GLOSSA/^Y. 



at its adult state, it is said to undergo metamorphosis. Insects 

 afford the most familiar example, but many other animals 

 undergo similar changes. 



Morphology (Gk. iwp^ri, form, and Uyos). — The study of the form 

 of animals. We study their form in order to draw conclusions as 

 to their relationships with one another, which are then expressed 

 by means of classification. It is merely a more new fashioned 

 name for comparative anatomy (see Anatomy) ; but its use is 

 closely associated with modern ideas regarding the inter-relation- 

 ship of different kinds of animals. 

 MoKPHOLOGicAii EQuiTiLEXT, i.e. homologue ; see Homologde. 

 The term morphology is also applied to the study of the form 

 of plants (Vegetable Morphology). 



Notochord (Gk. vQtos, back, and xop^i}, a string). — The round 

 string oJ tissue lying down the back of embryo vertebrates, under- 

 neath (ventrally to) the spinal cord. This structure, homologous 

 with the UBOCHOED of larval Ascidians, and with a similar dorsal 

 structure in Bakmoglossvs, is one of the proofs that the verte- 

 brates are descended from invertebrate ancestors. 



Nuclear Division. — See KAayoKisEsis. 



Nucleolus (diminutive of nucleus). — A small mass of firmer stuff 

 inside the nucleus of a cell, which is seen iu some cells, espe- 

 cially in those that are preparing for very active growth. 



Nucleus (Lat. nucleits, kernel, from mix, a nut). — The dark body 

 which is seen in the protoplasm of a cell. It takes up staining 

 more readily than the rest of the protoplasm, and is therefore 

 believed to consist of firmer material. 



Ontogeny (Gk. Svt-, at present existing, hence in the singular a being 

 or individual ; and yei'ea, birth). — The history of the development 

 of the Mdi'iidiiaJ, from the egg-cell onwards. The "ontogenetic 

 development" of any form of animal is spoken of in contra- 

 distinction to its " phylogenetic development." 



Organism (Gk. 6pyavoi>, an engine). — The machinery of a living 

 creature, by means of which its life is carried on. The machinery 

 remains when the life is gone, and a dead creature may still 

 be spoken of as an " organism." The simplest form of life 

 machinery is that of a single cell, and one cell only may compose 

 an " organism." 



Oviparous (Lat. ovum, an egg, and pario, to bring forth). — Produc- 

 ing the young from eggs. Examples : hen, frog, butterfly, snail. 



Phylogeny (Gk. ^OXoi/, a kind or race, and 7ej'ed, birth). — The process 

 of development by which any form of animal has been developed, 

 through progressive types, from its ancestral form. The way in 

 which the terms ontogeny and phylogeny are used can be best 

 explained by an example : the ontogenetic development of a frog 

 means its development from a tadpole to a frog ; but the phylo- 

 genetic development of the frog means its gradual evolution from 

 a fish-like ancestor, until it reached a stage in .which the adult 



