Pj. — The first parental generation, i.e. the parents of an F^; 

 Pg the grand-parents. 



Pachynema. — The chromosome thread at pachytene. 



Pachytene. — The third stage in the prophase of meiosis (q.v.) 

 during which the paired zygotene chromosomes each split 

 longitudinally into chromatids so that each bivalent of two 

 chromosomes is composed of four chromatids. 



Paedogamy. — Fertilization by gametes derived from the same 

 parent cell following many nuclear divisions. 



Paedogenesis. — Reproduction, either sexual or asexual, by 

 larval animals or by individuals which are not adult; neoteny. 



Paedomorphosis. — The process whereby a new form of 

 adult organization is produced which is endowed with high 

 potential for further evolution. A form of evolution in which 

 some of the more important steps have resulted from novelties 

 which first manifested themselves in early stages of develop- 

 ment of the ancestors (cf. de Beer). 



Pairing of Chromosomes. — (Active) the coming together 

 of chromosomes at zygotene or (passive) the continuance of 

 their association at the first metaphase of meiosis (Darling- 

 ton). Somatic . The lying of homologous chromosomes 



especially close to one another at metaphase of mitosis. (Dar- 

 lington). Secondary . The side-by-side association of 



bivalent chromosomes at meiosis. Zygotene Pairing. — See 



under Meiosis, Heterogenetic Pairing, Homogenetic , 



Homosynapsis. 



Pairing Segment. — (Of a sex chromosome). The portion of 

 a sex chromosome which is duplicated in the dissimilar chro- 

 mosome-partner. See under Partial Sex Linkage. 



Palaeogenetic. — Pertaining to the persistence of larval or 

 embryonic characters into adult life; neotenous. 



Palaeogenetics. — The genetic interpretation of phenomena 

 observed in fossils. 



Palingenetic Stages. — The stages which, during the de- 

 velopment of an animal from the egg, recapitulate the history 

 of the race. 



