PLATE 4 



EXPLANATION OF FIGURES 



Figures 26 and 27 from Arbacia. The rest from Strongylocentrotus. All 

 drawings made by means of a camera lucida. 



22 Monaster egg in later division cycle. X 2100. 



23 Monaster egg showing multiple asters. X 1600. 



24 Spindle in secotod division cycle of monaster egg. X 1900. 



25 Monaster egg showing chromosome vesicles. X2100. 



26 Arbacia egg treated with a narcotic showing pronuclei. X 2100. 



27 Arbacia egg treated with a narcotic showing centrosome. X 2100. 



494 



