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GLOSSARY 



OVOVIVIPARITY - condition in which egg con- 

 tains enough yolk to carry the embryo to 

 hatching. After this stage the larva is 

 liberated from the maternal organism with- 

 out receiving further nourishment. 



OVULATION - the release of eggs from the 

 ovary, not necessarily from the body. 



PAEDOGENESIS - relative retardation of the 



development of body structures as compared 

 with the reproductive organs; reproduction 

 during larval stage; precocious sex develop- 

 ment. 



PAEDOMORPHOSIS - introduction of youthful 

 characters into the line of adults. 



PALINGENETIC - term used for repeated or re- 

 capitulated stages which reflect the history 

 of the race (Haeckel). 



PARABIOSIS - lateral fusion of embryos by in- 

 juring their mirror surfaces and approx- 

 imating them so that they grow together 

 (see telobiosis). 



PARTHENOGENESIS - development of the egg 

 without benefit of spermatozoa; develop- 

 ment stimulated by artificial means. 



PARTHENOGENESIS, ARTIFICIAL - activation 

 of an egg by chemical or physical means 

 (e. g. , butyric acid, hypertonic solutions, 

 irradiation, needle prick, etc. ) 



PARTHENOGENESIS, FACULTATIVE - eggs 

 normally fertilized before development, 

 may, on occasion, develop when fertiliza- 

 tion is delayed before sperm penetration. 



PARTHENOGENESIS, NATURAL - maturation 

 of the egg leads to development without the 

 aid of spermatozoa (e. g. , some insects). 



PARTITION-COEFFICIENT - the factor which 

 determines the size of any part at any time 

 by parcelling out materials; relative capa- 

 city for various parts of the embryo to 

 absorb food from a common supply at dif- 

 ferent times. Such coefficients are expres- 

 sions of intrinsic growth potentials, so 

 balanced in normal development that no 

 single structure can monopolize the nutri- 

 ment to the detriment of other structures. 



PARTHENOGENETIC CLEAVAGE - fragmen- 

 tation 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 sperm 

 of the amphibia generally leaving a trail of 

 pigment taken in from the surface coat. 



PATH, PENETRATION - the path of the sperm 

 as it enters the egg before it veers into the 

 copulation path. 



PATHFINDERS - pioneering nerve fibers which 

 assume the task of growing into the unin- 

 vaded peripheral tissues (Weiss). 



PENETRANCE - the degree to which a group of 

 organisms expresses the presence of a 

 gene. (See Expressivity. ) 



PERIBLAST, CENTRAL - cells of syncitial na- 

 ture beneath and separate from the blasto- 

 derm of fish. 



PERIBLAST, MARGINAL - cells of syncitial 

 nature bounding the central blastoderm of 

 the fish or chick. 



PERMEABILITY - property of a membrane in- 

 dicated by the rate at which substances pass 

 through, the phenomenon involving four at- 

 tributes of mass, area, time, and concentra- 

 tion as 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 

 reciprocal of the hydrogen ion concentration 

 in gram-mols per liter. The negative value 

 of the power of 1 equivalent to the concen- 

 tration of hydrogen ions in gram-molecules 

 per liter. The neutral solution (neither 

 acidic nor basic) has a pH value of 7: pH 

 values less than 7 are acid and those more 

 than 7 are alkaline. 



PHENOCOPY - the imitation of a particular gene- 

 type by response to physiological factors in 

 the environment, but carrying no hereditary 

 implication. 



PHENOCRITICAL PERIOD - the period in the 



development of an organism when a particular 

 gene effect can be most easily influenced by 

 environmental factors. 



PHENOTYPE - the expressed genetic influences. 



PHOCOMELUS - failure of proximal portion of 

 appendages to develop, distal parts may be 

 normal. 



PHYLOGENY - 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 distinguishable region of mosaic eggs 

 which gives rise to later and specific organ 

 development. 



PLASMALEMMA - the outermost, thin, viscous 

 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 (Feul- 

 gen and Voit, 1924). It is not specific, how- 

 ever, as positive reactions are given by cer- 

 tain alkalis, aliphatic ketones, some unsat- 

 urated compounds (e. g. , oleic acid), weak 

 salts of strong bases (e. g. , acetates and 

 phosphates), some amino oxides and certain 

 catalytic oxidizing systems. 



PLASMODEMS - fine protoplasmic threads (pre- 

 sumably) connecting cells mitotically derived 

 from a parent cell; used in connection with 

 nnarginal cells in the blastodisc of fishes and 

 birds. 



PLASMODESMATA - protoplasmic bridges 



claimed (Paton, 1907) to be the means of 



