EMBRYO-SAC. 



[ 281 ] 



ENCALYPTA. 



fleshy secondary deposits, starch, oil, &c. 

 The position of the embryo in the albumen, 

 or the modes in which the embryo is folded 

 up within the seed-coats, are of great im- 

 portance in systematic botany, for the charac- 

 terization of families. Particulars regarding 

 these points, and the manner of examining 

 them, are given under the head of SEED. 

 The development of embryos is described 

 under OVULE. See also ORCHIDACE^:, 

 OROBANCHACE^E, CUSCUTA. 



BIBL. Henfrey - Masters, Sot. 634; 

 Brongniart, Ann. Sc. Nat. xii. 14, &c. ; 

 Jussieu, ibid. 2 se"r. xi. 341 ; St.-Hilaire, 

 Ann. Sc. Nat. 2 ser. v. 193; Duchartre, 

 Ann. Sc. Nat. 3 ser. x. 207 ; Hofmeister, 

 Abh. sacks. Ges. Wiss. vi. & vii. ; Sachs, 

 Sot. 553; and the BIBL. of the articles 

 OVULE, SEED, &c., above referred to. 



EMBRYO-SAC, OF PLANTS. A cell 

 which becomes enlarged into a sac in the 

 substance of the upper part of the nucleus 

 of the ovule or rudiment of the seed. In 

 the cavity of this are developed the germinal 

 vesicles (PI. 47. figs. 3, 4, 5), one of which 

 (occasionally more), after fertilization, gives 

 origin to the EMBRYO. The most common 

 condition of the embryo-sac is that of a large 

 ca,vity excavated in the nucleus, bounded by 

 its own cell-membrane, and containing abun- 

 dant protoplasm, and subsequently germinal 

 vesicles and endosperm-cells (see OVULE). 

 Not unfrequently, however, it becomes de- 

 veloped into diverse saccate processes, either 

 pushing their way through the substance of 

 the nucleus in variable directions (Scrophu- 

 lariacece &c.), or emerging from the micro- 

 pyle, coming to meet the pollen-tube ( Vis- 

 cwm), or even so much developed externally 

 that the embryo is formed and perfected 

 altogether outside the nucleus (Santaluni). 

 These and other conditions are further de- 

 scribed under OVULE. When the germinal 

 vesicle is fertilized, and is undergoing deve- 

 lopment to produce the embryo, the embryo- 

 sac often becomes completely filled with 

 endosperm-cells, at first free, but afterwards 

 adhering together through their crowded 

 condition. These may persist and form an 

 endosperm to the seed, as in Nuphar, where 

 there is an additional episperm formed out- 

 side the embryo-sac from the substance of the 

 nucleus. Albuminous seeds generally have 

 either an episperm or an endosperm alone. 

 In exalbuminous seeds the endosperm ori- 

 ginally existing inside the embryo-sac be- 

 comes absorbed through the pressure of the 

 growing embryo, the embryo gradually filling 



up the cavity, and by further expansion ob- 

 literating the embryo-sac and nucleus. See 

 ALBUMEN, of Seeds. 



In the Coniferse the embryo-sac, origi- 

 nally formed by the excessive expansion of 

 one of the cells near the apex of the nucleus, 

 becomes subsequently filled up by cellular 

 tissue, in the upper part of which become 

 developed the bodies called corpuscula, each 

 of which possesses a kind of secondary 

 embryo -sac of its own, in which the ger- 

 minal vesicles are developed (see GYMNO- 

 SPERMIA). 



The term embryo-sac might also be ap- 

 plied to the large cell at the base of the 

 archegonia of the FERNS, LYCOPODIACE^, 

 MOSSES. 



BIBL. See OVULE and GYMNOSPERMIA. 



EMPUSI'NA, Cohn. See SPORENDO- 



NEMA. 



EMYD'IUM, Doyere (Echiniscus, 

 Schultze). A genus of Arachnida, order 

 Colopoda, family Tardigrada. 



Char. Head furnished with appendages j 

 mouth conical, without appendages or ter- 

 minal sucker ; epidermis semisolid, present- 

 ing, especially on the upper surface of the 

 body, an evident annular division. 



E. testudo (PL 50. fig. 7). Reddish- 

 brown ; body ovoid, opaque ; snout conical, 

 presenting traces of division into three 

 rings ; head indistinctly divided into three 

 segments, the first and third presenting short 

 setiform filaments supported upon very short 

 tubercles, the second with a palpiform, blunt 

 and flattened appendage ; pharyngeal tube 

 very slender; styles straight ; bulb without 

 an internal jointed framework; eye-spots 

 small, oval, simple, most visible at the 

 under aspect of the body; trunk divided 

 into four simple rings, with spines and long 

 filaments; legs three-jointed, each with 

 large and strong claws, the posterior pair 

 with a kind of spur also at the back part of 

 the lower margin of the second joint; move- 

 ment excessively slow ; length, from the 

 end of the extended snout to the posterior 

 border of the fourth ring, 1-80". Found on 

 the moss covering tiled roofs ; common. 



E. spinulosum I 



E. granulatum \ 



BIBL. Doyere, Ann. Sc. Nat. xiv. p 279 



ENALLOSTE'GIA. One of D'Or- 

 bigny's orders of Foraminifera, having the 

 chambers alternate in two or three rows, 

 not spiral ; such as Polymorphina and Tex- 

 tularia. 



ENCALYP'TA, Schreb. Agenusof Ca- 



