4 EVOLUTION 



are obviously comparable to amoebae. Pas- 

 sive encysted cells are illustrated in some 

 forms of connective, skeletal and fatty tissue. 

 Thus the physiological classification of the 

 Protozoa is verified in the histology of the 

 higher animals, and is 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, echo- 

 ing 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 is flagellate, but there 

 are some exceptional amoeboid forms. Fi- 

 nally, the same cell-cycle is not only recog- 

 nizable in the reproduction 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 indicated by the 

 cell-cycle become more evident still when we 

 recognize that the three phases correspond 

 to the three possibilities of relatively pre- 

 ponderant anabolism, relatively predom- 

 inant katabolism, and a compromise between 

 these two. 



THE BEGINNING OF A BODY. The simplest 

 organisms are single cells physiologically 

 complete in themselves; they leave off where 

 higher creatures begin, that is to say, in a 



