132 bulletin: museum of comparative zoology. 



its apex. /3 is in an unfavorable position fur the observation of the 

 polar structures, a and /3 he in a common clear space, which shows a 

 constriction in the region between the two chromatic masses. This 

 common clear space is that formerly bounded by the mother-cell 

 membrane. The equatorial membrane, separating the two daughter 

 cells, has disappeared, y and 8 are unfavorably situated for study. 



Figures 34 and 35 represent two young cells whose sister cells could 

 not be identified. The chromatic masses are in the same condition, both 

 showing the polar concavity and the loosening of the chromatin into 

 large indistinctly separable granules. In both cases the lightly stained 

 polar region and its centrosome are present, and in both cases a thin 

 layer of lightly stained material extends from the polar region along one 

 side of the chromatic mass to its opposite end. In Figure 35 a slightly 

 clearer area bounded by a darker ring surrounds the centrosome. In 

 both figures a clear space surrounds the ellipsoidal body, which is com- 

 posed of the chromatic mass plus its polar structures. 



Figure 36 shows a group of cells from the posterior ventral border of 

 a regenerating brain. At the centre of the group is a young cell. The 

 chromatic mass is composed of indistinctly separated granules. On 

 one side of it is the lightly stained polar region within which lies a 

 dark granule, doubtless the centrosome. On the border of the lightly 

 stained region are two dark places, but focussing shows that they are 

 not distinct granules. There is no marked polar concavity of the chro- 

 matic mass. The surrounding clear space belonging to the daughter cell 

 is well defined. 



Above the dividing cell in Figure 29 is a young cell which shows the 

 swelling chromatic mass, the polar region, centrosome, and the clear 

 space about the entire stained object. 



Figures 37 and (Plate 6) 38 show later stages in the condition of the 

 chromatic masses ; it is not until such stages as these are reached that 

 answers can be confidently given to the questions, which already must 

 have suggested themselves. How much is nucleus and where is the 

 cytoplasm in Figures 31-36 1 Unless one is prejudiced by holding the 

 belief that the centrosome must lie outside the nucleus, there are two 

 possible interpretations of the object shown in Figure 32 b. It may be 

 that the nucleus is identical with the chromatic mass, or it may be that 

 the new nuclear membrane is represented by the elliptical outline which 

 encloses the chromatic mass together with the lighter region at its pole. 

 Upon careless observation with a dry objective, one would hardly hesi- 

 tate in saying that the elliptical outline is that of the nuclear membrane. 



