298 LEONARD P. SAYLES. 



regenerating tail (8 x n fj. and 3.5 fj., respectively). In the dorsal 

 portion of the terminal hypodermis many of the cells are ap- 

 parently in the process of breaking away from the epithelium 

 to migrate into the coelom where they form the cerebral ganglion 

 (Fig. 13). They become elongated and slender in this process 

 and develop an appearance which answers well the description of 

 cells which Krecker ('10, p. 441) believes are mesoderm cells 

 migrating anteriorly to form the new longitudinal muscles 

 " spindle-shaped cells with slightly granular cytoplasm and large 

 nucleus containing a deeply staining nucleolus." 



Regarding the activation of the ectoderm by the presence of 

 the neoblasts, there is no further evidence from anterior re- 

 generation. Neoblasts apparently migrate anteriorly and the 

 ectoderm is also transformed, similar processes to those occurring 

 at the posterior end. 



From these observations it is apparent that not only the ecto- 

 dermal and endodermal elements of the regenerated head are 

 derived in the same manner as at the posterior end but the 

 mesodermal structures as well. Just as in posterior regeneration 

 the neoblasts apparently metamorphose and migrate to the 

 wound region and the cells of the ventral portion of the hypo- 

 dermis become greatly enlarged. About ten or eleven segments 

 seem to be involved in the regenerative processes except in the 

 case of the ectoderm in which the changes are confined to the 

 immediate vicinity of the wound as in posterior regeneration. 



DISCUSSION. 



The production of new tissue in the anterior regeneration of 

 microdrilous annelids seems to be essentially the same as in the 

 posterior regeneration in these forms. Iwanow ('03) in Lum- 

 briculus variegatus and Krecker ('10) in Tubifex and Limnodrilus 

 describe the formation of the new mesodermal tissue from neo- 

 blasts at the posterior end but believe that there is a dedifferenti- 

 ation of old mesoderm to form the new at the anterior end. 

 The loss of contractile substance by some of the longitudinal 

 muscle cells at the cut surface, however, occurs at both ends. 

 The spindle-shaped cells abundant in the dorsal part of the bud 

 in anterior regeneration may be seen in Lumbricidus inconstans 



