THE MITOTIC CYCLE 



polarized ends densely contracted, which are thus heteropycnotic. 

 Schrader225 ]^^g shown that among the Pentatomidae (the Shield- 

 bugs) a bouquet stage is found only in those species which have 

 heterochromatin. 



Some workers (Smith^^®) see in this polarization merely the residual 

 orientation of the previous telophase, the persistence of which in 

 the interphase nucleus was first described by Rabl.^^' Hughes- 

 ScHRADER,^^^ however, has been able to prove that in some Mantids 

 (the 'praying insects') this explanation of the bouquet polarization 





Figure 40 Bouquet stage of an oocyte of Dendrocoelitm lacteum 



X 2933, from Gelei'^*'» {By courtesy. Arch. Z^llsforsch) . Polarization 



related to the position of the central body. 



cannot apply. Polarization is seen twice in the meiosis of the spermato- 

 cytes; first in leptotene, opposite the yet undivided centrosome. This 

 stage is succeeded by one in which the chromosomes lose their arrange- 

 ment and the division centre is then no longer seen. Later, in mid- 

 pachytene, two centrosomes reappear in their final positions at opposite 

 poles of the nucleus, towards which the ends of the chromosomes are 

 again pointed. Again, in the spermatocytes of the earwig Anisolabis, 

 ScHRADER^^^ has followcd the division of the centrosomes and their 

 gradual movement apart, during which he finds that each division 



112 



