120 



C. M. CHILD. 



16 



teristic of the anterior region occurs --that is the change is due to 

 the development in this region of a new " head " physiologically 



speaking. 



III. REGULATION IN VERY SMALL PIECES. 



By the use of pieces approaching the minimal size in which 

 regulation is possible, we have a means of determining the physi- 

 ological potences of the various regions more exactly than 



by any other method. I have 

 recorded the full history of a 

 / large number of species of this 

 kind from both species of Pla- 

 uaria, though mostly from the 

 Californian. 



Worms of rather large size were 

 selected and cut into eighteen to 

 twenty pieces by transverse or 

 nearly transverse cuts. Each 

 piece was numbered and isolated 

 and observed as 'an individual. 

 The results of these experiments 

 are rather remarkable. The 

 pieces from the region imme- 

 diately posterior to the old head 

 produce a large head and a very 

 small posterior end with the 

 pharynx near the latter (Fig. 1 1) 

 or sometimes no pharynx, /. c., 

 these pieces are so exclusively 

 "anterior" physiologically speak- 

 ing that sometimes they are incap- 

 able of giving rise to the pharny- 

 geal and postpharyngeal regions 

 at all, and in any case these re- 



Fics II-IQ gions are small. The pieces be- 



hind these show a smaller head 



and larger posterior end with the pharynx approaching the middle 

 and give rise to normal animals. But a region including the old 

 pharynx and extending for a short distance anterior and posterior 



