276 INTRODUCTION TO CYTOLOGY 



Huskins points out that those organisms in which the chromosomes are 

 reported to be split in the premeiotic telophase (p. 256) are either forms 

 with incomplete synaptic pairing or with no crossing-over, suggesting 

 that such doubleness is actually a hindrance to normal meiosis. This is 

 in accord with his observations on the inverse relation between splitting 

 and pairing in certain speltoid wheats and fatuoid oats. 



It is interesting to speculate upon the possible relation between the 

 synaptic "attraction" and the later "repulsion" which is suggested by the 

 behavior of the chromatids in diplonema and anaphase, but such specu- 

 lations have as yet little of a definite nature to support them. Attempts 

 to identify the immediate causes of meiosis and especially its origin in the 

 evolution of organisms can yield little of value until many other questions 

 have been answered by reliable observation. 



Fig. 160. — At left: Zea chromosome II folded back upon itself in meiotic prophase; 

 spindle-attachment region near one of the folding points. At right: Zea chromosome VIII 

 eynapsing with a homologue having a portion of one of its arms inverted; see Fig. 170. 

 {After McClintock, 1933.) 



That synapsis is not solely a matter of gene homology, but is influ- 

 enced by some condition pervading the nucleus or cell generally, is 

 indicated by derangements of synapsis caused by abnormal cultural 

 conditions or by a single Mendelian gene (p. 273). It is also strikingly 

 shown by the fact that non-homologous parts of chromosomes are some- 

 times observed in a close association to all appearances like that of homol- 

 ogous parts (McClintock, 1932a, 1933; on Zea). For example, when an 

 "interchanged" chromosome synapses with other chromosomes with 

 which it is in part homologous (pp. 329 to 333), the association which 

 begins between homologous parts may sometimes extend along the 

 threads in such a way as to bring non-homologous parts together. Fur- 

 thermore, a single chromosome normal in other respects may fold back 

 upon itself at various levels, one portion thus entering into close associa- 

 tion with the other (Fig. 160). Such association usually disappears 

 before diakinesis. It is rightly emphasized (McClintock, 1932c) that 



in the meiotic prophase, the first of these being the split which was delayed from the 

 premeiotic mitosis and taking effect in division //, while the second is that taking 

 effect in the first postmeiotic division. 



