CROSSING OVER AND CHROMOSOMES 107 



for the ordinary spermatogonia! (or somatic) cell-divi- 

 sions. Each chromosome vesicle begins to show a coiled 

 thread (Fig. 46, c). Each thread next becomes longer 

 and longer (Fig. 46, d) until the whole nucleus is filled 

 with them. One or both ends can often be seen at the 

 ''distal pole" of the cell, where deep-staining nucleoli 

 are present. The cells are now in the so-called thin 

 thread, or leptotene stages. 



The threads next come together in pairs beginning 

 at the distal end of the chromosomes (the zygotene stage, 



Fig. 47. 



-Formation of a thick thread after synapsis, a, b; and the following condensation 

 of a tetrad, c. (After VVenrich.) 



Fig. 47, a). When the fusion is complete and all the 

 threads are double (Fig. 47, 5), the stage is called the 

 thick thread or pachytene stage. There are half as many 

 threads now present as at the beginning. A longitudinal 

 split is present in the chromosome throughout these stages 

 along the line of fusion of the two thin threads. Wenrich 

 identifies the split as the "primary split." 



Another longitudinal split at right angles to the other 

 one soon appears (Fig. 47, c), thus forming tetrads, each 

 composed of four chromosomes. The tetrads next shorten, 



