AMITOSIS IN CELLS GROWING IN VITRO. 449 



however, a cell was found in which the nucleus divided directly 

 while being watched, and the following paragraph, extracted 

 from the protocol written at the time, is a brief description of the 

 process as observed. The drawings (Plate I.) were sketched 

 free-hand from observation at fifteen-minute intervals, and 

 afterward retouched by reference to fixed and stained cells of 

 similar morphology. 



8.45 P.M. A cell (Fig. i), growing from a 57-hour culture of 5-day chick heart, 

 presented an elongated nucleus with a concavity upon one side. The outline of this 

 concavity was indistinct on account of the fact that the centrosphere (c] was situ- 

 ated very close to it. 



9.00 P.M. (Fig. 2). The nucleus is now straight and the indentation is almost 

 obliterated. 



9.15 P.M. (Fig. 3). The general outline of the cell has changed, and this has 

 been followed by change in shape of the nucleus. At first there were two nucleoli 

 within the nucleus, the lowermost being paired, but at 9.30 (Fig. 4) the latter appears 

 as a dumb-bell-shaped body. The nucleus is now rounded. 



9.45 P.M. (Fig. 5). The nucleus is still rounded, and the nucleolar substance 

 consists of two masses close together. 



10.00 P.M. (Fig. 6). The nucleus has become elongated again. There is no 

 apparent cleft, but the left side, against which the centrosphere is resting is not so 

 distinct as the right. The central nucleolus now appears as a single structure and 

 some refractive substance, resembling another nucleolus, is seen in the lowermost 

 pole of the nucleus. 



During the next two fifteen-minute intervals (Figs. 7 and 8) a shortening of the 

 nucleus occurred, a shallow concavity to the left being noted. This concavity is 

 deeper at 10.45 (Fig. 9) its outlines being somewhat indistinct and the nucleus has 

 increased in length; across the middle of the nucleus is a refractive line. 



n.oo P.M. (Fig. 10). The concavity is seen with difficulty, and the line across 

 the nucleus persists. Fifteen minutes after this (Fig. n) the elongated nucleus 

 shows an indentation upon the right side, opposite the one upon the left, and it 

 appears to be undergoing constriction into two parts of equal size. Between the 

 two nuclear portions the refractive line is seen as before, and, from the appearance 

 shown in cells fixed and stained with iron hematoxylin (Fig. 24) this is evidently a 

 strand of mitochondria. 



11.30 P.M. (Fig. 12). The nuclear sacs are apparently quite separate, and 

 between them is the strand of mitochondria, and also part of the centrosphere, 

 this body, still undivided, having retained its position with reference to the nuclear 

 cleft. 



Judging from the behavior of this and other elongated, bent 

 and dumb-bell shaped nuclei it would seem that the nucleus may 

 return to its original rounded form provided the constriction has 

 not gone too far, but, if the degree of constriction passes a certain 

 critical point the nuclear sacs become completely separated. 



