y 



302 ELLINOR HELENE BEHRE. 



Child's morphological classification of the different degrees of 

 cephalic differentiation between normal and headless ('lie} is 

 followed in all the tabulations. Worms are classified as normal; 

 teratophthalmic, 'teratomorphic, anophthalmic, and headless. In 

 all experiments fifty pieces each' were used and the numerical 

 results are given in percentages unless otherwise stated. 



On the basis of a wide range of experimental data, Child 

 ('13^, '140, 'i^b, '16) has attempted a physiological analysis of 

 the factors concerned in head-frequency in this species, and his 

 conclusions are adopted in this paper. His analy- 

 sis is somewhat as follows: In each piece after sec- 

 tion two regions are to be distinguished physiolog- 

 ically, one which can be designated x, the region 

 at the anterior end of the piece from which the new 

 head is formed ; the other designated as y, consist- 

 ing of the remainder of the piece (see Fig. 5). The 

 region x undergoes more or less complete dediffer- 

 entiation as the result of section, its cells attain a more 

 or less embryonic condition, and from this embryonic tissue 

 the head gradually redifferentiates. The region y, on the 

 other hand, shows stimulation through the nerves after sec- 

 tion, but undergoes relatively little and only gradual dedif- 

 ferentiation. Within any single individual, the more posterior 

 the level of the body from which the piece is taken the greater 

 the stimulation of y (Child, '140). It has been shown that it 

 is. determined during this period of stimulation of y whether a 

 piece shall form a head or not (Child, '146) and that the more 

 posterior the level of the body from which the piece is taken the 

 lower the head-frequency (Child, 'iia, '16). 



Head-frequency may be experimentally altered and controlled 

 to a high degree (Child, 'lie, '16) and the results of experiments 

 along this line together with data noted above concerning the 

 regions x and y have led Child to the conclusion that a new head 

 is formed, not through correlation with and dependence upon the 

 rest of the piece, but so to speak in spite of it. The differentia- 

 tion of a head is determined by the embryonic cells of x, exactly 

 as in the embryo arising from an egg, and the region y can only 

 inhibit head development to a greatef or less degree provided its 



