CELLS IN DIVISION 



urethane, cleavage subsequently did occur, but only in an extremely 

 irregular fashion. In the delayed and irregular cleavages which follow 

 the inhibition of the spindle of sea-urchin eggs by podophyllin, however, 

 CoRNMAN and Cornman^^'^ conclude that ' furrow ' activity maintains a 

 relationship with chromatin and not with ' asters '. In the sand dollar 

 {Echinarachnius) , Fry,^**' compared the behaviour after activation of 

 nucleated and enucleated halves of the egg, and showed that the asters 

 divide and form a spindle only when they are in contact with chromo- 

 somes and that cleavage is always extremely irregular in an enucleated 

 half. DALcq^^" succeeded in depriving some of the achromatic figures in 

 Frog's eggs of all chromosomes by first treating the sperm with trypa- 

 flavine, which renders the male chromosomes incapable of division after 

 fertilization, and then cauterizing the animal pole of the tgg to eliminate 

 the maternal chromatin. Successive cycles of 'cineses achromosomiales' 

 could occur, but these were never followed by cleavage. In Arbacia 

 punctulata, however, Harvey^^^ succeeded in obtaining almost normal 



I CCCCGC8S8 



60 - j-^ 



^0- J j^ 



20- >^-c^ 



Figure 55. Displacement of particles on the surface 

 of the egg of the sea-urchin, Mesphilia globulus 

 to one side of the cleavage furrow as indicated, 

 expressed as percentage changes. The fertiliza- 

 tion membrane was first removed. From Dan 

 et alii^^^ {By courtesy, Protoplasma) . 



cleavages of activated 'red half eggs' which had no nucleus, and observed 

 a cleavage furrow in the normal relation to an amphiaster. Cleavage 

 of enucleate blastomeres within parthenogenetic embryos oi Artemia has 

 been described by Gross. ^*^ In European species of sea-urchins, Swann 

 (unpublished) found that only the most irregular divisions of enucleated 

 halves of eggs could be obtained. 



In normal cleavage, the mitotic spindle of late anaphase in some way 

 influences the progress of cytokinesis. If, as a slight abnormality, the 

 spindle is placed asymmetrically and lies nearest to the cell surface at 

 one point, the cleavage furrow will advance most rapidly in this direc- 

 tion. This fact was observed in the sea-urchin tgg by Teichmann.^*^* 

 It is also true of cells in tissue culture (Plate XII (i6d)), when the 

 advancing furrow bends the spindle before it. In the eggs of coelen- 

 terates, this is the normal sequence of events (Dan and Dan^^^). 



An approach to the problems of the dividing ^gg unrelated to its 

 structural features has been made by Rashevsky and his school of 

 mathematical biophysics. They postulate that the metabolism of the 



143 



