GLOSSARY 191 



Equatorial plane: plane that bisects at right angles the line connect- 

 ing the poles. 



Equipotential system: cell group, all parts of which have the same 

 potency. 



Exogastrulation: abnormal gastrulation, in the course of which the 

 archenteron evaginates, instead of invaginating. 



Explantation: the culturing of pieces of tissue outside the body. 

 Cf. implantation, transplantation. 



Extensive multiplicity: spatial multiplicity: a complexity of struc- 

 ture which is recognisably different in form or nature in differ- 

 ent regions of an organism or cell. Opp. : intensive multiplicity. 



False hybrid: hybrid formed by the fusion of an egg with a sperm 

 of another species, in which the nucleus of the latter does not 

 take part in development. 



Ferment: = enzyme. 



Fibroblast: young connective tissue cell. 



Follicle: multicellular covering of the egg in the ovary. 



Frontal: at right angles to the median plane, separating dorsal and 

 ventral halves. 



Gamete: germ cell. 



Gamone: fertilisation substance, formed by the gametes at fertilisa- 

 tion. Cf. androgamone, gynogamone. 



Gastrula: developmental stage, consisting of 2 (or 3) germ-layers. 



Gastrulation: development of a gastrula from a blastula. 



Gene: unit carrier of inheritable properties. Cf. determinants. 



Genotype: the inherited constitution of the individual. 



Germ-band: thickening of one or several germ-layers, from which 

 certain organs will develop. 



Germinal vesicle: the nucleus of the oocyte. 



Germ-layer: one of the cell-layers of the gastrula, each of which 

 will later develop into definite organs. 



Gradient: change of a scalar quantity from point to point. 



Gynogamone: fertilisation substance formed by the female germ 

 cells. Cf. androgamone, gamone. 



Haploid: containing a single set of chromosomes. This is the normal 

 condition in ripe germ cells. Cf. diploid, polyploid. 



Heterogeneous fertilisation: fertilisation by a sperm of another, 

 usually distantly related, species. 



Heteroplastic transplantation: transplantation into an individual of 

 another, closely related species. Cf. xenoplastic transplantation. 



Heterospermic merogony: development of a non-nucleated egg 

 fragment after fertilisation by a sperm from another species. 

 Opp.: homospermic merogony. 



Heterozygote : organism developing from a zygote formed by the 

 fusion of gametes that were dissimilar as regards the gene 

 under consideration. Opp.: homozygote. 



Histochemistry: the determination of the chemical composition of 

 cells and tissues. 



