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INTRODUCTION TO CYTOLOGY 



stance.^ In the living echinoderm egg it is found that the two asters 

 developed at the first cleavage division are regions in which the proto- 

 plasm has become decidedly more viscous, the peripheral and equatorial 

 regions at the same time showing a high degree of fluidity and active 

 streaming like that mentioned above. The growth of the two semisolid 

 masses results in a slight elongation of the egg and a cleavage furrow 

 develops in the more fluid region separating them, after which the asters 

 revert to a less viscous state. In a number of interesting experiments 

 Chambers has shown the dependence of the location of furrows on these 

 local differences in viscosity. When a dividing egg is cut in two obliquely 

 through the two asters, the pieces will continue to cleave along the 



Fig. 98. — Diagrams showing effect of bisecting cleaving echinoderm eggs. First row: 

 without disappearance of asters. Second row: with disappearance of asters. {After 

 Chambers, 1919.) 



fluid equatorial plane if the asters persist in the semisolid state; but if the 

 asters revert to the fluid condition, as often happens as the result of 

 rough handling, the cleavage furrow is obliterated, and the pieces pro- 

 duced by the cut divide symmetrically at the next cleavage (Fig. 98). 

 When the formation of a furrow at the first mitosis is prevented by 

 mechanical means, furrows develop between all four asters in the second 

 mitosis, cleaving the egg simultaneously into four blastomeres. Accord- 

 ing to Heilbrunn, "all agents which stimulate egg cells to segment cause a 

 gelatin or coagulation and ... all agents which prevent such gelation 

 prevent the division of the egg." In the plant cells mentioned in the 

 preceding section, it is probable that the furrows separate regions of 

 relatively high viscosity even though no asters in the ordinary sense are 

 present. 



7 Heilbrunn (1915, 1920a, 1921, 1925c), Chambers (1917, 1919). See Gray (1931). 



