GLOSSARY 



491 



nerve fiber growth; plasmodesmata sup- 

 posedly 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 (PoUister & Mirsky, 1944: Na- 

 ture 153:711). (See chromonucleic acid.) 



PLASTENS - (See mitochondria. ) 



PLASTICITY - the ability of early cell areas 

 (tissues) to conform to environmental in- 

 fluences, such plasticity disappearing at 

 the end of gastrulation. Syn. , pluripotency. 



PLASTIN - thread-like structural elements of 

 the cytoplasm which form a gel framework 

 by net formation. (Frey-Wyssling) 



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. 



PLEIOTROPISM - multiple effects of a single 

 gene due to effects upon metabolism. 



PLURIPOTENT - condition where cell or em- 

 bryonic area is amenable to several courses 

 of differentiation. An undetermined state. 



POIKILOPLOID - variable chromosome number. 



POIKILOTHERMOUS - cold-blooded; animals 

 which depend upon the environment to reg- 

 ulate their body temperature. Animals 

 lack temperature regulating mechanisms. 

 (E. g. , amphibia, fish). Opposed to homo- 

 iothermous. 



POLAR FURROW - space between blastomeres 

 of 40 cell stage due to shifting of the mitotic 

 axes in each of the blastomeres, generally 

 associated with spiral cleavage. 



POLARITY - stratefication; axial distribution; 

 assumption that behind any visible differ- 

 ences in the egg (cell or embryo) there is 

 an invisible arrangement of some (imag- 

 ined?) basic material. The type of polar- 

 ity 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. , Schicktungspolaritat. 



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 withdrawn during the intervals between 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 ento- 

 mesoblast and hence to mesoderm. Also 

 "yolk lobe, " although this lobe may actually 

 to devoid of yolk. 



POLAR PLASM - in determinate cleavage (e. g. , 

 annelid and mollusc eggs) some of the veg- 

 etative pole protoplasm may be identified 

 in early blastomeres by its particular con- 

 sistency. This may be the material of the 

 polar pole. 



POLE, ANIMAL - the protoplasmic portion of 

 a telolecithal egg from which the polar 



bodies are given off, in which the germinal 

 vesicle is found, and which has the highest 

 metabolism and gives rise to the principal 

 parts of the nervous system and sense or- 

 gans. Region of least yolk concentration. 

 Syn. , apical pole or hemisphere (See an- 

 imalization). 



POLE, VEGETAL - region of the egg opposite 

 the animal pole; region of lowest metabolic 

 rate; pole with greatest density of yolk in 

 telolecithal eggs, generally the endoderm 

 forming portion of the early egg. (See 

 vegetativisation. ) 



POLYDACTYLY - extra digits in hands or feet; 

 in man probably inherited. 



POLYEMBRYONY - natural isolation of blasto- 

 meres leading to the production of multiple 

 embryos; development of several embryos 

 from a single zygote. 



POLYHYDRAMNIOS - condition where the am- 

 niotic fluid exceeds two liters. 



POLYPLOID - possessing a multiple number of 

 chromosomes, such as triploid (3 times 

 the haploid number) tetraploid (4 times the 

 haploid), etc. Always more than the normal 

 diploid number of the typical zygote. (Wink- 

 ler, 1916.) 



POLYPLOIDOGEN - a chemical substance which 

 brings about the polyploid condition, usually 

 by inhibiting certain phases of nuclear divi- 

 sion. 



POLYSPERMY - entrance into the egg of more 

 than a single sperm, normally (e. g. , chick 

 and urodele) or under pathological condi- 

 tions (e. g. , Anura, Echinodermata, Mol- 

 lusca, etc. ) (Hertwig, 1887;Boveri, 1907; 

 Herlant, 1911). Normal polyspermy is 

 sometinnes called "physiological polyspermy' 

 while the abnormal is pathological, brought 

 about by chemical or physical conditions 

 (see Clark, 1936. Bio. Bull.). 



POST-GENERATION - regeneration out of newly 

 formed rather than already differentiated 

 tissues; restoration of parts of the embryo 

 by utilization of materials (unused) from an 

 injured (cauterized) blastomere (Roux). 



POTENCY - ability to develop embryologically; 

 capacity for completing destiny; ability to 

 perform an action; "future development 

 verbally transformed to an earlier stage 

 (Waddington). The test of potency is actual 

 realization in development. It is not the 

 same as competence. It is an explanatory 

 rather than a descriptive term (Roux, 1892) 

 for developmental possibility. " "A piece of 

 an embryo has the possibility of a certain 

 fate before determination, and the power to 

 pursue it afterwards." (Needham, 1942.) 



POTENCY, ACTIVE - cases of self-differentia- 

 tion where potencies are realized in isola- 

 tion even without inductive forces (Bautz- 

 mann, 1929). 



