RECONSTITUTIONAL PATTERNS IN EXPERIMENT 399 



INHIBITION OF RECONSTITUTION AND DESTRUCTION OF ZOOIDS 

 AS A RESULT OF ALTERED DOMINANCE 



According to Stevens (1902), pieces from the basal stem region of 

 Antennularia ramosa usually reconstitute hydranth-stem axes; those from 

 more distal levels, stolons. This difference appears to be due to different 

 relations of dominance at the different levels. At the proximal levels hy- 

 dranths are often absent from the lateral branches or in poor condition, 

 while farther distally they are present and in better condition, the more 

 distal branches being physiologically younger and their dominance more 

 effective in inhibiting development of hydranth-stem axes than that at 

 more proximal levels. 



In planarian reconstitution a developing head region may, under cer- 

 tain conditions, inhibit development of another head. For example, in 

 the well-known experiment involving decapitation and partial anterior 

 longitudinal spHtting of the body, a head develops on the anterior end 

 of each separated part, even if the two anterior ends represent different 

 body-levels (Fig. it,5, A, B). However, if a half-transverse section is also 

 made at the posterior end of the longitudinal split, thus removing one 

 of the two separated parts, head development is inhibited on the half 

 anterior cut end resulting from this section (Fig. 135, C). Apparently, in 

 this case the more posterior level of section on the one side is dominated 

 and determined as lateral, instead of as head, by the dominant region 

 at a more anterior level on the other side. In the case of B this dominance 

 is not effective in inhibiting head development, probably because it cannot 

 overbalance the gradient already established and intensified by the activa- 

 tion following section in the shorter anterior end. This experiment has 

 been used repeatedly by the writer in laboratory class work (see also 

 Goldsmith, 1939). The conditions in forms like Figure 135, A and B, 

 are comparable to those in bipolar forms of hydroids and planarians; 

 neither dominant region affects the other because the two opposed gra- 

 dients are more or less nearly equal. In the case of Figure 135, C, one 

 of the opposing gradients has been removed, and head development is 

 inhibited. It has been shown earlier in this chapter that planarian head 

 reconstitution can also be inhibited by grafts. 



A striking case of the effect of altered dominance appears in the inhibi- 

 tion, disintegration into cells, and complete destruction of zooids already 

 morphologically distinguishable in certain species of the rhabdocoel 

 Stenostomum (Child, 1903&; Van Cleave, 1929). In Figure 136, A, a chain 

 of S. grande sectioned at A' A' leaves the posterior part of zooid i.i. and 



