Nucleus and Chromosomes 47 



The first sequence in chromosomal evolution is seen at the late 

 prophase and early metaphase, while chromosomes are strongly cleft, 

 and two chromatids are coiled about each other in a relational coil 

 (Fig. 2.10) . The entire chromosome is straightened so that relational 

 coiling is easily perceived. Through the whole process of uncoiling, 

 the delayed metaphase permits observation at each stage. Since both 

 arms are held at one point, the centromere, the description of un- 

 coiling is made easier. Uncoiling, then, is the first step and ]:)egins 

 when the nuclear membrane disappears, unless action takes place 

 earlier in a precocious uncoiling, as was reported in the section above 

 under actions during prophase. The first step in the evolution toward 

 a c-pair is passed when the major relational coiling has been removed 

 (Fig. 2.10). 



Next, the further reduction is similar to the terminalization of 

 the chiasmata. The contacts of chromatids occurring originally at 

 several points, finally slip off at the end (Fig. 2.\0B) . The movement 

 begins at the centromere and proceeds to the end of each chromosome. 

 The last contact is at the very end of each chromosome. If both ends 

 are in contact, the characteristic figuie-8 obtains (Fig. 2.105) . Should 

 one end lose contact, and the other remain attached, a forceps type 

 develops (Fig. 2. IOC) . All the while uncoiling takes place, the 

 chromosomes are shortening. Usually the reduction is to one and 

 one-half times the regular length."' In one instance, actual measure- 

 ments for chromosomes of Petroselinum were 4.0 microns for control 

 and 1.5 microns for colchicine-treated chromosomes at c-metaphase."*-^ 



Finally the last stage is reached, when both ends separate and 

 move out as if there were actual repulsion of the two arms (Fig. 

 2. IOC) . The cruciform type has been seen a number of times in 

 plant,-^*^ insect,^" and mammalian cells cultured in vitro.^'^ Manuiials 

 receiving colchicine via injection have not generally shown cells with 

 the cruciform type. A maximum contraction is attained and the c- 

 pair is held together only at the centromere (Figs. 2 AD and 2. IOC) . 

 Thus the t\vo chromatids starting from prometaphase as a cleft 

 structure relationally coiled, are reduced until only the ends are in 

 contact. After these are released, there develops the typical X-shaped 

 structures (Fig. 2. IOC). This sequence has taken a longer time than 

 the control because an intactness period is ten times longer than 

 untreated mitosis. 



A stickiness of chromosomes prevents the X-shapes, or cruciforms. 

 Such physical changes are important to the falling apart of the c- 

 pairs."' 



Straightened chromosomes that are clearly marked at the centro- 

 mere (Fig. 2AD) improve the cytological and morphological studies 



