296 GLOSSARY OF EMBRYOLOGICAL TERMS 



Peristomiai Mesoderm — mesoderm of amphibian gastrula derived from 

 (ventral) lips of blastopore. Opposed to gastral mesoderm. 



Peritoneal cavity — body cavity (coelom). 



Peritoneum — coelomic mesothelium of abdominal region reinforced by 

 connective tissue. 



Perivitelline Membrane — See Vitelline Membrane. 



Perivitelline Space — space between vitelline (fertilization) membrane and 

 contained egg, generally filled with a fluid. 



Pfliiger's Law — dividing nucleus elongates in direction of least resistance. 



Phenotype — outward appearance of an organism regardless of its genetic 

 make-up. Opposed to genotype. 



Pigment Layer of Optic Cup — thin outer wall of primary optic cup, poste- 

 rior to retina, which never fuses with rods and cones of retina. 



Pineal — See Epiphysis. 



Pituitary — See Hypophysis. 



Placode — Plate-like thickening of ectoderm from which arise sensory 

 or nervous structures (e.g., olfactory placode). 



Plane — imaginary two-dimensional surface; may be frontal, sagittal, trans- 

 verse, median, or lateral. 



Plasmosome — a true nucleolus. (See Nucleolus.) 



Plectrum — See Columella. 



Plexus Choroid — Vascular folds in roof of prosencephalon, diencephalon, 

 and rhombencephalon. 



Poikilothermal — cold-blooded; animals whose body temperatures are sub- 

 ject to environmental changes because they lack regulating mechanisms. 

 Opposed to homoiothermal. 



Polar — pertaining, in most cases, to animal pole, although may refer to 

 vegetal pole, or both. 



Polar Body — relatively minute, discarded nucleus of maturing oocyte 

 (generally three). (Syn., polocytes.) 



Polarity — axial distribution of component parts; animal and vegetal poles; 

 stratification. 



Pole, Animal — region of egg where polar bodies are eliminated; ectoderm 

 forming portion of pre-cleaved egg. (Syn., apical or animal hemi- 

 sphere.) 



