Meiotic Division — 95 — Microchromosomes 



Merostathmokinesis. — Incomplete inhibition of the spindle 

 (usually as a result of treatment with colchicine or some 

 substance with similar effects). This may occur either through 

 the failure of the cytoplasm to behave normally, or through the 

 failure of the nucleus, this latter effect being termed hemikine- 

 sis. 



Mesomitosis. — A form of nuclear division effected within 

 the nuclear membrane, so that the cytoplasm takes no active 

 part in the process. 



Metabolic Differentiation, Theory of. — The theory that 

 sex determination is conditioned by the degree of metabolism 

 of the developing individual. 



Metabolic Nucleus. — A 'resting' nucleus, i.e. one which is 

 not dividing. 



Metabolic Stage. — The resting stage ; the phase in the nuclear 

 cycle when the nucleus is at rest in the sense of not dividing. 



Metacentric Chromosome. — An isochromosome, i.e. a chro- 

 mosome composed of two exactly similar arms united by a 

 centromere. 



Metagenesis. — Alternation of generations; an alternation of 

 a sexual with an asexual form, or, cytologically, the alternation 

 of a haploid with a diploid stage. 



Metakinesis. — Metaphase. See Mitosis. 



Metaphase. — The stage of mitosis (or meiosis) during which 

 the chromosomes lie in the equatorial plane of the spindle. 

 See Meiosis and Mitosis. 



Metaphase Pairing Index. — The proportion of metaphase 

 cells in which two particular chromosomes have paired, to the 

 total number of cells examined (cf. Tobgy). 



Metaplasia. — The production by the cells of one tissue, of a 

 different tissue. 



Metaplasm. — A general term for non-living inclusions in the 

 protoplasm. 



Metasyndesis. — Parasynapsis ; the side-by-side conjugation 

 of chromosomes at zygotene. 



Metaxenia. — The influence sometimes exerted by pollen on 

 the maternal tissues of a fruit, cf. Xenia. 



Methyl Testosterone. — See under Androgens. 



Metoestrus. — The period following oestrus. 



Microcentrum. — The central part of the astral system. 



Microchromosomes. — Originally a pair of fragment chromo- 

 somes in the Heniiptera characterized by much delayed synap- 

 sis. Now often used of any abnormally small chromosomes. 

 M-chromosomes. 



