BLACKMAN. — SPERMATOGENESIS OF THE MYRIAPODS. 503 



In later stages (Figures 19-21), when all of the chromosomes are free 

 in the nucleus, the nucleoli, much reduced' in size, may still be seen. 

 These nucleoli, although at first spherical (Figure 21), soon become 

 irregular in outline and rapidly disintegrate (Figures 19*^, 20). At 

 least a portion thus set free in the nucleus is used in the evolution of 

 the chromosomes. 



The chromatin segments of the reduced number as they arise from 

 the karyosphere are diffuse granular threads (Figure Ls). These soon 

 elongate, and the granules composing them become coarser. The seg- 

 ments assume a variety of shapes, but most often they are roughly 

 U-shaped or V-shaped, just as they were in the early spermatocytes 

 before they entered the karyosphere. At about their middle point, — 

 i. e., at the angle of the V, or at the loop of the U, — there is very 

 often a small gap in the chromatin thread (Figure 18), as has been 

 observed by several investigators on other material (Montgomery, :00, 

 :05 ; Sutton, :02 ; Blackman, :03, :05, 05*; Foot and Strobell, :05). 

 This undoubtedly represents the point at which the conjugating 

 chromosomes united during synapsis. 



Soon each segment undergoes a longitudinal splitting, resulting in 

 two more slender segments (Figures 19''-21). The halves thus result- 

 ing typically lie parallel to each other, but very often they are twisted 

 about each other, as is shown by several chromosomes in Figures 19^ 

 and 20. Less often the ends of the halves may be separated and drawn 

 in opposite directions, as is the case with several chromosomes in Figures 

 19" and 19". Tyjiically, however, the two threads lie near each other 

 and are approximately parallel. 



The next change in this double segment consists in the cleavage of 

 each of the halves at about their middle point, resulting in a four- 

 parted chromosome or tetrad. This is a transverse division of the 

 chromosome and occurs at the point where in preceding stages an 

 interruption in the chromatin was observed. Thus, when this division 

 is completed, it results in the separation of entire spermatogonia! 

 chromosomes. 



As soon as the transverse cleavage is established the chromatids 

 revolve upon each other in one of several ways, producing tetrads vary- 

 ing in shape, but all referable to one type. The more common method 

 is as follows : At the point of transverse cleavage the threads show a 

 tendency to bend at a more or less acute angle, and in each half-segment 

 the adjacent ends produced by the cross division become drawn out in 

 the same direction, but perpendicular to the axis of the original thread, 

 while the corresponding portion of the other half is drawn out in the 

 same plane, but in an opposite direction. This results in the typical 



