314 C. M. CHILD. 



posterior to a certain level, and once established, the second 

 zooid grows nore rapidly than the first. When the animal is 

 5-8 mm. in length a very short second zooid is present and in 

 animals sightly longer than this fission may be induced experi- 

 mentally (Child, 'io). In the posterior region of the second 

 zooid a third zooid arises early because the head region of the 

 second zooid is not sufficiently active to control more than a 

 short region; the extreme posterior region of the body often 

 seems to consist of a series of very short head regions, reminding 

 one of the series of minute segments which often appear in 

 the growing posterior region of various annelids. 



Under experimental conditions it is possible to delay or inhibit 

 the appearance of the second zooid in regulating pieces of to 

 accelerate it. For example, if we isolate the region anterior 



7 



FIGS. 6-7. 



to the level c or 2 in Fig. I, including the old head, it forms an 

 animal more or less like Fig. 6, in which the head and prepharyn- 

 geal region are disproportionately large. If now after the growth 

 and differentiation of the posterior new tissue is completed, we 

 isolate the posterior end at the level indicated by the dotted line 

 in Fig. 6, we find it does not produce a new head, i. e., it is physio- 

 logically as w r ell as morphologically a posterior end. If we feed 

 pieces like Fig. 6 so that growth occurs, we shall find that after they 

 attain a certain length the second zooid appears and after this 

 the posterior region of the animal shows a high capacity for head 



