INTERPRETATIONS OF MEIOTIC PHASE 41 



at the onset of the next. It has been shown by Loeb l that 

 the eggs of the sea-urchin may be stimulated to develop by 

 an artificial chemical stimulus. Unfertilised eggs may also 

 be induced to develop asters (the radiations surrounding the 

 centrosomes when present) by various stimuli ; particularly 

 by chemical stimuli. 2 In the centre of these artificially 

 produced asters, central granules may appear that are in- 

 distinguishable from normal centrosomes. On the other 

 hand, these central granules may be absent. These facts 

 suggest that not only may cell division be stimulated by 

 a chemical action, but that the real centre of force may be 

 where some chemical substance is present, and that the 

 appearance of a definitely visible body, the centrosome, is 

 not a matter of primary importance. This view is held by 

 many competent observers, and is compatible with what we 

 know of cell division. In any case, the fact remains that in 

 the cells of the higher plants no centrosome appears in the 

 midst of the radiations of the asters at the poles of the 

 division figures. There is simply a space there filled with a 

 fine reticulum, and apparently structureless. These spaces 

 take the place of the centrosomes during the process of cell 

 division. 



The Meiotic Phase. The meiotic division produces two 

 daughter cells with half the number of chromosomes present 

 in the somatic cells. In animals these two cells divide again, 

 and the resulting cells are converted into mature gametes 

 without any further division. In plants a number of divisions 

 may occur, all the cells retaining the reduced number of 

 chromosomes. In the division following the meiotic, the 

 chromosomes divide longitudinally, exactly as they did in 

 the somatic divisions. 



There are two interpretations as to the manner in which 



1 Loeb, J., " On the Nature of the Process of Fertilisation and the Artificial 

 Production of Normal Larvae," American Journal of Physiology, iii. 3, 1899. 



2 Morgan, T. H., ''The Action of Salt-Solutions on the Fertilised and Un- 

 fertilised Eggs of Arbacia and other Animals," Archiv fur Entvricklunysmechanik 

 viii. 3, 1898. 



