Male Pronucleus — 93 — Meiosis 



Mean. — The sum of a group of observations divided by the 

 number in the group. 



Mean-square Error. — See Appendix 1. 



Mediastinum Testis. — The central core of a testis. 



Mediocentric. — Having a median or sub-median centromere. 



Mega-evolution. — Macro-evolution, q.v. 



Megagamete. — Macrogamete ; the larger of two conjugating 

 gametes (typically the female). 



Megaheterochromatic. — A species or individual which dif- 

 fers from its group in having more heterochromatin. 0pp. 

 Microheterochromatic. 



Meganucleus. — Macronucleus; the larger of two nuclei in 

 a cell; the "vegetative" nucleus. 



Megasporangia. — Ovules, q.v. 



Megaspore. — The larger type of spore in heterosporous higher 

 plants. This, in the seed plants, gives rise to the embryo-sac. 



Megasporocyte. — The diploid cell in the ovary from which 

 the four haploid megaspores are formed — the embyro-sac 

 mother-cell. 



Meiocyte. — A cell whose nucleus has begun to divide by 

 meiosis. 



Meiomcry. — The state of having fewer than the normal 

 number of parts. Adj. Meiomeric. cf. Pleiomery. 



Meiosis. — A form of mitosis in which the chromosome com- 

 plement of the nuclei is reduced from the diploid (2n) number 

 to the haploid (n). Typically, meiosis consists of the following 

 stages : — 

 1st Division. 



(i) Prophase, (o) Leptotene: the chromosomes appear as 

 long, fine, single, unpaired threads 2n in number (6) Zygo- 

 tene: homologous chromosomes come together in pairs to 

 form n bivalents (c) Pachytene: each member of each paired 

 zygotene chromosome splits longitudinally into two chromatids 

 which remain closely associated {d) Diplotene: the four 

 chromatids (= one bivalent) formed in pachytene, move 

 apart in two pairs but these two pairs remain adherent in the 

 region of chiasmata {e) Diakinesis: Pronounced shortening 

 and thickening of the chromosomes now occurs, 

 (ii) Metaphase. The thickened chromosomes become ar- 

 ranged in the equatorial plane. 



(iii) Anaphase. Repulsion between centromeres (spindle at- 

 tachments) forces the chromosomes apart but the chromatids 

 remain together, one pair from each bivalent going towards 

 each pole. 



