382 PATTERNS AND PROBLEMS OF DEVELOPMENT 



for grafts, die when isolated; but when they are implanted in a planarian 

 body, other parts of the head may regenerate and induction of post- 

 cephalic regions in the host body may follow. With complete union of 

 dorsal and ventral epithelia of graft and host on all sides the graft may 

 induce a cylindrical, tubular outgrowth with eyespots variously situated 

 about the closed tip and with either dorsal or ventral epithelium inside 

 or outside; if the outgrowth is dorsal, ventral epithelium is inside; if 

 ventral, it is outside. Under certain conditions, when part of the cut sur- 

 faces of graft and host fail to unite, a tubular or funnel-hke outgrowth, 

 open at both ends, develops, eyespots may appear in it, and it may induce 

 postcephalic parts in the host. Also, when union between graft and host 

 is incomplete, a normal head may develop from the graft and act as in- 

 ductor. This is a common result with implantation at postcephalic levels. 

 Heteroplastic grafts of ganglionic pieces into the ganglionic region give 

 certain results of interest. Complete union with host usually results 

 with ganglionic grafts of D. dorotocephala into the ganglionic region of 

 D. tigrina. With grafts in normal orientation there is no outgrowth, little 

 development of new tissue, no evidence of induction, and locomotion and 

 reaction to food are normal within 12 hours after operation, though most or 

 all of the host ganglionic region was removed. In controls, with a window 

 in the ganglionic region similar to that in which the graft is implanted, nor- 

 mal reactions appear only after a week or more of reconstitution. Appar- 

 ently, the implanted ganglionic region of the one species attains domi- 

 nance very rapidly in the other. With reversed orientation of the hetero- 

 plastic grafts complete union is less frequent, tubular outgrowths with 

 eyespots may develop, some with short postcephalic induction, and a 

 secondary head may develop from a free cut surface of the host. These 

 forms often show further modifications resulting from partial distintegra- 

 tion or resorption of host head or graft outgrowth and may finally ap- 

 proach more or less closely a normal animal with single head. Appar- 

 ently either the graft head or the host head may dominate the other 

 component and inhibit it. Even with dorsi ventral reversal of hetero- 

 plastic grafts in the ganglionic region complete union may result. Such an- 

 imals advance with head raised and bent posteriorly. With incomplete un- 

 ion a graft head or combinations of graft and host tissue may develop. Ex- 

 tra eyespots frequently appear with ganglionic grafts in the ganglionic 

 region, even though there is no outgrowth, and with other than normal 

 orientation of the graft its polarity may be altered or reversed. These 

 results with ganglionic grafts in the ganglionic region are from Miller. 



