Polytypic — 121 — Progamic Sex Determination 



Pregnancy. — Gestation; the period between conception (fer- 

 tilization) and the birth of the offspring. 



Prehensile Flowers. — Flowers whose insect visitors grasp 

 the style and stamens so as to cover their venters with pollen 

 and so effect pollination (Carpenter). 



Preheterokinesis. — A form of meiotic division characterized 

 by the sex chromosome passing undivided to one pole in the 

 first spermatocyte division, cf. Postheterokinesis. 



Premunition. — Disease tolerance resulting from latent in- 

 fection occurring early in the life of the individual. 



Prepotence. — (i) The capacity, in a sire, of being able to 

 transmit his characteristics to his offspring. The term can 

 also be used of the dam but more commonly refers to the sire, 

 (ii) The capacity of a particular type of pollen to fertilize 

 the ovules of any specific plant despite simultaneous competi- 

 tion from the plant's own pollen. 



Pre-reduction. — See under Post-reduction. 



Presence and Absence Theory. — The theory that dominance 

 indicates the presence of the particular gene concerned and 

 recessiveness its absence. 



Pressure. — The force with which selection is working on an 

 organism e.g. low or high predator — , competition — , selec- 

 tion — , disease — . 



Primary Constriction. — Centromere, q.v. 



Primary Mutant. — A plant with one extra normal chromo- 

 some, i.e. a trisomic plant. 



Proanthesis. — The production of flowers before the normal 

 flowering season. 



Probability. — See Progeny in Appendix 1. 



Probable Error. — This is equal to the variance (Standard 

 error) multiplied by 0.67449. 



Prochromocentric Heterochromatin. — The heterochromatin 

 on either side of the centric constriction. 



Pro-chromosomes. — Separate masses of basichromatin in 

 the resting nuclei, or in the presynaptic stages, supposed to be 

 forerunners of the definitive chromosomes or centres for their 

 formation (Overton; Wilson). That part of the hetero- 

 chromatin which is situated on either side of the centromeric 

 constriction (Levan). See Heterochromatin. 



Procryptic Coloration. — Coloration of a protective nature. 



Progamic Sex Determination. — Sex determination which is 

 dependent on the size or physiological state of the ova as 

 opposed to normal sex determination which is largely governed 

 by the heterosoraes. " 



