I90 PATTERNS AND PROBLEMS OF DEVELOPMENT 



of these, differentially inhibited head forms appear (Abeloos, 1930); 

 in others, an essentially normal head form or no head at all devel- 

 ops on pieces of any length posterior to a certain level. ^ Apparently, 

 if the initial activation of the head-forming cells occurs in these latter 

 forms it is not inhibited physiologically, even in short pieces. It is per- 

 haps of interest to note that in these species there is more regeneration 

 of new tissue at the anterior ends of pieces than in Dugesia; consequently, 

 the developing head and ganglia are isolated to a greater degree from the 

 old nerve cords and so from any nervous factor originating at the posterior 

 cut surface and inhibiting head development. In the dendrocoelids Proco- 

 tyla and Dendrocoelum rate of head development decreases from anterior 

 levels of section to a region near the middle of the body; posterior to this 

 level head development has not been observed, irrespective of length of 

 piece or presence of a posterior cut surface, although some new tissue is 

 formed; but posterior ends regenerate at all except extreme anterior 

 levels. Apparently, activation of the cells at the anterior end of the pieces 

 following section decreases more steeply from anterior to posterior levels 

 in these forms than in Dugesia and at a certain level becomes insuffi- 

 cient to initiate development of ganglia and head, though still sufficient 

 under the dominance of more anterior levels to develop a posterior end. 



SECONDARY DIFFERENTIAL MODIFICATIONS OF THE PLANARIAN HEAD 



Not infrequently pieces undergoing reconstitution in water, but with 

 heads inhibited by the physiological factor, show secondary modifications 

 at the anterior end, suggesting a conditioning to, or a recovery from, the 

 earlier inhibition, more probably the latter, since the physiological in- 

 hibiting factor is present only temporarily following section. For example, 

 in anophthalmic forms like Figure 63, / or /, further development often 

 occurs after a week or more, resulting in forms like Figure 72, ^ and B. 

 In the case of Figure 72, ^, the median region of the anterior new tissue 

 has undergone elongation; in Figure 72, 5, not only elongation but de- 

 velopment of a median cephalic lobe and eye has occurred. In this case 

 the outgrowth finally attains a condition permitting development of a 

 teratomorphic head. Development of a normal or teratophthalmic head 

 from these primarily anophthalmic pieces has never been observed. 



extremely short pieces, 1/16 or less of the body length, under natural conditions. Animals, ap- 

 parently of the same species, from another locality several hundred miles from the first showed 

 a head-frequency gradient essentially like that of D. dorotocephala, according to data obtained 

 by Professor T. Minoura. 



* Sivickis, 1931a, 1933; Buchanan, 1933. 



