REGENERATION IN LUMBRICULUS. 297 



fifth day (Krecker, '10). After the third day it is difficult to 

 say just what does happen in the bud region. Many meta- 

 morphosed ectoderm cells are present and things are so confused 

 that it is difficult to distinguish the various types of cells, par- 

 ticularly those as similar in appearance as are the neoblasts and 

 the much enlarged cells from the ventral hypodermis. 



The process of formation of new septa and longitudinal muscles 

 by cells known to be derived from neoblasts has not been 

 observed. The fact that neoblasts metamorphose in considerable 

 numbers and are frequently seen migrating along the nerve cord 

 is, nevertheless, evidence which seems to indicate that they play 

 some part in anterior regeneration in Lumbriculus. If such is 

 the case, the mesodermal structures are formed in the same 

 manner in both types of regeneration, just as are the ectoderm 

 and endoderm. 



Ectoderm. 



As in posterior regeneration only those cells in the immediate 

 vicinity of the wound take part in the formation of the new 

 ectodermal structures. There occurs a similar enlargement of all 

 these cells during the first day or so, no one region undergoing 

 any more extensive change than any other. Iwanow ('03) in 

 his discussion of the formation of the new nerve elements at the 

 anterior end describes a marked enlargement of the ectoderm 

 cells. Krecker (' 10, p. 433) says that the ectoderm cells " undergo 

 no such metamorphosis" as at the posterior end, explaining this 

 on a basis that no neoblasts migrate anteriorly to stimulate it to 

 activity. As in the case of posterior regeneration, however, by 

 the second day there are cells on the ventral side which have 

 quite large nuclei and nucleoli, and by the third day they have 

 reached a considerable size (Fig. 16). The area over which this 

 marked enlargement extends is greater than in posterior regenera- 

 tion. Large nuclei are present in cells found over a considerable 

 portion of the ventral and ventro-lateral hypodermis and even 

 somewhat dorsally in the terminal portion. The largest cells, 

 nevertheless, are confined for the most part to the ectoderm in 

 the vicinity of the mid-ventral line. This region extends from 

 the end of the old nerve cord anteriorly. The nuclei and nucleoli 

 measure about the same as in the transformed cells of the 



