SEGMENTAL 



1312 



SELENIUM 



S., Rivinian (of the tympanic ring), that portion of 

 the temporal bone between the two points of attach- 

 ment of its tympanic portion to its squamous portion. 

 S., Sacral (of the pelvic floor), that portion which 

 " extends from the sacrum to the posterior vaginal 

 wall." (D.B.Hart.) S., Schmidt-Lautermann's, 

 the elongated pieces making up the medullary sub- 

 stmce of nerve-fibers, several pieces being included 

 within each internode. 



Segmental (seg - men' '- tal) \_segmentum, a segment]. 

 Pertaining to, or of the nature of, a segment ; made up 

 of segments. S. Duct, the duct of the pronephros, 

 q.v. S. Organs. See Nephridia. S. Sense- 

 organs, a series of organs formed by the temporary 

 or permanent union of the sensory ganglia ; the ear, 

 eye, and nose possibly belong to this class ; the 

 branchial sense-organs of Beard, and the ganglionic 

 sense-organs of Minot. 



Segmentation (seg-men-ta'-shun) \_segmentum, a piece 

 cut off]. In biology, the process of cleavage or divi- 

 sion. In embryology, the term is restricted by usage 

 " to the production of cells up to the period of devel- 

 opment when the two primitive germ-layers are clearly 

 differentiated and the first trace of organs is beginning 

 to appear." (Minot.) Merogenesis. S. -cavity, the 

 central space in the blastula stage of the segmentation 

 of an ovum. S. -cells, homogeneous indifferent cells 

 formed by the repeated division of the fecundated 

 egg-cell, and which compose first of all the solid 

 mulberry germ. (Hseckel.) S., Centro-lecithal, a 

 form of segmentation in which the spheres enclose a 

 central nutritive yolk. S., Complete, holoblastic 

 segmentation. S., Direct, amitosis, or direct cell- 

 division. S., Discoidal, a form of segmentation in 

 which the germinal disc alone is involved. S., Du- 

 plicative, segmentation peculiar to the gonococcus, 

 marked by an interval between the two segments. 

 S., Free, cleavage of zymoplasts. S., Germ, seg- 

 mentation of the impregnated ovum, or of the first 

 embryonic segmentation-sphere, or blastosphere. S., 

 Holoblastic, segmentation in which all the con- 

 tents of the ovum undergo cleavage. S., Incomplete, 

 S., Meroblastic, segmentation in which only a por- 

 tion of the contents of the ovum, the formative yolk, 

 undergoes cleavage, the other portion, or food-yolk, 

 being a reserve store of food for the developing 

 embryo. S., Metameric, division of the embryo into 

 metameres. S. -nucleus. See under Nucleus. S., 

 Partial. See S., Incomplete. S., Protovertebral, 

 division of the mesoblast on each side of the notochord 

 into somites, or protovertebrse. S., Regular, seg- 

 mentation in which the spheres are equal in size and 

 symmetrically arranged. S. -sphere, one of the cells 

 of an ovum during the early stages of segmentation. 

 See Blastosphere and Aforula. S., Total. See S., 

 Holoblastic. S., Unequal, a variety of segmentation, 

 in which, after cleavage of the ovum into four equal 

 segments, the spheres of one pole are smaller and 

 more numerous than those of the other. 



Segmentellum (seg-men-tel' '-urn) [segmentum, a seg- 

 ment]. A blastula. 



Segnitia, Segnities (seg-nish'-e-ah, seg-nish'-e-ez) [L.]. 

 Torpor ; sluggishness, as of the bowels ; languor. 



Segond, Angles of. See Angle. 



Segregate (seg' '-re-gat) [segregare ; se, by one's self ; 

 grex, a flock]. To separate or set apart ; separated 

 from each other. S. Fecundity. See Homo- 

 gamy. 



Seguin's Signal-symptom. The warning of the be- 

 ginning of the epileptic attack, consisting in affection 

 of the muscle or muscles habitually first involved in the 

 epileptic convulsion. 



Seiage (sa'-ahzh) [Fr.]. In massage, a pressing with 

 a to-and-fro movement, similar to the action of a saw. 

