Z-chromosomes. — See W-chromosomes ; X-chromosomes. 



Zoidiophily. — Pollination by means of animals. 



Zoogamete, — A motile gamete. 



Zoogamy. — Sexual procreation in animals. 



Zoogonous. — Viviparous ; giving birth direct to live young, as 

 opposed to the production of eggs. 



Zoogony. — Vivipary. 



Zoophily. — Pollination by animals. 



Zygonema. — The chromosome thread during the zygotene 

 phase, vi^hen the chromosomes have come together in pairs. 



Zygosis. — Conjugation. 



Zygosome. — A newly fused chromosome pair produced by 

 syndesis (zygotene) ; mixochromosome. 



Zygotaxis. — Attraction as between gametes of opposite sex. 



Zygote. — The cell which results from gametic fertilization 

 and, by extension, the individual which eventually grows from 

 this cell. 



Zygotene. — (i) The stage in the prophase of meiosis during 

 which homologous chromosomes come together in pairs, (ii) 

 The paired chromosome threads at this stage. 



Zygotic Incompatibility. — A form of homomorphic incom- 

 patibility iq.v.) in which the incompatibility reaction depends 

 directly on the genetic constitution of the zygote producing 

 the male gamete, rather than that of the male gamete itself, 

 together with the constitution of the female zygote, (cf. 

 Mather), cf. Gametic Incompatibility. 



Zygotic Lethal. — A lethal factor whose effect is only apparent 

 in the embryo, larva or adult as distinct from a Gametic 

 Lethal which renders inviable any gamete which carries it. 



Zygotic Meiosis. — Meiosis which takes place in the first two 

 divisions after fertilization, at the beginning of the life cycle 

 of an organism; initial meiosis; zygotic reduction. 



Zygotic Mutation. — A mutation occurring in the zygote 

 shortly after fertilization. 



