, and spindle orientation has to be spe- 

 cific. Nothing is known about the mecha- 

 nisms involved. The recent emphasis on 

 autonomous cytoplasmic units (plasmagenes) 

 has revived the idea of differentiative mitosis 

 (cf. Ephrussi, '51). A decrease in the relative 

 rate of reproduction of autonomous cyto- 

 plasmic imits may result in a loss of these 

 from the cell and can thus alter the char- 

 acteristics of the cell (Lwoff and Dusi, '35; 

 Sonneborn, '46; Ephrussi, '51; Spiegelman, 

 Delorenzo and Campbell, '51). A similar 

 process has been suggested as a possible ori- 

 gin of tumors (Potter et al., '50). Where 

 identical plasmagenes or their precursors 

 exist in large numbers the random separa- 

 tion during cvtokinesis assures their dis- 

 tribution to both cells. They may reproduce 

 either in the interphase or during mitosis. If 

 a specific plasmagene occurs in small num- 

 bers its reprodiTction has to be synchronized 

 closelv with cell division in order not to get 

 lost (plastids in lower plants). 



CYTOPLASMIC CHANGES DURING 

 MITOSIS 



Marked physical and chemical changes 

 have been observed in the cytoplasm during 

 mitosis. Rhythmic changes in viscosity were 

 demonstrated in many types of cells by dif- 

 ferent techniques through the work of Heil- 

 brunn on eggs, Carlson in grasshopper neuro- 

 blasts, Zimmermann and Kostoff in plant 

 cells (references in Heilbrunn, '52a). There 

 is general agreement that the viscosity is 

 high in prophase (in eggs it increases after 

 activation: mitotic gelation of Heilbrunn). 

 It then decreases to a minimum in metaphase 

 and anaphase, to increase again before cyto- 

 kinesis. Related to the changes in viscosity 

 is the rounding off observed in elongated 

 epithelial cells and in irregularly shaped 

 cells such as fibroblasts. According to Lettre 

 ('51) this is the result of the lowered level 

 of ATP in dividing cells and can be pro- 



