REGENERATION IN LUMBRICULUS. 



neoblasts is of considerable significance." This case may be 

 equally well explained in accordance with the view of an in- 

 dependent transformation. The factor which brings about the 

 metamorphosis of the neoblasts and ectoderm is either absent or, 

 because of the physiological condition of this particular indi- 

 vidual, produces no effect. It is reasonable to suppose that, if 

 the same underlying cause brings about like changes in these 

 two types of cells and if they are equally susceptible to activation, 

 one type will not undergo a process of transformation when the 

 other does not. In all other cases given by Krecker for posterior 

 regenerates he himself states (p. 435) that "the evidence adduced 

 has to do only with instances in which neoblasts are found about 

 the metamorphosed ectoderm cells." Admittedly neoblasts are 

 present in all cases where the ectoderm cells are greatly enlarged, 

 but an explanation for this is easily found. Neoblasts migrate 

 only along the ventral nerve cord and so they are found in the 

 ventral region of the bud. The ectoderm cells which are greatly 

 enlarged are those which are to give rise to the nerve cord in 

 the ventral region. The close proximity of these two types of 

 cells in the regenerating bud seems to be due to the fact that 

 both bear a definite relation to the nerve cord those of the 

 hypodermis to form the new portion of the nerve cord in the 

 bud and the neoblasts using it as a pathway to the wound 

 region. In the discussion of anterior regeneration his statement 

 that no metamorphosis of the ectoderm and no migration of the 

 neoblasts occurs in this type ot regeneration will be considered. 



The development of the setigerous glands and the production 

 of the setae have for the most part been neglected in the work on 

 regeneration. Bergh ('90) and Penners ('23) both give some- 

 thing of a description of their formation during embryonic 

 development. In regeneration it is difficult to observe the early 

 stages due to the great number of cells scattered throughout 

 the growing bud. By the fifth day the cells of the glands in 

 the two or three segments of the bud nearest the old tissue 

 stand out clearly since at this time the other cells of these 

 segments have become arranged in a more orderly fashion. The 

 nuclei of the hypodermal cells near the old tissue have begun to 

 take on the appearance typical of the older cells. They are 



