no 



FUNDAMENTALS OF CYTOLOGY 



chiasmata, where two of the chromatids have exchanged portions by 

 crossing over (Fig. 81). A tetrad with one chiasma thus has the form of 

 an X, while one with two at or near its ends appears as an 0. When there 

 are several, the regions between them form a series of such openings, one 

 of which usually includes a region on ])oth sides of the kinetochore. The 

 number and the location of chiasmata may differ characteristically in 

 different organisms. In general there are more in long chromosomes 

 than in short ones. 



During the diplotene stage the tetrads continue the shortening begun 

 in the pachytene stage. This involves a coiling of the chromonemata 



Fig. 79. — Pachytene stage in micro- Fig. 80. — Diplotene stage in speraiato- 



sporocyte of maize. Chromosome 6 at- cyte of scorpion (Tityus hahiensis). The 

 tached to nucleolus above; chromosome 1 members of one bivalent are very loosely 

 looped over 6; chromosome 3 at upper left. associated. The large black spots are 

 {After B. McClintock.) extraneous material in the preparation. 



{Courtesy of F. G. Brieger and E. A. Graner.) 



within the matrix, which begins to be more evident at this time. With 

 such changes the chromosomes pass gradually into the diakinesis stage. 



Diakinesis Stage. — This stage is characterized by the presence of 

 compact tetrads lying well spaced out in the nucleus, often near its 

 membrane. This is therefore a favorable stage for counting the chromo- 

 somes. They have continued to shorten as their chromonemata have 

 continued to coil, sometimes into both major and minor spirals. The 

 matrix has become abundant, giving them smoother contours. They 

 have the form of X's, V's, O's and other more complex shapes depending 

 upon the number and location of chiasmata. Since the matrix tends to 

 form a common mass about any two closely associated chromonemata, the 

 tetrads often look merely double instead of ciuadruple. Commonly th(; 

 one having the nucleolus organizers is in contact with the nucleolus. 



In many organisms the tetrads show fewer chiasmata than they did in 

 the diplotene stage. This is due to a process called tenninalization , in 



