RAND : NERVOUS SYSTEM OF LUMBRICID^. 149 



and containing at its centre a prominent centrosome with some radia- 

 tions, is seen at the polar concavity of the nucleus. The sister cell was 

 immediately adjoining, but its axis was in an unfavorable position. 



Figure 56 represents a cell in which the resting condition has 

 been regained. There is still a slight flattening of the nucleus on one 

 side, — the remains of the polar concavity. The centrosome, imbedded 

 in a mass of cytoplasm denser and more finely granular than that of the 

 rest of the cell, lies on the flattened side of the nucleus and directly 

 opposite the flattened region. The sister cell was identified in the next 

 section. It showed similar conditions as to the nucleus, slightly concave 

 on one side, and the presence of a denser mass of cytoplasm at this 

 region of the nucleus, but no definite centrosome could be seen. 



A study of the earliest stages of regeneration may reveal that the 

 new epidermis over the cicatrix owes its origin to the subepidermal 

 cells rather than to the columnar cells of the old epidei'rais. The occur- 

 rence of mitosis in the subepidermal cells many segments back of the 

 one injured recalls the fact of groups of actively dividing cells in gan- 

 glia remote from the cut end of the worm. The subepidermal cells 

 may act as " Ersatzzellen," receiving an impulse toward mitosis even in 

 segments remote from the injury. 



IX. Some Peciiliar Mitoses. 



I desire to call attention briefly to certain dividing cells in a worm of 

 seven days' regeneration. Directly under the new epidermis was a 

 small mass of cells with nuclei like those of the epidermis. Abundant 

 mitoses were found among these cells. Figure 57 shows a group of the 

 cells with a dividing cell in the raetaphase. All stages of mitosis could 

 be found, presenting a series of conditions exhibiting no unusual fea- 

 tures. There were telojjhases with chromatin solidly massed at the 

 poles and a barrel-shaped interzonal region, the whole figure being 

 imbedded in a mass of dense cytoplasm. Later stages showed the for- 

 mation of an equatorial membrane, the constricting of the interzonal 

 filaments and some evidence of a Zwischenkorper. The old cell 

 membrane and the equatorial membrane eventually disappear, but at 

 all stages there is some dense cytoplasm collected about the daughter 

 chromatic masses. The cytoplasm of the resting nuclei is indefinitely 

 outlined ; there are no cell membranes. 



Among these cells a few cases were found which exhibit exceptional 



