112 FUNDAMENTALS OF CYTOLOGY 



stage. The results of X-ray studies suggest that it is present below the 

 limit of visibility much earlier. In any event it represents the plane of 

 anaphasic separation in the first postmeiotic mitosis. In a plant this 

 would be the first mitosis in the spore, in an animal the first mitosis in the 

 fertilized egg. 



Anaphase I. — The tetrads now separate into dyads which begin to 

 move apart toward the opposite poles of the spindle. Often there 

 appears to be considerable resistance in the region of a chiasma to the 

 disjunctive forces, so that the tetrad may elongate and assume an odd 

 shape (Fig. 82). Eventually the two dyads become free. Meanwhile the 

 two chromatids composing each of them commonly widen out from each 



J 



Fig. 82. — Late metaphase / in microsporocyte of peony (Paeonia), showing the five 

 tetrads about to separate into dyads. In the third the kinetochores are at the points above 

 and below; a chiasma is present in each arm. In the fourth and fifth the kinetochores are 

 at the sharp angles; subterminal chiasmata are present at the equator. {After K. Sax.) 



other except at the kinetochore (Fig. 84). Thus a dyad with a nearly 

 terminal kinetochore appears as a single V, while one with a median 

 kinetochore is a double V. As already pointed out, genetical evidence 

 indicates that the two chromatids of a dyad, at least in the region of the 

 kinetochore, are as a rule sisters. This interpretation has further cyto- 

 logical support in the chromosomes of an amphibian, w^hich show^ the 

 kinetochore of the dyad not yet divided at this time, although it has two 

 kinosomes and two tractile fibers. Its division is completed in //. In 

 maize also tw^o tractile fibers can be seen extending from each dyad in I. 

 At the end of the anaphase the two groups of dyads form compact groups 

 at the poles. 



Telophase I and Interkinesis. — The polar groups of chromosomes at the 

 close of anaphase / nearly always undergo a certain amount of telophasic 

 transformation similar to that seen in somatic nuclei. In most cases, 

 however, the alteration is not carried far enough to form fine-textured 

 metabolic nuclei, the individual chromosomes often being discernible at 

 least in part up to the beginning of prophase //. The abbreviation of 

 interkinesis, or stage between divisions / and II, reaches an extreme in 



