194 GLOSSARY 



Pharynx: part of the intestine, immediately caudal to the mouth 



cavity. 

 Physiology: the causal science of the phenomena of life. 

 Placode: thickening of the ectoderm. 

 Plasmagene: corpuscle in the cytoplasm which in certain respects 



corresponds to the genes in the nucleus. 

 Pluteus: larva of a sea urchin or brittle star, with a calcareous 



skeleton consisting of radially protruding rods. 

 Polar bodies: small cells expelled from the egg in the course of its 



maturation divisions. 

 Polarity: condition of the body in which an axis connecting two 



different poles can be distinguished. 

 Polyploid: containing more than two sets of chromosomes. Cf. tri- 



ploid, pentaploid. 

 Polyspermy: the penetration of several sperms into a single egg. 



Cf. dispermy, monospermy. 

 Postpharyngeal: caudal to the pharynx. Opp.: prepharyngeal. 

 Posttrochal: basal to the band of cilia in the trochophore larva. 



Opp.: pretrochal. 

 Potency: power of development. 

 Predetermination: first phase in the process of determination, during 



which the fixation of the fate of the cells is still labile. 

 Preformation: presence in the egg of the structure that will become 



unfolded during the development of the embryo. 

 Prepharyngeal: rostral to the pharynx. Opp.: postpharyngeal. 

 Pretrochal: situated above the band of cilia in the trochophore larva. 



Opp.: posttrochal. 

 Pronucleus: nucleus of a gamete. 



Protoplasm: the substance of which living cells consist. 

 Reaction system: cell group reacting to an induction with a definite 



developmental process. 

 Reductional division: nuclear division in the course of which the 



chromosome number is halved. 

 Regenerate: cell group formed by regeneration for the restitution 



of lost organs or parts of the body. 

 Regeneration: the reformation or restitution of lost organs of parts 



of the body. 

 Regulation: in biology, this term is applied in particular to processes 



that lead to a return to the normal situation after a disturbance. 

 Reorganisation: restitution by transformation or shifting of material. 



Cf. Morphallaxis. 

 Rostral: pertaining to the front end. 



Scalar quantity: quantity to which a numerical value can be attri- 

 buted, but which is not directional. 

 Somatoblast: cell from which an important part of the body of the 



embryo will arise. 

 Sperm aster: aster in the egg cytoplasm arising from a sperm that 



has penetrated into the egg. 