 It is practised with the ulnar border of the hand, or 

 the palmar side of the hand. 



Seidlitz Powder (sed'-litz). Pulvis effervescens com- 

 positus. See Potassium. 



Seignette Salt. Potassium and sodium tartrate, KNaCJ 

 ^O^ + 4H 2 0. It crystallizes in large prisms with 

 hemihedral faces. See Rochelle Salt. 



Seller's Treatment. See Treatment, Methods of. 



Seirospore (si' - ro - spbr) [attpd, garment; OTvopd, 

 seed]. In biology, a special organ of non-sexual 

 propagation occurring in some genera of Alga- (Cera- 

 miacece) . 



Seizure (sez'-ttr) [ME., seisen, to seize]. The sudden 

 onset of a disease or an attack. In surgery, the 

 grasping of a part to be operated upon. 



Sejugous (se-ju'-gus) [sex, six ; j'ugum, a yoke]. Tn 

 biology, having six pairs of leaflets. 



Sel [L.]. Salt. S. alembroth, a solution of mercuric 

 chlorid and ammonium chlorid, each gr. x,in distilled 

 water, Oj (Imperial). S. amarum, S. amer, mag- 

 nesium sulphate. S. ammonia, S. ammoniac, S. 

 ammoniacum, ammonium chlorid. S. ammoniac 

 martial, ammonio-chlorid of iron. S. ammoniacal 

 nitreux, ammonium nitrate. S. de Chrestien, gold 

 and sodium chlorid. S. commune, S. culinare, 

 sodium chlorid. S. digestif, potassium chlorid. S. 

 digestif de Vichy, sodium bicarbonate. S. d'Epsom, 

 magnesium sulphate. S. de Figuier. See .S". de 

 Chrestien. S. de Glauber, sodium sulphate. S. de 

 Perse, sodium borate. S. de saturne, lead acetate. 

 S. secret de Glauber, ammonium sulphate. S. de 

 Seidlitz, magnesium sulphate. S. de Seignette, 

 potassium and sodium tartrate. S. de soude, sodium 

 carbonate. S. vegetale, potassium tartrate. 



Selection (se-lek' -shun) [selectus, p.p. of seligere, to 

 choose]. In biology, the process of choosing from 

 a number. S., Artificial, the artificial choice, 

 definitely planned, of such forms of animals or plants 

 as will by differentiation develop and reproduce 

 given or desired characteristics. S., Natural, " the 

 preservation of favorable individual differences and 

 variations and the destruction of those which are in- 

 jurious " (Darwin " Survival of the fittest"). S., 

 Physiologic, " the selection of those varieties, the 

 individuals of which are fertile among themselves, 1 tit 

 sterile or less fertile with other varieties and with the 

 parent stock. This has been called segregate fecundity 

 by Gulick, and homogamy by Romanes " (De Var- J 

 igny). S., Sexual, the selection by females, among I 

 varying and competing males, of the strongest or most 

 attractive. 



Selene (se-le'-ne) [oeItjvti, moon]. The white spot some- 

 times occurring on the finger-nails. Cf. Lunula. 



Seleniasis (sel-en-i' -as-is) [atv.i/rr/, moon]. Lunacy; 

 epilepsy ; somnambulism. 



Seleniasmus (sel-en-i-az'-mus). See Seleniasis. 



Selenic (se-lcn'-ik) [geatjvtj, the moon]. A compound j 

 containing selenium combined directly with three 

 atoms of oxygen. S. Acid, H, 2 Se0 4 , a dibasic acid, j 

 resembling sulphuric acid in its properties. 



Selenid (sel'-en-td) [ae?i]vr], the moon]. A compound) 

 containing selenium. 



Selenion (se-len'-e-on). See Selenium. 



Selenite (sel'- en -it) [aeH/vrj, moon]. I. A salt of 

 selenous acid. 2. A translucent form of calcium! 

 sulphate. 



Selenium (se-le'-ne-um) [oetyvt/, moon]. Se = 79-4!, 

 usually bivalent, sometimes quadrivalent or hexavalt nt. 

 A rare element, resembling sulphur in its properties, j 

 See Elements, Table of. 



