CHAPTER VII 



DIFFERENTIAL MODIFICATION OF DEVELOPMENT: 

 OTHER ANIMAL GROUPS 



INVERTEBRATES 



SLIGHT differential inhibition of larval development has been pro- 

 duced with a number of chemical agents in several species of poly- 

 chete annelids (Child, igi'jd). In these modifications apical and 

 segment-forming regions are most inhibited, as might be expected from 

 their greater susceptibility to lethal concentrations (pp. 120-29). How- 

 ever, the regions of the trochophore body are differentiated so early and 

 the period from fertilization to swimming trochophore is so short in the 

 species used that the alterations in form and proportions by external 

 agents are not great, though they are clearly evident. Some modifications 

 of later larval stages are apparently secondary, but they are so slight 

 that their significance remains uncertain. 



In reconstitution of pieces of oligochete annelids, particularly the micro- 

 drilous forms, physiological factors determining head frequency appear 

 to be very similar to those in planarians. As Hyman (1916a) has shown, 

 some microdrilous species regenerate a head at any body-level; in others 

 head regeneration occurs only at levels near the anterior end of the body, 

 irrespective of length of piece; and in still others, notably Lumhriculus, 

 head frequency in pieces depends on level of body and length of piece, 

 much as in Diigesia { = Euplanaria). According to Hyman, the physio- 

 logical factor inhibiting head development in the shorter pieces of Lum- 

 hriculus results from the posterior section, as it does in Dugesia, and is a 

 stimulation of the piece. Apparently, also, head development may be dif- 

 ferentially inhibited in Lumhriculus. Hyman distinguishes microprosto- 

 mial and aprostomial anterior ends and certain outgrowths apparently 

 intermediate between head and tail or beginning development as one and 

 undergoing partial transformation into the other. In pieces which give 

 low head frequency in natural environment head frequency is increased 

 by temporary exposure to an inhibiting agent (e.g., KCN), exactly as in 

 Dugesia. This rather close parallelism in certain aspects of reconstitu- 



247 



