1 88 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



Evidently it acts chiefly on the physiological, supposedly nervous, factor 

 which inhibits head development; but, instead of making this factor more 

 effective, it decreases its effectiveness in some way, perhaps by general 

 stimulation of the nervous system with resulting functional disorganiza- 

 tion (F. S. Miller, 1937). With 

 certain concentrations and ex- 

 posure periods strychnine is so 

 highly effective that pieces 

 from the more posterior levels 

 of the anterior zooid, largely 

 acephalic in the controls, de- 

 velop a high percentage of nor- 

 mal heads. 



In pieces so long that there 

 is no physiological inhibition of 

 head development in controls, 

 cyanide, various anesthetics, 

 caffein in certain concentra- 

 tions, CO2, hydrogen ion, and 

 low temperature all decrease 

 head frequency more or less; 

 that is, inhibited head forms 

 appear in significant percent- 

 ages, instead of high normal 

 frequency. 



The head-frequency gradi- 

 ent of Dugesia a gills and D. 

 tigrina, both of which have a 

 posterior-zooid region, is es- 

 sentially like that of D. doroto- 

 cephala, except that in D. ti- 

 grina the physiological inhib- 

 iting factor is apparently less 

 effective and the gradient is less steep.'' Certain other triclad species with- 

 out posterior zooids show a decrease in frequency of head development 

 from anterior to posterior levels over the whole body length. In some 



7 Head frequencies under natural conditions have been determined in the forms earher 

 known as Planaria lata (Sivickis, 1923), Euplanaria jnacidata (Watanabe, 1935/'), and E. 

 novangliae (Child) , all of which are now regarded by Hyman as D. tigrina (seep. 41, footnote 7). 

 A Japanese planarian from one locality was found to give differentially inhibited heads only in 



A B C D E 



Fig. 69. — Head frequencies of 1/8 pieces from an- 

 terior zooids of animals conditioned to CO^ and non- 

 conditioned animals under different experimental 

 conditions. L, nonconditioned, reconstituted in water; 

 L', conditioned, reconstituted in water; M, noncon- 

 ditioned; M', conditioned, reconstituted in carbonate- 

 free water, pH 4.15; N, nonconditioned; N', condi- 

 tioned, reconstituted in carbonate-free water, pH 4. 5, 

 plus 0.5 gm. of CaCU per liter. Indices as ordinates 

 against body-levels as abscissae (from Rulon, 19360). 



