6o6 



H. S. 'Jennings 



2 In a line or race which I called am, the course of events was 

 as follows : The ancestor am was a short individual, seeming to 

 lack almost completely the posterior half of the body. In the 

 first two fissions the anterior product was in each case a normal 

 individual, while the posterior product was more or less abnor- 

 mal, with a blunt irregular posterior end. In the fourth genera- 



4 



Fig. 9 A number of generations in the history of the race am, showing the shifting, transformations, 

 and gradual disappearance of the spine. The numbers at the left indicate the generations figured. The 

 spine first appeared in generation 4 and disappeared in generation 11. 



tion there were two abnormal individuals, one of which bore a 

 short spine projecting from its aboral surface, at about the middle 

 of the posterior half of the body (Fig. 9, *). 



In the fifth generation the anterior individual was normal, while 

 the posterior one bore the spine a little farther forward than in 



