METABOLIC GRADIENTS 79 



DEVELOPMENTAL GRADIENTS IN AGAMIC AND 

 EXPERIMENTAL REPRODUCTION 



Among the lower animals and most plants new indi- 

 viduals arise, not only by the process of gametic or sexual 

 reproduction, but by various agamic or asexual pro- 

 cesses, such as division, budding, etc. These processes 

 vary greatly in different forms and even in the same 

 individual under different conditions, but their essential 

 feature is the formation of a new individual from a part 

 of a pre-existing individual, a process which usually in- 

 volves more or less dedifferentiation and redifferentiation 

 in a new direction. Although these agamic reproductive 

 processes differ more or less widely from embryonic 

 development, the metabolic gradients characteristic 

 of the individual either persist from the original indi- 

 vidual or arise anew in each case, and developmental 

 gradients of some sort appear in relation to them. 



In the formation among animals of new individuals 

 by budding, as, for example, in the hydroid, Pennaria 

 (Figs. 25-27), the hydranth becomes distinguishable 

 first, the stem later, and closer examination shows that 

 apical regions of the hydranth are somewhat in advance 

 of basal. In Figs. 26 and 27, for example, the apical 

 tentacles are more advanced in development than the 

 basal. 



In the flatworm, Sknostomum, division occurs after 

 the body attains a certain length, the first visible indica- 

 tion of the new individual being the appearance of a new 

 head-region (Fig. 28) at a certain distance from the 

 original head. This new head-region acquires control 

 of parts posterior to it and finally separates as a new 

 animal. By continued division before separation of 



