ALTERNATION OF GENERATIONS 



l8l 



mesoderm and endoderm — as inactive and ' unalterable ' acces- 

 sory idioplasm. These only become active, and cause budding, 

 when they have reached some definite part, such as the ovarv or 

 proliferating stolon. 



The development of the higher medusas or Acalephce by 

 strobilation can easily be traced from the above processes. In 

 these animals the sexual forms arise asexually : the polype 

 becomes divided into disc-shaped portions, and so comes to 

 resemble a pile of saucers, each disc eventually being trans- 



FiG. II. — Development of Medusee by strobilation — i, the young larva; 2-5, its 

 development into a polype; 7, a polype viewed from the oral pole; 6, 8, 

 and 9, transverse division of a polype into disc-shaped portions; 10, the con- 

 striction of these portions into young Medusae; 11 and 12, a young Medusa. 

 (From Hatschek's ' Lehrbuch der Zoologie.') 



formed into a medusa. If the medusa underwent division, the 

 process would be one of simple regeneration : the diflferentiation 

 of one of these discs into a medusa depends on a mechanism in 

 the idioplasm exactly similar to that which gives rise to the process 

 of regeneration in a worm the anterior end of which has been 

 cut oflf, or which has undergone spontaneous division. The 



