METABOLIC GRADIENTS 



8i 



A 



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

 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 llat- 

 worm, Stcnostomum: 

 Fig. 28, stage of two 

 zooids; Fig. 29, chain of 

 several zooids. 



