226 X. M. STEVENS. 



in practically every nucleus, indicating that the paired threads 

 are at no time so thoroughly fused as even apparently to lose their 

 identity. This is further indicated by the earliest prophase 

 stage (Fig. 28) where the paired strands begin to untwist. The 

 following prophase stages consist of further untwisting and longi- 

 tudinal contraction of the paired homologous chromosomes. In 

 the synapsis stage (Figs. 25 and 26) it is impossible to tell whether 

 threads of equal length form the pairs, but in the prophase pairs 

 this is perfectly evident (Figs. 29 and 30). The untwisting and 

 contraction frequently proceed at different rates in different 

 pairs in the same nucleus and in different nuclei of the same cyst, 

 so that one can easily compare the various stages of the process 

 and be perfectly sure that the untwisting is continuous. There is 

 no secondary fusion of paired threads such as frequently occurs 

 in cases where a precocious longitudinal split appears in a 

 telosynaptic bivalent and then closes up before the rings and 

 crosses are formed (see Blattilla germanica, Stevens, '05). Fig. 

 30 shows various stages in the formation of the definitive chromo- 

 somes of the first maturation mitosis from the parasynaptic 

 threads of a stage a little later than that of Fig. 28. The paired 

 chromosomes untwist and contract simultaneously. Some re- 

 main united at one end (b and d) while in other cases union of a 

 pair at one or both ends is a secondary phenomenon and may 

 even occur after the spindle has formed (a and c}. That there is 

 much variation in the form and size of the 18 bivalents in meta- 

 phase is shown in Figs. 3, 7, 8, 9, 30, 31, 32 and 33. The most 

 frequent forms are rings, E's and crosses, though one or more 

 pairs of straight rods may be found in nearly every spindle. 

 Most of the chromosomes are attached to the spindle fibers at 

 or near the middle of each univalent member of the pair, so that 

 the separated chromosomes pass to the poles of the spindle in 

 the form of V's (Figs. 31 to 33). In the case of the double rods 

 the fibers are attached at the ends. Many of the chromosomes 

 are partly or wholly split longitudinally in the anaphase (Fig. 33). 

 There are of course two kinds of second spermatocyte equatorial 

 plates containing 18 and 19 chromosomes respectively (Figs. 34 

 and 35), X appearing in the form of a large V (Fig. 35) in one 

 half of the cells. Division of the chromosomes is here longi- 

 tudinal as seen in Fig. 36. 



