20 INTRODUCTION. 



represents a slight pause in the process of mitosis. For this reason it 

 is the stage most easily obtained and most frequently observed. 



METAKINESIS. 



Up to the stage of the mature spindle, as in Fig. 7, H, each 

 chromosome is seen to consist of two daughter segments oriented in 

 one of the ways described above. As soon, however, as these seg- 

 ments begin to separate in metakinesis, each splits longitudinally in a 

 plane at right angles to the longitudinal splitting which took place in 

 the prophase. In some instances, and when the chromosomes are 

 viewed from the end, each is seen to be composed of four rods, the 

 four granddaughter segments, placed side by side in pairs, forming a 

 tetrad, Fig. 9, A. As a rule the granddaughter segments cannot be 

 definitely recognized until the daughter segments have separated 

 somewhat. Having almost or quite separated, the daughter segments 

 are seen to be in the form of a V, although it never should be for- 

 gotten that V's do not invariably result. As the result of the second 

 longitudinal splitting, each typical V-shaped daughter chromosome 

 consists of two granddaughter segments which adhere or are even 

 fused at the ends to which the spindle fibers are fastened, while the 

 opposite ends diverge (Fig. ,9, B). It frequently happens that the 

 opposite ends of the granddaughter segments do not diverge, but lie 

 more or less in contact side by side, so that the retreating daughter 

 chromosomes consist of two applied rods (Fig. 9, F, the middle pairs). 

 In some cases, as already mentioned, the ends of the granddaughter 

 segments forming the angle of the V fuse, so that the V appears to be 

 one piece formed by bending. The bent or contorted condition of the 

 granddaughter segments during metakinesis is due to the previous 

 twisting of the daughter chromosomes upon each other. 



If the chromosomes be in the form of rings, as shown in Fig. 8, E, 

 it is evident that the separating daughter chromosomes may also be in 

 the form of a V or U, but such V's and U's will be produced by a 

 bending of the daughter segments. This is true in a great many cases 

 in Lilium and in other plants, among both monocotyledonous and 

 dicotyledonous species. In such cases each U or V is invariably 

 double, as the result of the second longitudinal fission that is, the 

 granddaughter segments are U-shaped and closely applied to each 

 other (Fig. 9, F, right and left). Sometimes these granddaughter seg- 

 ments may separate slightly, giving the impression of two similar 

 daughter chromosomes lying one just beneath the other. This is one 

 of the several phenomena that have led to erroneous interpretations 

 of the chromosomes. 



