370 SUMMARY OF CURRENT RESEARCHES RELATING 'J'" 



rejuvenescence. Its amount depends on the degree of reconstitutiona] 

 change, md the size and region of the piece. It disappears as the 

 animal increases in size and age. , 



Head-regeneration in Pieces of Planaria.* — C. M. child continues 

 his studies on ' ; the dynamics of morphogenesis and inheritance in 

 experimental reproduction," dealing in the present instance with head- 

 determination in Planaria dorotocephala. He lias previously shown 

 that the frequency of head-format ion in isolated pieces of tin- Planarian 

 varies with size of piece, region of body, and various external conditions 

 which can be controlled experimentally. He now finds that whether or 

 not a head shall arise at the anterior end of an isolated piece is deter- 

 mined so fixedly during the first six or eight hours after section thai 

 head-formation cannot afterwards he prevented by conditions which do 

 prevent it when acting immediately after section. Head-determination 

 undoubtedly begins immediately after section. 



The period during which head-determination occurs is the period oi 

 stimulation following section, and in general the more a piece is 

 stimnlated by section the less likely it is to produce a head. Head- 

 formation is a process opposed or antagonistic to the maintenance of 

 the piec-. 



The development of a new head on a headless piece is not the 

 restitution of a missing part, but the first step in the development of a 

 new individual. 'Whether a head is to develop or not depends primarily 

 on whether the cells which give rise to new tissue at the anterior end 

 of the piece become physiologically isolated to a sufficient degree to 

 develop independently of other parts of the piece, or whether other 

 parts prevent this development. In the former case a head arises, in 

 the latter the piece remains headless. 



If the group of cells which gives rise to the embryonic tissue at the 

 anterior end of the piece is designated x, the similar group at the 

 posterii >r end, z, and the remainder of the piece, y, the head-frequency 

 in pieces may be expressed in a very simple form, viz., head-frequency = 



— — -. Tail-frequency, on the other hand, is directly proportional 



rate y 



to rate y. 



In pieces of considerable length the new head is localized at the 

 anterior end of the piece because the axial gradient determines that the 

 ••ells at this end are physiologically isolated to a much higher degree 

 than the cells at the posterior end. A group of cells developing inde- 

 pendently at a transverse-cut surface gives rise to a head, but when 

 developing in subordination to other pans, gives rise to a posterior end. 

 In short pieces, biaxial heads, biaxial tails, or reversal of polarity may 

 occur according to the relations between the rates of the regions x, >/■ 

 and z. The author concludes that "a single fundamental reacti n 

 system is the basis of development and inheritance in each species, race 

 or individual. The apical or head region, or the dominant part of that 

 region, represents the fundamental reaction more nearly than any other 

 p in .if !.!i- organism.*' 



*^Joum. Exper. Zool., xvii. (1914) pp. 61-79 (2 tigs.). 



