segmentate (segment at)

[L. segment a tus, ornamented with strips of tinsel, lit. having segments, Brit., II. 292. segmentation (seg-men-ta's hon), n. [ to sphere; a blastocele.- Segmentation nucleus, the nucleus of an impregnated ovum or germ-cell, resulting from fusion of a male and a female pronucleus, and ca pable of undergoing segmentation.-Segmentation of the vitellus, embryol., yolk-cleavage; morulation; the first proce of germination of the ovum of any metazoic animal, by which the original single cell of which the ovum primitively consists becomes converted, wholly or in part, into a ma of similar cells, constituting a morula or mulberry-ma . The cells thus formed are specified as cleavage- cells, blastomeres, or segment ell a. Segmentation goes on in different cases with some variations, chiefly due to the presence of food-yolk and the position of this yolk relatively to the formative yolk (see centrolecithal, ecto lecithal). Total segmentation is nece arily restricted to holoblastic ova; it is distinguished from the partial seg. mentation of meroblastic ova (see holoblastic, meroblastic ), the terms meaning respectively that all, or that only some, of the yolk segments. Total segmentation is equal or regu lar when the whole germ-cell divides into two similar cleavage-cells, and these into four, and so on, the resulting gastrula being the arch i gastrula. Total segmentation is unequal or irregular when the cleavage-cells are unlike one another; it results in the a mph i gastrula. The partial segmentation of meroblastic eggs is always unequal, and either discoidal with formation of a disco gastrula, or su perficial and forming a per i gastrula. Total equal segmentation is also styled primitive, primordial, and palinge netic, the modifications introduced in unequal and partial segmentation being described as kenogenetic. Other descriptive rather than definitive, are used by different IMG:content-0518.png:[blocks in formation] IMG:content-0519.png:[merged small][ocr errors] ged surface differing from an ordinary rack in that it is ed to and operating, the follower of curved, and works a, rack; b, segment-gear; c, lever at d; e, connecting-rod shown as join connected with b and pivoted to frame a hand-pre . by oscillating on a center instead of reciprocating cating in slides or guides. E. H. Knight. segment-saw (segment-sa), n. 1. A circular saw used for cutting veneers from squared logs, consisting of a conical disk having the apex central with the arbor, and very thin firmly toothed segmental saw-plates fastened to the outer margin of the disk. Such a saw having a diameter of 60 inches would be about 16 inches thick at the arbor-the object being to bend the veneers out like a thin shaving as they are sawed from the log. 2. In surg., same as Hey's saw. See saw1. segment-shell (seg'ment-shel), n. A modern projectile for artillery, usually in the form of a conical or oblong shell for rifled guns, in which an inner cylinder of thin iron contains the bursting-charge, and this is contained in an outer shell composed of segmental pieces which are either thrown in all directions on the bursting of the shell, or thrown forward, according to the arrangement made: the whole is cased in lead for transportation and loading. segmentum (seg-men'tum), n.; pl. segment a (-tä). [NL. use of L. segmentum, segment: see segment.] In an at. and zool., a segment, as an arthromere, a metamere, a diarthromere, an antimere, an actinomere, a somite, etc. IMG:content-0520.png:[blocks in formation] segni ties, slowne , tardine ,