76 GENERAL BOTANY 



more definite form and arrangement within the nuclear cavity. 

 In some instances they seem to form a continuous band of chro- 

 matin substance, which is called the spirerne (ft). In other cases 

 definite rodlike masses, called chromosomes (c), emerge directly 

 from the nuclear network. In either case the conversion of the 

 network of the resting nucleus into the definite masses known 

 as chromosomes is most easily understood by supposing that 

 after the chromatin granules have condensed to form either the 

 spireme or the isolated chromosomes, the remaining portions of 

 the original network break down and leave the chromosomes free. 



As soon as the chromosomes are formed in the above manner 

 they shorten and thicken and finally take up a definite position 

 at the periphery of the nuclear cavity next to the nuclear 

 membrane. The chromatin rods are now ready for the equal 

 division of their substance preparatory to the formation of 

 daughter nuclei. Since the further changes in the chromo- 

 somes during prophase are concerned with the spindle, they 

 will be described in connection with the following discussion 

 of that structure. 



The spindle which is concerned with the equal distribution of 

 the chromatin substance in nuclear division may for convenience 

 be termed the first spindle, to distinguish it from the second spin- 

 dle, which is concerned with the building of the cell wall which 

 divides the cell as a whole. In such cells as those found in grow- 

 ing root tips the first spindle makes its appearance as two fibrous 

 masses of cytoplasm at the opposite ends, or poles, of a nucleus in 

 prophase (c). The fibers, which are composed of living cytoplasm, 

 become more conspicuous as the spindle continues its forma- 

 tion at either pole of the nucleus. Finally two half spindles are 

 formed, each with a conical apex and a broad base. The base of 

 each half spindle fits over one pole of the nucleus like a skull- 

 cap, and the fibers composing these half spindles appear to extend 

 from its apex, or pole, to the nuclear membrane (c?). When the 

 half spindles are fully formed, the nuclear membrane gradually 

 disappears, beginning at the poles of the nucleus, and the half 

 spindles elongate across the nuclear cavity. They ultimately 

 unite to form a complete single spindle, with a bulging equatorial 



