METABOLIC GRADIENTS 



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the flatworm Planaria (Fig. 30) a piece such as a or &, 

 or almost any other piece, cut from 

 the body will develop into a whole 

 animal of small size by the forma- 

 tion of a new head at one end and a 

 new tail at the other and a trans- 

 formation and redifferentiation of 

 the internal organs of the piece into 

 those of a whole animal as indi- 

 cated in Figs. 31-33. In the out- 

 growth of the new tissue at the 

 two cut surfaces the axial gradients 

 appear as gradients in rate of 

 growth. Fig. 31 shows that the 

 outgrowth of new tissue is more 

 rapid at the apical than at the 

 basal end of the piece and more 

 rapid in the median than in the 

 lateral region of each cut surface, 

 and Fig. 34, a side view of the 

 piece, shows more rapid outgrowth 

 at each end in the ventral than in 

 the dorsal region. In this case the 

 axial gradients in the piece persist 

 from the parent individual, and the 

 head arises at the apical end of 

 the piece, the tail at the basal end. 

 In other cases of experimental 

 reproduction from isolated pieces 

 the axial gradients appear either 

 in the same or in some other way 

 according to the kind of individual and the conditions. 



Figs. 28, 29. — Asexual 

 reproduction in flat- 

 worm, Stenostomum: 

 Fig. 28, stage of two 

 zooids; Fig. 29, chain of 

 several zooids. 



