234 



INTRODUCTION TO CYTOLOGY 



In the case of most animals, and apparently in certain plants also, 

 the split which is to function in the homceotypic mitosis may develop dur- 

 ing diakinesis or even much earlier, the result being the formation of 

 chromosome tetrads. This introduces another element of complication 

 which will be touched upon later (p. 243). 



i 



Fig. 86. 



A, parasynapsis in Allium fistulosum. B, parasynapsis in Osmunda regalis. C, nucleus 

 of oocyte of Scyllium canicula (Selachian) in "growth stage." D, single chromosome in 

 growth stage, showing the fine subdivision of its substance. (A and B after Gregoire, 1907, 

 C and D after Marechal 1907.) 



The diakinesis stage is terminated by the dissolution of the nuclear 

 membrane and the formation of the spindle, upon which the bivalent 

 chromosomes, whether secondarily split or not, now become arranged. 

 Because of the peculiar form and consistency of the heterotype chromo- 

 somes the mitotic figure presents a striking contrast in appearance to the 

 ordinary figure of somatic cells. This is especially true as the chromo- 

 somes are drawn into various curious shapes as their anaphasic separation 

 begins. The two univalent components of each bivalent chromosome 

 eventually become free from each other and pass to the two daughter 

 nuclei, bringing about reduction. During the anaphase the separating 



