SPERMATOGENESIS OF THE DRAGON-FLY. 279 



apart are brought close together. At first the long axis of the 

 loop coincided with the plane of union in synapsis, but with the 

 bending of each autosome the long axis is reversed and lies at 

 right angles to the original long axis. The ends where the chromo- 

 somes united do not bend, but remain extended to form side 

 arms. A cross is thus produced by ftris process, the upper half 

 composed of one univalent autosome and the lower half of 

 another. When the signet ring condenses it assumes the same 

 form as the others; all finally become condensed crosses showing 

 a less dense area in the middle. 



Figs. 39, 40, and 41 show cells containing a number of crosses, 

 and in Figs. 40 and 41, the sex-chromosome which has retained 

 its round compact form through all the stages of the growth 

 period appears. In some prophases as many as nine different 

 sizes of crosses which would form bivalent chromosomes of the 

 primary spermatocyte could be counted. The largest bivalent 

 autosome comes from a cross in which the secondary long axis 

 is much extended and the united ends of the univalent auto- 

 somes bulge only slightly to form exceedingly short side arms 



(Fig. 41). 



(c) First Spermatocyte Division. 



The chromosomes, as last described, have become bivalent 

 masses; with only an area less dense in the center to indicate 

 the former central split. As just described, each bivalent^has 

 four projecting parts corresponding to the arms of a cross. 

 Hence, if a bivalent is cut exactly in two, one half when viewed 



from the cut surface shows two ends connected by a cross piece 

 below the level of the ends. It is difficult to give an adequate 

 idea by a description but the text figure makes this clear by 

 picturing the result in cross-section of different cuts through the 

 bivalent. 



