MEIOSIS 1 1 1 



which the four chromatids, after opening out two by two to give the 

 diplotene .stage, continue to open into the chiasma on its proximal side 

 while the opening on its distal side closes, the result being a gradual 

 movement of the chiasma along the tetrad, even to its end. 



The meiocytes, or cells in which all the foregoing prophasic changes 

 occur, are in most cases relatively large cells with lafge nuclei. Their size 

 increases through a portion if not all of the prophase. In plant sporocytes 

 the increase is moderate, while in animal spermatocj^tes it is often greater 

 and involves a temporary ''diffusion" of the chromosomes at about the 

 diplotene stage. The animal oocyte undergoes an enormous inci-ease in 

 size at this stage, developing most of the features that are to characterize 

 the egg which it eventually becomes. The oocyte nucleus becomes wery 

 large during this "growth period," its chromosomes sending out thready 

 processes in all directions and losing their stainability- This suggests a 

 synthetic function comparable to that performed during the metabolic 



^'^ 



Fig. 81. — Tetrads in advauced diplotene .stage from .spermatocytes of grassliopper. They 

 show, respectively, one, two, three, and four chiasmata. {After F. A. Janssens.) 



stage, when the chromonemata are again in extensive contact with the 

 other substances in the nucleus. Eventually the chromosomes again 

 become compact and stainable and assume the form characteristic of the 

 diakinesis .stage. 



Metaphase I. — At the close of the diakinesis stage the achromatic 

 figure is developed, and the tetrads become arranged in its equator with 

 their kinetochores facing its two poles. In a lateral view of the metaphase 

 figure they appear about as they did at late diakinesis except for the more 

 clearly evident location of their kinetochores and sometimes their greater 

 compactness. When viewed from the direction of the spindle pole they 

 can easily be counted unless they are very long and crowded. Large 

 chromosomes show their coiled chromonemata particularly well at this 

 stage and at anaphase, many of the best studies on minute structure 

 having been made on such chromosomes. For example, some genera of 

 plants show a "double-coiled" condition in their chromonemata only at 

 this time. 



Of special interest is the fact that longitudinal doubleness (the so-called 

 tertiary split) may become visible in the chromonema of each chromatid 

 at metaphase and anaphase /, the tetrad therefore having eight lialf- 

 chromatids. In other cases it is first seen during division //. Occasion- 

 ally it has been demonstrated as early as diakinesis or even the diplotene 



