148 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



VIII. Mitosis in the Subepidermal Cells of the Old 

 Epidermis. 



Figures 53-56 (Plate 8) represent subepidermal cells, or basal cells of 

 the epidermis. These cells occur wedged in between the deep ends of 

 the columnar epidermal cells. In one worm of seven days' regeneration, 

 many dividing cells were found among the subepidermal cells in five or 

 six segments back of the injured segment. However, no mitosis was 

 found in the elongated columnar epidermal cells of the same animal. 



Figure 53 represents a cell in which the division is complete. The 

 new nuclear membranes have been formed. The nuclei are cup-shaped, 

 and the greater mass of chromatin is collected on the concave side of the 

 nucleus. At the concavity of the nucleus in the lower cell may be 

 seen the conical spindle region with a minute centrosome at its apex. 

 In the upper cell there is a slightly denser region immediately outside 

 the concavity of the nucleus, and at the centre of this denser region is 

 the centrosome. Several indistinct granules lie at the edge of the 

 denser region. There are some weak radiations from both centrosomes. 

 A dividing cell membrane is present. This membrane is represented as 

 if the cell were seen as a transparent object. The lower of the lines, mi'., 

 is the intersection of the membrane with the upper surface of the old 

 cell membrane ; the upper line, mb'., is its deeper intersection. On the 

 dividing membrane and in the axis of the old spindle (whose former 

 position is marked by a darker region extending between the two nuclei) 

 is a small dark mass, perhaps a "Zwischenkorper." 



Figure 54 shows two sister cells. In one the concavity of the deeply 

 cup-shaped nucleus, as well as a considerable space immediately outside 

 of it, is filled by a mass of dense cytoplasm. The centrosome, an 

 intensely staining granule with weak radiations, lies just outside the 

 concavity of the nucleus. The condition of the young nucleus is char- 

 acteristic. The chromatin is massed in a nearly solid layer on the 

 concave side of the nucleus, while more or less isolated masses of chro- 

 matin extend from this layer to the equatorial surface of the nucleus. 

 The upper nucleus of Figure 54 is seen in the direction of its (spindle) 

 axis. The mass of chromatin which lines the cavity is seen as a nearly 

 complete circular band. 



A young cell whose nucleus has increased in size and lost something 

 of its concavity is shown in Figure 55. A finely granular region 

 separated from the outer coarser cytoplasm by a narrow clear space, 



