Knight — 134 — Dictionary 



Semi-apospory. — The production of unreduced spores as a 

 result of a pseudohomeotypic division (Fagerlind). 



Semi-dominance. — A reduced dominance characterized by 

 the production of an intermediate phenotype in individuals 

 heterozygous for the gene concerned; partial dominance. 



Semi-heterotypic Division. — A first meiotic division which, 

 through defective pairing, gives rise to a single nucleus — the 

 'restitution' nucleus. 



Semi-lethal Factors. — See under Lethal Factors. 



Seminal Vesicles. — Glands, located at the ends of the vasa 

 deferentia, but connected, each by a duct, to the urethra. The 

 seminal vesicles, at one time thought to act as a storage cavity 

 for semen, secrete a fluid which is believed to provide a medium 

 for transport of spermatozoa. 



Sepal. — An individual segment of the calyx. 



Sepalody. — The production of sepal-like structures in place 

 of petals. 



Separation Theory, Wagner's. — The hypothesis that the 

 formation of a real variety (Darwin's "incipient species"), 

 can succeed in nature only where some individuals can cross 

 the previous borders of their range and segregate themselves 

 for a long period from the other members of their species (cf. 

 Mayr, 1942). 



Septivalent. — See under univalent. 



Serotinous. — Late flowering. 



Sertoli Cells. — The "nurse" cells to which the spermatids 

 become attached whilst undergoing their development to 

 spermia, which later become spermatozoa. 



Service Period. — The period between calving and subse- 

 quent conception. 



Sesquidiploid. — A triploid produced by crossing a tetraploid 

 species with a diploid species. 



Set of Chromosomes. — The complement of chromosomes oc- 

 curring in a gamete. 



Sewal Wright Effect. — Non-adaptive differentiation into 

 different types due to "drift" (q.v.) followed by random fixa- 

 tion in small isolated populations. 



Sex Chromosome. — A chromosome which has no truly simi- 

 lar homologue in the heterozygous sex and which is closely 

 bound up with sex determination. See W-, X-, Y-chromosomes. 



Sex Controlled Inheritance. — Sex-limited, q.v. 



Sex Determination. — The genetic and allied phenomena which 

 during ontogeny determine the sex of the adult organism. 



Sex Glands. — Testes in the male and ovaries in the female. 



