386 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



of this kind may show dupUcation of eyespots and of prepharyngeal, as 

 well as of postpharyngeal, regions. One case observed showed duplication 

 through sixteen successive fissions (Miller). 



A point of considerable interest is that the induced reversal of polarity 

 in the host is a gradual progressive process, extending over several weeks. 



A 



D 



Fig. 131, A-D. — Heteroplastic planarian ganglionic grafts in posterior zooid region. A, B, 

 Dugesia tigrina novangliae to D. dorotocephala; C, D, D. tigrina to D. dorotocephala. A, DDAP, 

 union complete, 21 days; outgrowth, reorganization of digestive tract and development of 

 pharynx posterior and anterior to graft; after section at .YA' tail instead of head developed on 

 piece containing graft. B, DDAP, union complete, 14 days; induction of reorganization and 

 pharynx development posterior and anterior to graft. C, DDAA, 133 days; induced reorgani- 

 zation and pharynx development posterior and anterior to graft. After fission at A , originally 

 anterior to the graft, a tail instead of a head reconstituted, and this part became a second 

 posterior end, as in the figure, and fusion between the two posterior ends A and P gradually 

 took place. D, same animal as C at 216 days, after fusion of the two posterior ends; only main 

 branches of digestive tract are shown (from J. A. Miller, 1938). 



Section anterior to the graft a few days after it is implanted is usually 

 followed by regeneration of a normal head. Section at a later period may 

 result in a differentially inhibited head or complete inhibition of regenera- 

 tion, and after still later section a posterior end instead of a head de- 

 velops. Evidently there is a gradual extension anteriorly of the graft 

 dominance until it is blocked by the original dominance of the host. In 

 the light of other data this gradual extension of dominance suggests a 



