GLOSSARY 



679 



Mu-ta'tion (L. mutare, to change). A change 

 in a gene in a gamete which results in a new 

 hereditary variation (trait). Broadly used 

 to include all kinds of hereditary variations 

 resulting from gross chromosome changes. 

 Mutations may also occur in somatic cells, 

 but these are not hereditary unless the in- 

 dividual reproduces asexually. 



Mu'tu-al-ism (L. mutuus, exchange). An asso- 

 ciation of two species which is beneficial to 

 both of them. 



My'e-lin (Gr. myelos, marrow). The fatlike 

 substance surrounding the axis cvlinder of a 

 medullated nerve fiber. 



My'o-fi"bril (Gr. mys, muscle; L. fibrilla, a 

 small fiber). One of the numerous longitu- 

 dinal small fibrils contained within the pro- 

 toplasm of the muscle cell or fiber. 



My'o-neme (Gr. mys, muscle; nema, thread). 

 A type of contractile fibril in certain Pro- 

 tozoa. 



Nai'ad (L. naias, water nymph). An aquatic 

 gill-breathing nymph. 



Na'res ( L. naris, nostril ) . The openings of the 

 air passages, both external and internal, in 

 the head of a vertebrate. 



Na'sal (L. nasus, nose). Pertaining to the nose 

 or nostrils. 



Nat'u-ral par'the-no-gen"e-sis (L. natura, na- 

 ture. Gr. parthenos, virgin; genesis, origin). 

 Development of an egg without fertilization 

 during the normal life cycle of an animal, as 

 in aphids. 



Nat'u-ral se-lec'tion. The "selection" in nature 

 of those individuals or species best fitted to 

 survive in a particular environment; the 

 elimination of the "unfit." 



Nem'a-to-cyst (Gr. nema, thread; kystis, blad- 

 der). One of the stinging capsules found in 

 coelenterates; each is produced by a single 

 cell, a cnidoblast. 



Ne-phrid'i-o-pore (Gr. nephros, kidney; poros, 

 passage). The external opening of an ex- 

 cretory tubule or nephridium. 



Ne-phrid'i-um (Gr. nephros, kidney). A tubu- 

 lar excretory structure characteristic of 

 many invertebrates, such as the annelids. 



Neph'ro-stome (Gr. nephros, kidney; stoma, 

 mouth). The ciliated funnel-shaped opening 

 at the inner end of a nephridium through 

 which the liquid wastes enter the tubule. 



Neu'ral (Gr. neuron, nerve). Pertaining to the 

 ncr\ous svstcm. 



Neu'ral ca-nal' (Gr. neuron, nerve; L, canalis, 

 canal). A canal through the vertebrae; the 

 canal formed by the neural arches. 



Neu'ral plate, groove, and tube (Gr. neuron, 

 ner\c). Three successive phases in the de- 

 velopment of the central nervous system in 

 vertebrates. 



Neu'ral spine (Gr. neuron, nerve. L. spina, 

 spine). The dorsal projection of a vertebra. 



Neu'ri-lem"ma (Gr. neuron, ncr^•c; lemma, 

 coNcring). The outermost sheath of a ner\e 

 fiber; also spelled neurolemma. 



Neu'ro-coele (Gr. neuron, nerve; koilos, hol- 

 low). The cavity in a chordate nerve cord. 



Neu'ron or neu'rone (Gr. neuron, ner\e). A 

 nerve cell, including the cell body and all 

 of its processes. 



Niche (ecologic) (O.F. nichiere, to nestle). 

 The constellation of environmental factors 

 into which a species fits or which is required 

 by a species. 



Node of Ran'vi-er ( L. nodus, knot. After Louis 

 Ranvicr, French histologist) . The place on 

 a nerve fiber where the medullary sheath is 

 constricted. 



No'to-chord (Gr. notos, back; chorde, string). 

 Characteristic cylindrical rod of supporting 

 cells in the chordatcs, found dorsal to the 

 digestive tract and ventral to the nerve cord; 

 either surrounded or supplanted by the 

 vertebrae in most vertebrates. 



Nu-cle'ol-us (L. diminutive of nucleus, kernel) . 

 A spherical, well-defined body found within 

 the nucleus of many kinds of cells. 



Nu'cle-us (L., kernel). It is a specialized pro- 

 toplasmic body within the cell which con- 

 tains chromosomes. 



Nu-tri'tion (L. nutrimentum, nourishment). 

 The sum of the processes concerned in the 

 growth, maintenance, and repair of the liv- 

 ing body as a whole, or of its constituent 

 parts. 



Ob-jec'tive (L. objectivus, from ohjicere, to 

 place before). The lens or combination of 

 lenses of a microscope nearest the object 

 under obscr\-ation. 



Oc-cip'i-tal (L. occiput, back of the head). 

 Pertaining to the base of the skull of a 

 vertebrate. 



