PHYSIOLOGY OF RECONSTITUTION OF PLANARIA LATA. 



143 



at level aa (Fig. 25), and at intervals the anterior ends of a certain 

 number of them — e.g., fifty — are cut off as short pieces at the level 

 bb and their head frequencies recorded after reconstitution. Omit- 



Fig. 25. Outline indicating levels of section, aa and bb, in experiment on 

 head determination. 



ting tables and graphs, the most important results obtained are as 

 follows : In the short pieces isolated at once in the usual manner 

 the chief head frequencies are : normal, 40 per cent. ; teratophthal- 

 mic, 17 per cent. ; acephalic, 30 per cent. In short pieces cut from 

 anterior ends of long pieces after five hours head frequencies are : 

 normal, 80 per cent. ; teratophthalmic, 6 per cent. ; acephalic, 2 per 

 cent. In short pieces cut in the same way after twenty-three hours 

 100 per cent, normal heads appeared. 



This experiment shows that under ordinary conditions factors 

 determining head frequency are to some extent effective during 

 the first few hours after section, for during this period the altera- 

 tion of head frequency characteristic of long pieces by great de- 

 crease in length of piece becomes progressively less. This, of 

 course, does not mean that head frequency can not be altered in 

 other ways after this time ; it merely means that conditions deter- 



