ii4 



THE EVOLUTION OF SEX. 



infusorian has its encysted chapter, a gregarine its amoeboid stage, and 

 a rhizopod may begin as a mobile ciliated spore; for each group, while 

 decerning one phase of the cycle, retains embryonic reminiscences of 

 the others. 



A conviction that the triple division really meant much would 

 grow in our student's mind, if he passed from the Protozoa to the 

 cells which compose higher animals. There he would find active 

 ciliated cells in most of the classes — from the ciliated chambers which 

 lash the water into a sponge, to the cells lining the air-passages in 

 man; passive encysted cells would be illustrated in some forms of 

 connective, fatty, and skeletal tissue; while the white blood-corpuscles 

 would be at one recognized as amoebae. Extended observation here 

 also would show him the cells passing from one phase to another. 



Fig. 34. — The cyst of Protomyxa busting, the flagellate young stages becoming at once amoeboid, 

 eventually to unite in a composite amoeboid mass, or "plasmodium." — After Haeckel. 



His rough classification of the Protozoa would be verified in the his- 

 tology of higher animals, and would reappear in the study of their 

 diseases. He would be thus at length in a position to say that how- 

 ever these three phases were brought about, the forms characteristic 

 of them were of such wide occurrence through Nature as to justify his 

 restatement of the familiar cell-theory in terms of a larger conception, 

 that of the cell-cycle; that is to say, from the conception of the cell as 

 a unit-mass of living protoplasm, amoeboid, encysted, or ciliated, as 

 the case might be, he would come to regard these forms as the pre- 

 dominant phases of a cycle, — primeval, certainly, in the history of the 

 organic world, and largely so even in the individual cell. 



All this time, however, our student has remained a morphologist, — 

 his use of terms, like active and passive, simply expressing change of 

 place. Not on this path of structural observation alone is it possibe 



