19 a 



20 a 



20 b 



Plate XIII 



Living spindles in polarized light (reduction 9/9*5). 



jVb. 19 Egg of the sea-urchin Psammechinus miliaris, X 450. compensated, fertiliza- 

 tion membrane removed. (0) 56 minutes after fertilization — metaphase. (b) Same 

 egg at 62 minutes — late anaphase. 



J\fo. 20 Chick fibroblast, X 800. (o) Compensated to show spindle bright, (b) Com- 

 pensated to show spindle dark, small aster visible at left hand spindle pole. 



{Continued on opposite page.) 



