108 



Cellular Structure and Activity 



Miihldorf, '51). In animal cells this is com- 

 monly accomplished by the cleavage fvuTOw, 

 a circular groove in the cell sm-face that 

 gradually deepens and cuts the cell in two. 

 What determines the formation of this fur- 

 row and its position in the cell, and how is 

 the formation accomplished? The experi- 

 mental analysis so far indicates clearly that 

 several factors are involved and that different 

 ones predominate in different cells. 



Elongation of the Spindle. As a general 

 rule cleavage depends on the elongation of 

 the cell (mitotic elongation, cf. Churney, 

 '36), and the furrow is formed at a right 

 angle to this elongation. In most tissue cells 

 where asters are relatively small this elonga- 

 tion is dependent on the stretching of the 

 spindle. Elongation by itself does not assure 

 a cleavage furrow, but where it is sup- 

 pressed the cell does not divide. If the spindle 

 is prevented from stretching in its long axis, 

 for instance through sticking of chromo- 

 somes, it may spread out laterally and stretch 

 the cell at a right angle to the normal 

 axis. In this case a furrow appears vertically 

 to the new dii'ection of elongation and splits 

 off an anuclear bud (Ris, '49). Cell elonga- 

 tion may be accomplished independent of 

 the spindle by centrifugation. Irrespective of 

 the orientation of the cell the cleavage fur- 

 row again cuts through the narrow region 

 (Harvey, '35). 



Function of the Amphiaster. In cells with 

 large asters, such as blastomeres of many 

 eggs, mitotic elongation and cleavage may 

 occur in the absence of a spindle (Fank- 

 hauser, '34; Harvey, '36; Briggs et al., '51). 

 In such cells the growing amphiaster is 

 probably responsible for mitotic elongation 

 (Gray, '27b). The role of the aster for the 

 initiation of the cleavage furrow was stressed 

 by Dan ('48). By studying movements of 

 the cell surface with kaolin particles he 

 found that the surface gradually stretches 

 during cleavage except in the region of the 

 furrow, where it first contracts and then ex- 

 pands. He explains this by assuming that 

 aster rays are anchored in the cell cortex 

 and cross in the equatorial region. As the 

 asters move apart in anaphase, owing to 

 spindle elongation, the aster rays pull in the 

 surface of the equatorial ring, thus initiating 

 the furrow. The increased birefringence dur- 

 ing anaphase of the aster rays crossing in the 

 equator lends support to this hypothesis 

 (Inoue and Dan, '51). 



Function of the Cell Cortex. Certain 

 observations indicate that the cleavage fur- 

 row may be formed independently of both 



spindle and aster and thus suggest a defi- 

 rute autonomy of this structure, fainter ('18) 

 xound tliat in sea urchm eggs treated with 

 phenyl urethane cleavage occurs in the ab- 

 sence of asters. Harvey ('35) displaced tlie 

 amphiaster to one side by centrifugation and 

 Carlson ('52) did the same by micromanipu- 

 lation, without affecting the position of the 

 cleavage furrow. According to Marsland 

 ('51) tlie cell cortex increases in viscosity 

 before cleavage and the cleavage furrow is 

 part of the cell cortex that is particularly 

 thick and more solidified. The study of 

 plasmolysis in the sea-inxhin egg by Mon- 

 roy and Montalenti ('47) also indicates that 

 the viscosity of the cortex is low in metaphase 

 and high before cytokinesis. Like other cy- 

 toplasmic gels the cortex can be solated by 

 increased hydrostatic pressure which thus 

 inhibits cleavage or reverses it, if in prog- 

 ress, Wilson ('51), on the other hand, claims 

 that in Chaetopterus the cell cortex decreases 

 in viscosity during division. The gel nature 

 of the cleavage furrow was demonstrated 

 dramatically by Chambers ('38), who de- 

 stroyed one half of the dividing egg and 

 found the furrow remaining intact. He also 

 showed that only egg fragments containing 

 cortical material can divide. In the am- 

 phibian egg Schechtman ('37) has studied 

 the cleavage furrow in detail and concluded 

 that it originates as a localized growth of the 

 cortex toward the egg interior. His view is 

 supported by Waddington's experiments on 

 the frog egg ('52). Here the furrow can 

 grow and deepen even if it is isolated from 

 the egg interior by a cellophane strip. 



Changes in the cell cortex are also in- 

 dicated by the "bubbling" so evident in many 

 films of dividing cells. It is most pronounced 

 near the spindle poles and may be due to 

 a thinning of the cortex in that region. The 

 progress of the cleavage furrow is accom- 

 panied by actual contraction of the ring of 

 cortical gel (Lewis, '51). Such contraction 

 has also been observed in the furrow of the 

 sea urchin (Scott, '46). 



The Cleavage Substance. Cornman and 

 Cornman ('51) have suggested that as the 

 membrane dissolves a substance is released 

 from the nucleus that spreads to the cor- 

 tex and initiates the furrow. Similar ideas 

 have been expressed by others, for instance 

 Dalcq, Costello, Beams, and Conklin (for 

 references see Cornman and Cornman, '51). 

 The observation that cytasters develop only 

 after breakdown of the germinal vesicle 

 (Yatsu, '05), and the fact that asters are 

 necessary for division of anucleate cells, sug- 



