A. — Angstrom, 0.0000001 mm. 



AI, All. — The first and second anaphases, respectively, in 

 meiosis. 



Aberration. — An irregularity in chromosome distribution 

 during heterotypic or homotypic cell division (Winters). 



Abiogenesis. — Spontaneous generation of living organisms 

 from non-living matter. 



Abiogeny. — Abiogenesis, q.v. 



Abortion. — A miscarriage; arrested development of an organ. 



Acarpous. — Devoid of fruit. 



Acceleration. — The speeding-up of the time of action of a 

 gene so that the character it controls develops earlier in the 

 life cycle than it did in ancestral forms. 



Accessory Chromosomes. — Sex chromosomes, q.v. See, 

 also, under W-, X- and Y-chromosomes. 



Accidental Evolution. — Evolution which confers no selec- 

 tive advantage and which owes its origin to mutations of 

 more or less neutral effect. 



Acclimation. — Adaptation to climatic change on the part 

 of the individual. The physiological adjustment or increased 

 tolerance shown by an individual organism to a change in the 

 surrounding environment. Sec Acclimatization (Carpenter). 



Acclimatization. — The adjustment or increase in tolerance 

 shown by a species in the course of several generations in a 

 changed environment. See Acclimation (Carpenter). 



Acentric. — Lacking a centromere. 



Acentric-dicentric Translocation. — Aneucentric transloca- 

 tion, q.v. 



Acentric Inversion. — An inversion of a segment of a chromo- 

 some which does not involve the centromere, cf. Paracentric 

 Inversion. 



Achievement Quotient. — The "educational age" divided by 

 the "mental age". (The child is assigned to an "educational 

 age" on the basis of tests made on subjects taught in school; 

 for "mental age" see under Intelligence Quotient). 



