3o8 



SOME PROBLEMS OE CELL-ORGANIZATION 



divisions the asters disappear, yet the irregular nuclear divisions con- 

 tinue, nuclear spindles with distinct centrosomes being formed at each 

 division, but apparently without relation to the older asters, and they 



D 



■/■'■\\} 



mm^ 





Fig. 151. — Formation of centrosomes and asters in unfertilized echinoderm-eggs. [A, B, 

 MORGAN ; C-E, R. Hertwig.] 



A. Arbacia, after 4% hours in 1.5% solution of sodium chloride, then 5 hours in sea-water; 

 scattered chromosomes and asters. B. Asters formed after 6V2 hours in NaCl. C-E. Echinus 

 after treatment with 0.5 % strychnine-solution, showing various forms of astral formations (fan- 

 shaped aster, half spindle, and complete mitotic figure). 



are believed by Morgan to arise de novo from the ^gg substance.^ 

 In the meantime irregular cleavage of the ^gg occurs, though no 

 embryo is produced.^ Loeb, however, in the remarkable experiments 



1 '99, p. 479. 



- Morgan makes the important observation, which harmonizes with that of Boven, 

 reported at page 108, that the divisions occur with respect to the number and position of the 

 nuclei, not of the asters, concluding that the former must therefore play an essential rdle as 

 centres of division, and that the activity of the asters is in itself not sufficient to account 

 for division of the cytoplasm. 



