sperm-cell — 139 — Splitting 



pole before the rest of the chromatid; centromere; insertion 

 region; kinetochore. 



Spindle Elements. — The 'fibres', striations or elements of 

 which a spindle is made up. See Spindle Fibres, 



Central Spindle Element. — A spindle element which is 

 not related to any of the chromosomes and which forms the 

 'core' of the spindle surrounded by the other elements which 

 lie parallel to it (White). 



Spindle Fibres. — Fibres which appear to connect the centro- 

 meres to the poles of the spindle during cell division. These 

 are possibly artefacts of fixation which correspond positionally 

 with some latent 'structure' of the spindle cytoplasm but which 

 do not exist as definite fibres until coagulated by chemical 

 reagents. 



Spindle Spherule. — A specialized part of the centromere. 



Spiral. — A coil of the chromosome thread (chromosome or 



chromatid), at mitosis or meiosis. Internal , a coil within 



a single chromatid between prophase and anaphase. Relational 



, coiling of two chromatids or chromosomes round one 



another. Major , the larger internal coil at meiosis. 



Minor , the smaller internal coil, Relic , the coiling 



which survives at telophase and prophase. Super , larger 



coils derived in prophase from the rearrangement of relic 



spirals. Molecular , the coiling within the chromosome 



thread which conditions internal and relational spirals (Dar- 

 lington). 



Spiral Cleavage. — A form of cell division which the fertilized 

 ovum undergoes in certain Annelida, MoUusca, Nemertina 

 and Platyhelmia in which the daughter cells are formed from 

 the old cells in certain regular positions which follow a 

 definite rule. 



Spiralization. — The assumption of an internal (but not a 

 relational) spiral by the chromatids in mitosis and meiosis 

 (Darlington). 



Spireme. — (i) Prophase, (ii) The chromosomes during pro- 

 phase. 



Splitting. — Chromosomes are spoken of as 'split' when chro- 

 matid changes result from X-raying. The earliest stage when 

 chromatid changes are produced is called the 'beginning of 

 splitting'; the period when both chromatid and chromosome 

 changes occur together is called the 'duration of splitting'; 

 'time of splitting' refers to the time when chromosome and 

 chromatid changes are produced equally often (Nomenclature 

 of Newcombe, 1942). 



