MEIOSIS 261 



distinct about each of the four chromonemata, so that the whole tetrad is 

 obviously quadruple (Fig. 154). In other cases (Figs. 151, e; 156) it 

 appears to form a common mass about any two chromonemata lying 

 close together and not to divide until the chromonemata are separated 

 later in meiosis. This means that such chromosomes at diakinesis are 

 tetrads with respect only to their chromonemata.^- It has now been 

 shown that each of the four chromonemata in the large tetrads of Zea 

 (McClintock), Tradescantia (Nebel), and Trillium (Huskins) is longi- 

 tudinally double in the late prophase or metaphase. This suggests 

 either that the threads are double before synapsis, or that two splittings 

 must occur in the meiotic prophase (Huskins, 1932a). 



Before proceeding further with the history of the tetrads, mention 

 should be made of a remarkable phase through which they pass during 

 the late pachynema and diplonema stages in many meiocytes. During 

 the prophase of the first meiotic mitosis the cell and its nucleus usually 

 become considerably enlarged. In case this growth continues well into 

 the latter part of the prophase, the chromosomes may show a marked 

 tendency to recede from their compact form toward the more finely 

 divided condition of the metabolic stage. In the sporocytes of plants 

 this alteration is ordinarily not noticeable; the diplonema threads may 

 be irregular in outline, but usually they seem to pass into the diakinesis 

 stage with little change other than shortening and thickening. In 

 growing animal spermatocytes this modification of the threads is in 

 some cases carried much further, so that a characteristic "diffuse stage" 

 ensues. It is in the much more extensive "growth period" of the animal 

 oocyte that it is most pronounced. At this time the cell increases 

 enormously in size and develops most of the differentiation which is to 

 characterize the egg, and as it does so the pachynema or diplonema 

 threads become profoundly altered in appearance. They send out 

 thready processes in all directions, assume an irregular brush-like form, 

 and lose their basichromatic character partially or completely. These 

 changes seem to be associated with the metabolic processes concerned 

 in cell growth. When the growth is completed, the original staining 

 capacity returns while the chromosomes again assume a compact form 

 and pass into the diakinesis stage. 



The prophase of the first meiotic mitosis comes to a close with the 

 development of the achromatic figure. If centrosomes are present in the 

 cell (animal spermatocytes and the meiocytes of certain lower plants), 

 asters develop about them in the cytoplasm. With or without an inward 

 shrinking of the nuclear membrane the karyolymph becomes organized 

 as a spindle figure, as already described (p. 151). The tetrads take up 



12 The claim is made, however, that in some cases each of the four spiral chro- 

 monemata is accompanied by its own individual matrix, the several matrices being in 

 contact but not continuous (Nebel, 1932; on Tradescantia and Zebrina). 



