TRANSITIONS IN MORPHOLOGICAL CHARACTERS 113 



This agrees with many observations made upon these rotifers 

 by Dr. Powers. The period before the transition is variable in 

 length. As to the cause of the mutation at this stage, it will be 

 discussed further in another portion of this paper. 



Influence of qualitative food change 



» 



Owing to a temporary decline in our Paramecium food culture 

 an effort was made, in the case of the pedigree series F, to sub- 

 stitute Oxytricha, which were at hand in large numbers. At 

 first the rotifers ignored this new food, their digestive tracts 

 remaining entirely empty. No young were produced in the gen- 

 eration thus fed for a period of three days and it was feared that 

 the race would be lost. But on the fourth day, that is, on Febru- 

 ary 19, they suddenly began to gorge themselves with the new 

 food, their stomachs expanding until they resembled white balls. 

 On February 20 a saccate individual was born in the direct line, 

 followed by individuals transitional towards the humped type. 

 The young were likewise fed on Oxytricha and they, in turn, 

 produced young of the full humped type. The size of each 

 individual as well as the total number of progeny produced 

 were both greatly increased. 



At this time the supply of Oxytricha began to run low and after 

 ten days Paramecia were again resorted to for food. The humped 

 individuals bred true for a number of generations; but their size, 

 as well as the number of young, gradually decreased until on 

 March 2, or seven days after the supply of Oxytricha ran out, 

 the saccate type reappeared. This experiment indicated the 

 possibility of qualitative changes in nutrition as a factor deter- 

 mining the sudden transition in type which we were seeking to 

 explain. 



Influence of Euglena upon Series F and G 



On March 6 a supply of Euglena viridis was obtained and it 

 was determined to try this flagellate as food, in order to determine 

 whether mutation could be again induced by a qualitative change 

 in diet. As in the Oxytricha experiments, one to two drops of 

 this culture, green with the flagellates, was added to the culture 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 15, NO. 1 



