368 ANIMAL BIOLOGY. 



of mammals. Embryonic or foetal appendages are provisional 



structures (such as the allantois and amnion) in connection with 



embryonic or foetal life. 

 Eucystment. A phenomenon occurring among lowly organisms when they 



become motionless and are surrounded by a coat, case, or cyst. 

 Endoderm. The inner layer of the hydra as opposed to ectoderm. Endo- 



plasm or endosarc, the inner substance of a protozoon, as opposed to 



ectoplasm or ectosarc, its outer substance. Endoplast, a term applied 



to the nucleus of the protozoa. 

 Epibole. The overgrowth of epiblast cells over the other cells of the 



segmented ovum, as in the case of the frog. 

 Evagination. See Invagination. 

 Eversion. A term applied to the opening out and expansion of the 



peristome of the vorticella. 

 Excretion. The process by which waste products are got rid of or expelled 



from the body. 



Facet. (1) A smooth surface of bone on which another bone works, e.g. 



tubercular facet of a vertebra ; (2) the polygonal areas of the insect's 



or crustacean's eye. 

 Femur. (1) The thigh of a vertebrate ; (2) the thigh-bone ; (3) the first 



joint in the leg of a cockroach. 

 Fertilisation. The union of a spermatozoon with the ovum. The process 



is often called impregnation. 

 Fenestra ovalis and Rotunda. Membranous "windows" in the periotic 



bone in connection with the organ of hearing. In the fowl the two 



often open externally into a fenestral recess. 

 Fission. The splitting of a cell or a lowly organism into two more or 



less similar parts. The cleavage of the ovum is a special form of 



fission. 



Foetus. Foetal membranes. See Embryo. 

 Folding off of the embryo. The process by which the embryo (e.g. fowl) 



is constricted from the yolk-sac. 

 Fontanelle. A space occupied by membrane. 

 Foramen. A hole in the skull or in a bone through which nerves or 



blood-vessels pass. 

 Fronto-parietal. A bone of the frog's skull, also called par ieto -frontal. 



Ganglion. A group of nerve-cells, e.g. Gasserian ganglion, sympathetic 

 ganglia. 



Gastrula. The two-layered condition of the embryo, resulting from a 

 pushing in or invagination of one portion of the blastosphere. 



Germinal layers. The cell-layers of the embryo, hypoblast, epiblast, and 

 mesoUast. Germinal vesicle, the nuclear portion of the ovum con- 

 taining nucleolar germinal spots. The germinal disc is a thilkened 



