STUDIES ON THE DYNAMICS OF MORPHOGENESIS 283 



have been observed only rarely and in very small pieces, i. e., 

 in pieces smaller than those which develop as headless. These 

 forms are the last term of the whole — headless series: we turn 

 now to the whole— •tailless series. 



'Tailless heads.' Just as headless tails occur under certain 

 conditions, so do tailless heads occur under certain other condi- 

 tions. Moreover, like the headless pieces these exhibit the most 

 various degrees of Haillessness, ' from the condition of fig. 13 

 with wide open posterior wound and a mass of new tissue differ- 

 ing visibly in no way from the earlier stages of tail formation, 

 but never developing further and never functioning like a tail, 

 to the condition shown in fig. 14 where the wound closes almost 



W 



Figs. 13-16 'Tailless heads.' (figs. 13 and 14) and "biaxial heads." (figs. 15 

 and 16). 



without any development of new tissue. Here, as in the headless 

 pieces, the absence of the tail is not due to the method of closure 

 of the wound, but the closure of the wound is the result of the 

 more or less complete absence of growth at this end. 



'Biaxial heads'. As the headless pieces give place with de- 

 creasing size and under certain other conditions to biaxial 

 tails, so the tailless pieces give place to biaxial heads (figs. 15 

 and 16). These may be merely heads alone, as in fig. 16, or they 

 may include the prepaaryngeal regions more or less completely 

 (fig. 15) and in some cases one or both may possess a pharynx. 

 Not infrequently they develop a common tail in later stages from 

 the lateral region midway between the two ends. 



Cases falling under each of these six different heads have been 

 described by other investigators, but the relation between the 

 frequency of their occurrence and various internal and external 

 factors has never been clearly shown. In the present paper we 



