DIFFERENTIAL DEVELOPMENTAL MODIFICATION. I i8i 



Head frequencies. — ^The frequency of occurrence of the various degrees 

 of differential inhibition of the anterior end in a particular lot of pieces 

 of the same length from the same body-level of animals of a certain 

 length and as nearly as possible in similar physiological condition has been 

 called, for convenience, "head frequency." In such lots head frequency 

 shows a high degree of constancy, but it is a rather sensitive indicator of 

 differences in physiological condition and in external environment. With 

 animals of a certain length from a particular locality, which probably 

 signifies similar nutritive condition, possibly in some cases conditioning 

 to some external factor, and with similar laboratory environment, head- 

 frequency values differ in definite directions with length of piece, level of 

 body, and conditions under which reconstitution occurs. In pieces from 

 animals of different length the values also differ in definite directions, with 

 fraction of body length represented. Moreover, they differ in character- 

 istic ways with differences in nutritive condition of the animals and with 

 differences in temperature and other factors of the environment of the 

 stocks preceding section. Animals from different localities or from the 

 same locality at different seasons of the year may give characteristically 

 different frequencies. Acclimating or conditioning animals to certain en- 

 vironmental factors preceding section alters the frequencies. And finally, 

 so far as comparison is possible, different species of the American genus 

 Dugesia { = Eu plana ria), and at least some species of the genus Planaria, 

 show characteristic species differences in head frequency. "> 



HEAD FREQUENCY IN RELATION TO CERTAIN PHYSIOLOGICAL FACTORS 



The relation between head frequency, length of piece, and body-level 

 is shown graphically for Dugesia dorotocephala in Figure 66, in which the 

 head-frequency indices for pieces 1/3, 1/4, 1/6, and 1/8 of the post- 

 cephalic body length are plotted as ordinates against body-levels as ab- 

 scissae. As the graph shows, head frequency decreases from the anterior 

 level to the region of the fission zone, that is, in the anterior zooid, scarcely 

 at all in 1/3 pieces, more in 1/4, still more in 1/6, and most in 1/8 pieces. 

 At the most anterior level the frequency is the same in pieces of all 

 lengths, but decreases more steeply as length of piece decreases. From the 

 region of the fission zone it increases posteriorly. With still shorter pieces 

 the decrease and increase are still steeper, but with sufficiently short 

 pieces the most anterior and posterior levels show a decrease in frequency, 

 as compared with longer pieces (Child and Watanabe, 1935a). In other 



1 For the literature chiefly concerned with planarian head frequency, the method used for 

 obtaining a "head-frequency index," and the appHcation of statistical methods to evaluation 

 of head-frequency data, see Appendix VII (p. 745). 



