152 INTRODUCTION TO CYTOLOGY 



uneven thickness, which soon begins to straighten out (Fig. 52, P). At 

 the same time the material composing the thread becomes more evenly 

 arranged throughout its length, so that the chromosome eventually takes 

 the form of a single slender thread. All of these changes — condensa- 

 tion, straightening, and equalization in thickness — may be seen going 

 on simultaneously in different chromosomes of the same nucleus, or even 

 in different portions of a single chromosome. 



The formation of the slender prophase chromosomes from the retic- 

 ulum in the above manner was first described in detail by Gregoire and 

 Wygaerts (1903) and Gregoire (1906), and new cases have since been 

 added. The above writers, together with Nemec (1910), Digby (1910), 



A rv 



J 



N 



J i 



R *<<? 



Fig. 52.- — Somatic mitosis in Tradescantia virginiana: prophases. 



At N are shown cross sections of chromosomes in the stage shown in Fig. 51, M. 

 X 1900. {After Sharp, .1920.) 



and Miiller (1912), believe that the separated portions of the reticulum 

 may also condense directly into the slender threads without passing 

 through the very irregular zigzag stage above described. It is probable 

 that both methods are followed in different cases, direct condensation 

 possibly being the rule in small nuclei. The view of Bonnevie (1908, 

 1911) concerning the origin of the zigzag threads has already been 

 mentioned in the paragraphs on the telophase. According to this worker 

 and to certain others (Wilson 1912a6) the chromatic material forms a 

 spiral thread within the chromosome during the telophase, this thread 

 uncoiling and emerging from the chromosome in the following prophase. 

 It is true that the zigzag threads occasionally have a strikingly regular 

 spiral aspect, but in view of the many other aspects observed and the 

 process which is known to give rise to them, it is probable that the 



