PHYSIOLOGY OF THE CELL. 



27 



Reproduction. Cells reproduce themselves in the higher animals in 

 two ways by direct division and by indirect division, or karyokinesis. In 

 the former the nucleus becomes constricted, and divides without any special 

 grouping of the nuclear elements. It is probable that this occurs only in 

 disintegrating cells, and never in a physiologic multiplication. In division 

 by karyokinesis (Fig. 2) there is a progressive rearranging and definite 

 grouping of the nucleus, the result of which changes is the division of the 

 centrosome, the chromatin, and the rest of the nucleus into two equal por- 

 tions, which form the nuclei. Following the division of the nuclei, the 

 protoplasm divides. The process may be divided into three phases: 



Close Skein 

 (viewed from 



the side). 

 Polar field. 



Loose Skein (viewed 

 from above i. e., from 

 the pole). 



Mother Stars (viewed from the side). 



Spindle. 



Mother Star (viewed Daughter Star Beginning Completed 



from above). Division of the Protoplasm. 



FIG. 2. KARYOKINETIC FIGURES OBSERVED IN THE EPITHELIUM OF THE ORAL CAVITY OF 

 A SALAMANDER. The picture in the upper right-hand corner is from a section through a dividing 

 egg of Siredon pisciformis. Neither the centrosomes nor the first stages of the development of 

 the spindle can be seen by this magnification. X 560. (Stohr.) 



i. 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 segments or loops, chromosomes, having their closed 

 ends directed toward a common center, the polar field, while the other 

 ends interdigitate on the opposite side of the nucleus the antipole. 

 Th polar field corresponds to the area occupied by the centrosome. 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 centrosome divides into two portions, 



