SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 153 



formation of spirals in the manner described by Bonnevie is at least 

 very exceptional. 



The true longitudinal splitting of the chromosomes is initiated in 

 the slender threads of the early prophase (Figs. 52 and 53, Q). As soon 

 as a thread becomes sufficiently equalized in diameter small vacuoles 

 appear along its axis and rapidly develop into a more or less continuous 

 split. Not all the threads, nor even all portions of the same thread, 

 undergo the change at the same time. If we consider the whole nucleus 

 at once, the processes of condensation, straightening, equalization, and 



FIG. 53. Somatic mitosis in Vicia faba: prophases. 

 Stages P, Q, and S correspond with P, Q, and S of Fig. 52. X 1650. 

 1913.) 



(After Sharp, 



splitting are all going on simultaneously; only in a given small portion of 

 a thread do they follow in definite sequence. Furthermore, as soon as 

 the threads become equalized they at once begin to shorten and thicken, 

 so that when vacuolation and splitting are a little delayed they occur in 

 somewhat heavier threads. 



The manner in which the vacuoles develop into the complete split 

 should be carefully noted. The vacuoles quickly form openings which 

 extend completely through the chromosome, so that the latter soon takes 

 the form of two parallel strands connected by heavy cross pieces repre- 

 senting the portions between the original vacuoles (Figs. 52 and 53, S). 



