THE MITOTIC CYCLE 



sister chromatids, however, remain in contact; and this 'pre-metaphase 

 stretch' is still well in advance of metaphase, for there is yet no congres- 

 sion. The chromosomes are then by no means fully nucleinated and 

 are drawn out to an almost fantastic extent. This stage is brought to 



^v^^-»fi»*-^'- 



Figure 42 Stages of spermatogenesis in Stagmomantis Carolina x 2700. From Hughes- 

 ScHRADER^^* {By courtesy, Biol. Bull.), a and b The second polarization of the 

 bivalents. In b terminal chromomeres are applied to the nuclear membrane under 

 the central body, c Early stage of premetaphase stretch. The nuclear membrane 

 has gone. Upper bivalents are still polarized, others are opening as their centromeres 

 orient to the poles, d Mid-stage of premetaphase stretch. 



an end by contraction of the chromosomes, during which their centro- 

 meres again approach each other; at the same time, each bivalent 

 comes into the normal equatorial position of metaphase and the whole 

 spindle then shortens. 



These remarkable observations present us with the question whether 



114 



