298 GLOSSARY OF EMBRYOLOGICAL TERMS 



Primitive Groove — groove through center of primitive streak, bounded by 

 primitive folds and terminated anteriorly by primitive pit and pos- 

 teriorly by primitive plate. 



Primordial Germ Cells — diploid cells which are destined to become germ 

 cells (e.g., oogonia and spermatogonia). (Syn., primitive germ cells.) 



Primordium — See Anlage. 



Proctodeum — ectodermal pit in region of future cloaca which invaginates to 

 fuse with hindgut endoderm to form anal or proctodeal plate, later to 

 rupture and form anus. 



Pronephric Capsule — mesodermal connective tissue covering of pronephric 

 masses derived from adjacent myotomes and somatic mesoderm. 



Pronephric Chamber — portion of amphibian coelomic cavity open ante- 

 riorly and posteriorly but closed ventrally by development of lungs. 



Pronephric Duct — outer portion of pronephric nephrotomes which de- 

 velops a lumen connected posteriorly with mesonephric or Wolffian 

 duct. (Syn., segmental duct.) 



Pronephric Tubules — lateral outgrowths of the most anterior nephrotomal 

 masses which acquire cavities in amphibia, connected with pronephric 

 duct. Possibly become infundibulum of oviduct. 



Pronephros — embryonic kidney of all vertebrates, extending from second 

 to fourth somites of frog embryo and consisting of as many primitive 

 tubules as somites concerned; completely lost in all adult vertebrates 

 except a few bony fish. (Syn., head kidney.) 



Pronucleus — egg nucleus after polar body formation and sperm nucleus 

 after entrance of spermatozoon into egg. 



Prophase — first stage in mitotic cycle when spireme is broken up into 

 definite chromosomes, prior to lining up on metaphase (equatorial) 

 plate. 



Prosencephalon — See Forebrain. 



Prosocoel — cavity of prosencephalon. 



Proximal — nearer the point of reference, toward main body mass. 



Pupil — opening into secondary optic vesicle, occluded in part by lens, and 

 regulated in diameter by ciliary muscles of iris. 



Ramus Communicans — connection between sympathetic ganglion and 

 spinal nerve, as numerous as ganglia in any vertebrate; probably orig- 

 inating from crest cells. Ramus means branch. 



