THE GENERAL OUTLINE OF METOSIS 63 



posed becoming ' spiralized ' ; that is to say each 

 prometaphase or metaphase chromatid at mitosis 

 consists of a cylindrical ' spring ', possibly enclosed 

 in some sort of sheath or pellicle. By special tech- 

 nical methods (exposure of the living material to 

 ammonia vapour or fumes of strong acids before 

 fixation) it has been shown ^^' ^^ that at meiosis 



Fig. 15. — Diagram of a bivalent at the first meiotic division 

 showing the major and the minor spiral. The spindle 

 attachments are subterminal and there is only one 

 chiasma. 



each chromatid probably forms a double spiral 

 (Fig. 15), each g3rre of the ' major ' spu'al being made 

 up of several gyres of the ' minor ' spiral. This may 

 also be the case at mitosis, but so far only one spu-al 

 has been observed at ordinary somatic divisions. 



Metaphase of First Meiotic Division 



At mitosis each chromosome is attached to the 

 spindle by a single spindle attachment (or one which 



