GREAT STEPS IN EVOLUTION 85 



sponges, the " flame-cells " of the lower worms, 

 the ciliated epithelium lining our air-passages, 

 being three familiar illustrations. The white 

 blood corpuscles are obviously comparable to 

 amoebae. Passive encysted cells are illus- 

 trated in some forms of connective, skeletal 

 and fatty tissue. Thus the physiological 

 classification of the Protozoa in verified in the 

 histology of the higher animals, and in further 

 corroborated in the study of their diseases. 

 In a certain kind of " sore throat " the 

 ciliated cells of the windpipe sink into an 

 amoeboid phase, echoing a normal change in 

 the life-history of the simplest Protists. The 

 young ovum is often amoeboid, the mature 

 ovum is encysted; the typical spermatozoon 

 In flagellate, but there are some exceptional 

 amoeboid forms. Finally, the same cell- 

 cycle is not only recognizable in the repro- 

 duction of the lower plants, but is plain in the 

 higher cryptogam, and vestigial in the flower. 

 And the deep significance and historical 

 importance of the lines of differentiation indi- 

 cated by the cell-cycle become more evident 

 still when we recognize that the three phases 

 correspond to the three possibilities of rela- 

 tively preponderant anabolism, relatively 

 predominant katabolism, and a compromise 

 between these two. 



