GLOSSARY OF EMBRYOLOGICAL TERMS 295 



Ovogenesis — See Oogenesis. 

 Ovogonia — See Oogonia. 



Ovulation — release of egg from ovary, not necessarily from body. 

 Ovum — Latin for egg. 



Pachytene — stage in maturation when allelomorphic pairs of chromosomes 

 are fused (telosynapsis or parasynapsis) so as to appear haploid, during 

 which process crossing over may occur; stage just prior to diplotene. 

 Term means thick or condensed. (Syn., diplonema.) 



Paedogenesis — reproduction during larval stage; precocious sex develop- 

 ment. 



Pancreas — digestive and endocrine glands arising as single posterior and 

 single anterior primordia in vicinity of liver. 



Parthenogenesis — development of an egg without benefit of spermatozoon. 



Parthenogenesis, Artificial — initiation of development of an egg by artificial 

 means. 



Parthenogenesis, Natural — maturation of eggs of some forms leads directly 

 to development without aid of spermatozoa. 



Parthenogenetic Cleavage — fragmentation of protoplasm of old and un- 

 fertilized chick eggs, originally thought to be true cleavage. 



Path, Copulation — See Copulation Path. 



Path, Penetration — initial direction of sperm entrance into egg, often shift- 

 ing toward egg nucleus along a new copulation path. (Syn., entrance 

 path.) 



Perforatorium — See A crosome. 



Pericardial Cavity — cavity or membrane sac which encloses heart, repre- 

 senting a cephalic portion of coelom within embryonic body. (Syn., 

 parietal cavity.) 



Pericardium — thin mesodermal membrane which encloses pericardial cav- 

 ity and heart. 



Perichondrium — mesenchymal layer immediately around forming cartilage. 



Perichordal Sheath — thin, mesodermal (sclerotomal), continuous sheet of 

 tissue immediately around notochord. 



Periosteum — mesenchymal layer, often originally perichondrium, which 

 will be found immediately around forming bone. 



