12 



THE STRUCTURE OF CELLS 



normal circumstances, they are absent. Morgan showed that 

 concentrated solutions of salts could induce the appearance of 

 centrospheres with radiations in the eggs of certain echinoderms, 

 and Loeb further proved that by adding magnesium chloride in 

 appropriate quantity to sea-water, the eggs of sea-urchins could be 

 brought into such a state that when replaced in normal salt water 



FIG. 6. 



Actiiwsphaerium eichornii. A and B, nuclear origin of the centrosome, which arises at one 

 end of the nucleus. C, D, further stages in the mitosis (A-D refer to the first polar mitosis). 

 E, diaster of a somatic mitosis, in which no centrosome is present. (After B. Hertwig.) 



they underwent the normal embryonic segmentation. Wilson, 

 investigating the cytology of the process, confirmed the results, 

 and ascertained that the treatment caused the formation of centro- 

 spheres which seemed to direct the cell-divisions. And R. Hertwig 

 long ago showed that at least the early stages of a parthenogenetic 

 segmentation could be similarly induced by the action of strychnine. 

 Hence there is a considerable body of evidence to show that the 

 centrosomes are structures which, though physiologically the signs 



