390 



BIOLOGY: GENERAL AND MEDICAL 



gradually close, until a hollow prolate spheroid is formed. 

 This soon becomes more and more prolonged into a 

 cylinder, at one end of which tentacles and an oral 

 aperture develop, the other end remaining closed to 

 form the foot. During these transformations no food 

 is consumed, and, therefore, no growth is possible; the 

 adjustment must be accomplished through a rearrange- 





I 



V 



\J 



Fia. 144. Regeneration in Planaria. o-e, Planaria maculata: a, normal 

 worm; 6, 6 1 , regeneration of anterior half ; c, c 1 , regeneration of posterior half ; 

 d, cross-piece of worm; d 1 , d z , d 3 , d 4 , regeneration of same; e, old head; e l , e 1 e*, 

 regeneration of same; /, Planaria lugubris;/ 1 , regeneration of new head on pos- 

 terior end of same. (After Morgan.) 



ment of the structures already present. The new hydra 

 thus formed soon grows to the normal size, however, 

 after feeding again becomes possible. 



H. V. Wilson found that when simple sponges were cut 

 into bits, rubbed up and passed through bolting cloth, 

 the dissociated cells settling to the bottom of the water in 



