REGENERATION IN LUMBRICULUS. 289 



Twelve hours after a worm is cut the wound has healed and in 

 those ectoderm cells which are brought into a terminal position 

 by this process the nucleoli have begun to enlarge (Fig. 10). 

 The cells themselves are beginning to elongate somewhat and 

 accompanying this process the nuclei are changing to the oval 

 shape more or less typical of elongated cells. The nuclei them- 

 selves are not appreciably enlarged at this time but the nucleoli 

 have increased from mere dots to quite evident structures about 

 i i in diameter. Cells of this type are not confined to any 

 definite part but make up the entire terminal portion of the 

 hypodermis. After one day of regeneration the enlargement has 

 gone still further but there is no marked difference in size between 

 cells in the ventral portion and those dorsally located. The 

 average size of the nuclei at this time is 4.5 x 7.2 /JL and of the 

 nucleoli 1.4 /x. 



On the second day, however, characteristic differences make 

 their appearance (Fig. n). The cells which are dorsal or lateral 

 show little if any change from the preceding day. In the median 

 ventral region just posterior to the end of the nerve cord many 

 of the cells have grown considerably but all gradations in size 

 can be found between the largest of these and the cells found 

 elsewhere in the bud. In the larger cells the nucleoli are oval 

 and about one third the dimensions of the nuclei. The average 

 size of ten of the larger nuclei found in two adjacent sections is 

 6.1x8.4^, with the nucleoli of the same cells averaging 1.9 

 x 2.8 (j.- On the following day the nuclei of the dorsal cells are 

 of about the same size as on the second day but many of the 

 nucleoli have enlarged to 2 ^. In the ventral region the changes 

 are even more marked. A number of the cells have enlarged 

 enormously and have nuclei of about 8 x 1 1 n, with nucleoli 

 averaging about 3.5 /j.. It is this stage of the hypodermal cells 

 to which Krecker applies the term "metamorphosed" ectoderm 

 and which may be most easily confused with neoblasts. Krecker 

 suggests that the fact that the neoblasts come to lie near such 

 cells is probably the reason why so many workers have thought 

 the neoblasts to be products of the ectoderm, which seems very 

 plausible. 



Krecker ('10, p. 436) suggests, as a theory of the formation of 



