246 THE president's address. 



axial thread throughout the length of the shaft. 

 X 1,780. 

 Fig. 17. Young spicule with subsidiary whorl developed between 

 the median and basal whorls, instead of, as usual, 

 between the median and apical whorls, x 1,050. 



,, 18. Stage in which the median and subsidiary whorls are 

 represented by rings of conical projections ; with 

 accessory silicoblast {a.s.). x 1,050. 



,, 19. Stage showing the appearance of the second sub- 

 sidiary whorl (s.w. 2) on the apical prolongation; 

 other lettering as before, x 1,050. 



,, 20. Fully developed or nearly fully developed spicule; 

 a.p., apical prolongation, other lettering as before. 

 X 1,050. 



21. Fully developed spicule in which the chief subsidiary 

 whorl is missing, x 1,050. 



22. Abnormal spicule with branch coming off from apical 

 , prolongation and chief subsidiary whorl imperfect. 



X 1,050. 



23. Apical whorl of a fully developed spicule, x 1,050. 



24. Median whorl of a fully developed spicule, x 1,050. 



)» 



>j 



Journ. Quekett Microscopical Club, Ser. 2, Vol. XIII., No. 80, Apnl 1917. 



