THE CENTROSOME 



307 



the changes of mitosis, the chromatin-network giving rise to a group 

 of chromosomes and a spindle, or more frequently a fan-shaped 

 half-spindle, arising from the achromatic substance. In some cases 

 not only a complete spindle appeared but also asters at the poles, 

 though no centrosomes were observed (Fig. 151). Morgan's experi- 

 ments along the same lines were mainly performed upon the sea- 

 urchin Arbacia, but included also the eggs of Asterias, Sipunculus, 

 and Cerebratnlus (Figs. 150, 151). In these eggs numerous asters 

 may arise in the cytoplasm, if they are allowed to lie some time in sea- 



fe^^k^^SMX^^^ 









B ' C 



Fig. 150. Formation de novo (?) of centrosomes. [A, B, MEAD; C, MORGAN.] 



A. Unfertilized egg of Chcetopterus with " secondary asters " developed a few minutes after the 



egg is placed in sea-water. B. Slightly later stage with two definitive polar asters and centrosomes. 



C. Large " sun " (transformed polar aster) containing numerous small " secondary asters " and 



centrosomes, from unfertilized egg of Cerebratulus after 22 hours in 1.5 % sodium chloride 



solution. 



water or treated by weak solutions of sodium or magnesium chloride. 

 These asters often contain deeply staining, central granules indistin- 

 guishable from the centrosomes of the normal asters ; and, what is of 

 high interest, such of them as lie near the nucleus take part in the 

 irregular nuclear division that ensues, forming centres toward which 

 the chromosomes pass. These divisions continue for some time, the 

 chromosomes being irregularly distributed through the egg, and giving 

 rise to nuclei of various sizes apparently dependent upon the number 

 of chromosomes each receives. After a variable number of such 



