34 HUMAN PHYSIOLOGY. 



chromatin, and the rest of the nucleus into two equal portions, which 

 form the nuclei. Following the division of the nuclei, the protoplasm 

 divides. The process may be divided into three phases : 



1. Prophase. The centrosome, at first small and lying within the 

 nucleus, increases in size and moves into the protoplasm, where it 

 lies near the nucleus, surrounded by a clear zone, from which 

 delicate threads radiate through an area known as the attraction 

 sphere. The nucleus enlarges and becomes richer in chromatin. 

 The lateral twigs of the chromatin cords are drawn in, while the 

 main cords become much contorted. These cords have a general 

 direction transverse to the long axis of the cell, and parallel to 

 the plane of future cleavage. They are seen as V-shaped seg- 

 ments or loops, chromosomes, having their closed ends directed 

 toward a common center, the polar Held, while the other ends 

 interdigitate on the opposite side of the nucleus the anti-pole. 

 The .polar field corresponds to the area occupied by the centro- 

 some. This arrangement is known as the close skein; but as the 



- process goes on, the chromosomes become thicker, shorter and less 

 contorted, producing a much looser arrangement, known as the 

 loose skein. During the formation of the loose skein, the centro- 

 some divides into two portions, which move apart to positions at 

 the opposite ends of the long axis of the nucleus. At the same 

 time delicate achromatin fibers make their appearance, arranged 

 in the form of a double cone, the apices of which correspond in 

 position to the centrosome. This is known as the nuclear spindle. 

 During the prophase the nuclear membrane and the nucleoli dis- 

 appear. 



2. The Metaphase. The two centrosomes are at opposite ends of 

 the long axis of the nucleus, each surrounded by an attraction 

 sphere, now called the polar radiation. The chromosomes become 

 yet shorter and thicker, and move toward the equator of the nucleus, 

 where they lie with their closed ends toward the axis, presenting 

 the appearance, when seen from the poles, of a star, the so-called 

 mother star, or monaster. While moving toward the equator of 

 the nucleus, and often earlier, each chromosome undergoes longi- 

 tudinal cleavage, the sister loops remaining together for a time. 

 Upon the completion of the monaster, one loop of each pair passes 

 to each pole of the nucleus, guided, and perhaps drawn by the 

 threads of the nuclear spindle. The separation of the sister seg- 

 ments begins at their apices, and as the open ends are drawn 



