122 P. B. SIVICKIS. 



representing the same body region — are kept together in one con- 

 tainer. The usual number of pieces in each lot is fifty, each taken, 

 of course, from a different individual of the size used in that ex- 

 periment. The pieces are allowed to undergo reconstitution and 

 examination is made and results recorded after twelve to fourteen 

 days, at which time reconstitution is so far advanced that no fur- 

 ther change in the character of the form produced will occur. 

 Since the forms produced fall into certain groups or types, as 

 described in a following section, the results can be tabulated to 

 show the frequency of each type in each lot. In this species, as 

 in P. dorotocephala, the head is the most characteristic distinguish- 

 ing feature of the different forms produced, except in very short 

 pieces. The basis of tabulation is, therefore, form and structure 

 of the anterior end, and the tables show the frequencies of the 

 different forms and are called, for convenience, head frequency 

 tables, this term having been used for similar data on P. doroto- 

 cephala. From the tabulated head frequencies graphs are con- 

 structed by a method described below, and in this way the head 

 frequencies in different regions, in pieces of different lengths, and 

 in animals of different size or physiological condition may be 

 directly compared. 



The relation of head frequency to region of body and length of 

 piece is shown by comparing pieces of different lengths and from 

 different body levels of animals of the same size, and therefore 

 approximately of the same physiological age. The relation of 

 head frequency to physiological age is determined by comparing 

 the results obtained with pieces from animals of different lengths. 

 In my experiments animals of two standard lengths have been 

 chiefly used: full-grown animals, 11-13 mm. long, physiologically 

 old, and with low rate of respiration ; and young growing animals, 

 4-6 mm. long, with much higher rate of respiration. The smaller 

 size is the smallest which can be conveniently used for such experi- 

 ments because of the difficulty of cutting pieces of equal size in 

 the smaller individuals. Some experiments have been performed 

 with sizes intermediate between these two extremes, some with 

 animals raised from eggs and some with animals raised from cut 

 pieces. 



