352 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



like C and D of Figure 119, most of the piece represents one or more 

 posterior zooids, and fission often occurs anterior to the middle; that is, 

 the dominance of the inhibited head does not extend over the whole 

 length of the piece, often not over the anterior half. 



Short planarian pieces, one-tenth or less of the body length, from levels 

 near the head (A' and Y of Fig. 119, A) often develop as tailless forms or 

 forms with very small posterior outgrowths and no pharynx or mouth 



zzx 



Y 



Fig. 119, A-G. — Scale of organization in reconstitution of Dugesia dorotocephala. A, 

 outline, indicating body-levels of pieces X, Y, Z; B, reconstitution of piece Z in normal en- 

 vironment; C, D, decrease in scale of organization of preoral region of Z-pieces under inhibiting 

 conditions. E-G, physiological inhibition of posterior development in pieces shorter than scale 

 of organization; E, head+.Y; F and G, X or I' {B-D, from Child, 19296). 



(Fig. 119, F, G). Short pieces, including the original head, are also often 

 tailless (Fig. 119, E). 



Table 8 gives percentages of forms, either tailless or with tail inhibited, 

 reconstituting from 1/16 pieces: A" and Y (Fig. 119, A), 1/8 pieces; 

 X + Y, without the original head; and A' and A' + F with the head. 

 According to the table, posterior development is absent or inhibited in 

 64 per cent of A'-pieces, in 30 per cent of F-pieces, and not at all in pieces 

 X + Y and head + A' + Y. Moreover, in A^-pieces with regenerating 

 head posterior development is much more inhibited (64 per cent) than in 

 pieces with posterior ends at the same level but with the original fully 

 developed head present (8 per cent). Apparently, in these short pieces the 



