ELEMENTARY VITAL PHENOMENA 



205 



cell-division are the simplest elements that development demands ; 

 in fact, in the whole living world there is no development without 

 growth and cell-division. In all Protista that reproduce by spore- 

 formation, there occurs a development expressing itself in com- 

 plex changes of form. In this case the spores, which are totally 

 unlike the mother-cell, must pass through a series of changes of 

 form until they become like it. The development of the Protista 

 has been little studied. Nevertheless, Rhumbler ('88) has 

 followed completely and with great care that of the infusorian 

 genus Colpoda. Colpoda is a small bean-shaped infusorian, the 

 surface of whose whole body is ciliated (Fig. 84, A). In spore- 



FIG. 84. Development of Colpoda cucullus. (After Rhumbler.) 



formation the body surrounds itself with a thick envelope or 

 cyst (), within which by giving off water the body constantly 

 diminishes its volume. Finally it extrudes all undigested food- 

 particles and draws itself together into a ball ((7), which loses its 

 cilia and surrounds itself by a second smaller envelope (D). The 

 contents of this second envelope (E) break up into single spores, 

 which together with a remnant consisting of useless material 

 burst the capsule and freely wander out (F). From each spore () 

 a new individual develops by the spore transforming itself into a 

 small amoaba-like being which creeps about, takes food, grows 

 (H, J, K, L\ develops a long flagellum with which it swims (M), 

 and finally contracts into a small spherical cell (N), which covers 

 its surface with cilia (0), and by further growth gradually assumes 

 the form of a Colpoda (P, Q, R). Thus the developmental cycle is 

 completed. 



