hjk GLOSSARY 



PAETHENOGENETIC CLEAVAGE - fragmentation of protoplasm of old and unfertilized chick eggs, originally 

 thought to be true cleavage. 



PATH, COPULATION - path along which the pronuclei approach each other, the aperm of the amphlhia general- 

 ly leaving a trail of pigment taken In from the surface coat. 



PATH, PENETRATION - the path of the sperm as It enters the egg hefore It veers Into the copulation path. 



PATHFINDERS - pioneering nerve fibers which assume the task of growing Into the uninvaded peripheral 

 tissues (Weiss). 



PENOTRANCE - the degree to which a group of organisms expresses the presence of a gene. (See Expressivity.) 



PERIBLAST, CENTRAL - cells of ayncltlal nature beneath and separate from the blastoderm of fish. 



PERIBLAST, MARGINAL - cells of ayncltlal nature bounding the central blastoderm of the fish or chick. 



PERMEABILITY - property of a membrane indicated by the rate at which substances pass through, the 



phenomenon involving four attributea of maaa, area, time, and concentration aa well as the nature of 

 the environment. 



PFLUGER'S LAW - the dividing nucleus elongates in the direction of the least resistance. 



pH - method of stating the measure of the hydrogen ion concentration, expressed as the log of the recipro- 

 cal of the hydrogen ion concentration in gram-mols per liter. The negative value of the power of 10 

 equivalent to the concentration of hydrogen iona In gram-mDlecules per liter. The neutral solution 

 (neither acidic nor basic) has a pH value of J: pH values less than 7 are acid and those more than 

 7 are alkaline. 



PHENOCOPY - the Imitation of a particular genetype by response to physiological factors In the environ- 

 ment, but cariylng no hereditary implication. 



PHENOCRITICAL PERIOD - the period in the development of ein organism when a particular gene effect can be 

 most easily influenced by environmental factors. 



PHENOTYPE - the expressed genetic Influences. 



PHOCCMELUS - failure of proximal portion of appendages to develop, diatal parts may be normal. 



PHYLOGEIiy - series of stages in the history of the race; the origin of phyla. 



PLACODE - plate or button- like thickening of ectoderm from which will arise sensory or nervous structures 

 (e.g., olfactory placode). 



PLANE - (See "section".) 



PLASM - a dlatinguiahable region of mosaic eggs which gives rise to later and specific organ development. 



PLASMATiT'WMA - the outermost, thin, vlacous layer of the ectoplasm in the fertilized egg which does not 

 change by centrifugation. 



PLASMAL REACTION - related to the presence of fat and aldehydes in the cytoplasm (Feulgen and Voit, 192'*). 

 It is not specific, however, as positive reactions are given by certain alkalla, aliphatic ketones, 

 some unsaturated compounds (e.g., oleic acid), weak salts of strong bases (e.g., acetates and phos- 

 phates), some amino oxldea and certain catalytic oxidizing aystems. 



PLASMODEMS - fine protoplasmic threads (presumably) connecting cells mitotically derived from a parent 

 cell; used in connection with marginal cells in the blaatodiac of fishes and birds. 



PLASMODESMATA - protoplasmic bridgea claimed ( Paton, 1907) to be the means of nerve fiber growth; plas- 

 modeamata supposedly incorporated into the substance of the axone during its origin. 



PLASMONUCLEIC ACID - one of the two types of nucleic acid, this one occurring in the cytoplasm, , in the 

 plasmosome (nucleolus), and possibly in minute quantities in the chromatin (Pollister & Mlrsky, 

 I9I1IH Nature 155:711). (See chromonuclelc acid.) 



PLASTENS - (See mitochondria.) 



PLASTICITY - the ability of early cell areas (tissues) to conform to environ mental Influences, such 

 plasticity disappearing at the end of gaatrulatlon. Syn. , plurlpotency . 



PLASTIN - thread-like structural elements of the cytoplasm which form a gel framework by net formation. 

 (Frey-Wysaling) 



PLATEAU'S LAWS - not more than three (3) planes can meet at any one edge and not more than four edges can 

 meet at any one point. Reference is made to cleavage planes. 



PLEICTROPISM - multiple effects of a single gene due to effects upon metabolism. 



PLURIPOTENT - condition \rtiere cell or embryonic area la amenable to several courses of differentiation. 

 An undetermined state. 



POIKILOPLOID - variable chromosome number. 



POUCILCTHEHMOUS - cold-blooded; animals which depend upon the environment to regulate their body tempera- 

 ture. Animals lack temperature regulating mechaniama. (E.g., amphibia, fish). Opposed to 

 homol othermous . 



POLAR FURROW - space between blastomeres of Uo cell stage due to shifting of the mitotic axes in each of 

 the blastomeres, generally associated with spiral cleavage. 



POLARITY - strateficatlon; axial distribution; assumption that behind any visible differences in the egg 

 (cell or embryo) there is an invisible arrangement of some (imagined?) basic material. The type of 

 polarity may be Inherent, predetermined, while the direction of polarity may be conditioned by the 

 environment. Related to the animal-vegetal and anterior-posterios axes. (See gradient.) Syn., 

 Schl cktungspolari tat . 



POLAR LOBE - lobe which remains attached to one blastomere into which it is periodically withdrawn during 

 the Intervals between mitoses, and which gives rise to the entomesoblaat and hence to mesoderm. Also 

 "yolk lobe," although this lobe may actually to devoid of yolk. 



POLAR PLASM - In detemdnflte cleavage (e.g., annelid and mollusc eggs) some of the vegetative pole proto- 

 plasm may be identified in early blastomeraa by Ita particular conaistency. This may be the material 

 of the polar pole. 



