GLOSSARY, 



animals in which the primary body-layers can still be obviously 

 distinguished in the adult ; e.g. hydra, jelly-fish. In the higher 

 animals they are so folded and complicated that this is difficult. 

 Egg. — The name given to the structure in which the young of many 

 animals are inclosed. An egg contains the germ, i.e. ovum, or 

 egg-cell, with food-yolk to supply it with nourishment during 

 growth, and a protective covering. In the hen's egg this last 

 consists of albumen (white of egg) supplemented by a hard shell ; 

 in some fishes' eggs the " shell " is leathery. The egg-cell or 

 ovum is sometimes shortly referred to as the egg. 



Endoderm (Gk. IvSov, within, and Sip/j,a, skin). — The name given 

 to the inside layer of the body when the outside is called 

 ectoderm. 



Endoplast (Gk. Mov, within, and TrXdoros, what has been formed or 

 moulded, i.e. a shape). — Literally, a structure or shape seen 

 inside an organism. An old name for the nucleus, still met 

 with in descriptions oiE certain unicellular organisms. 



Enteron (Gk. hrepov, in plural the intestines). — The digestive 

 cavity of the Calenteraia, which is not differentiated into body- 

 cavity and alimentary canal. 



Epiblast (Gk. iirl, upon, i.e. at the top of, and /SXdo-Tos, germ). — The 

 outer of the three "germinal layers," from which the skin and 

 the nervous system and sensory organs are developed. 



Evolution (Lat. evolutio, unrolling, e.p. of a scroll). — The gradual 

 unfolding or opening out of something previously hidden, packed 

 away in small compass ; hence, especially, the process of gradual 

 growth by which the more complex types of living beings are 

 believed to have originated from the more simple ones. 



Germinal Layers. — The embryo or germ, e.g. of a chick, is formed 

 of three layers, called the germinal layers. See p. 30. 



Histology (Gk. io-ros, from iVxTj/ii, to set up — the upright beam of 

 a loom, hence the warp, that was fixed to it, and the whole web ; 

 and \6yos). — The study of the tissues of living bodies (see Tissue). 



Homologous (Gk. 0/165, like, and \6y05), agreeing with. — Organs 

 derived from the same part of the body are said to be homo- 

 logous with one another, although they may be quite different in 

 appearance and in use. Example : the lungs of air-breathing 

 vertebrates are the homologue of the swim-bladder of the fish, 

 each being originally a cavity branching from the inside of the 

 throat. 



Hypoblast (Gk. i/tto, under, and pKiuros, germ). — The lower, or inner 

 of the three "germinal layers," from which is derived the 

 glandular lining of the alimentary canal and of its appended 

 glands. 



Individual, — In the case of colonial or compound organisms, such as 

 those occurring among the Ccelenterata, it is usual to speak of 

 the whole stock of the colony, including all the different nnits> 

 that have been produced by budding, as one " individual.'.' , .The 



