EMBRYO. 



[ 280 ] 



EMBRYO. 



cover and protect the subjacent pair of 

 membranous wings of these insects when 

 not in use. 



The elytra may be regarded as consisting 

 of an elongated, depressed fold of the inte- 

 gument, comparable to the web between the 

 fingers, or that of the bat's wing. Four 

 structures are distiuguishable in them : — 

 1, an outer, firmly adherent, epidermic layer, 

 composed of minute cells, frequently un- 

 distinguishable, or at least only to be de- 

 tected in parts ; this layer is continued 

 around the margins of the elytra, so as to 

 cover their inferior surface also, forming, 2, 

 the inner epidermic layer, in which the cells 

 are stated to be less distinct, more rounded, 

 and more closely placed than those in the 

 upper layer, hence presenting a more di- 

 stinctly angular form ; this layer is easily 

 detached from the elytra, and its surface 

 next the body of the insect is frequently 

 furnished with a number of very minute 

 hairs, or spiniform papillfe directed back- 

 wards (PI. 34. fig. 2). Beneath the outer 

 epidermic layer is 3, a layer of dark resinous 

 pigment ; whether contained in cells or not 

 has not been determined. 4, an intermediate 

 portion, composing the principal thickness 

 of the elytrum, representing the two fused 

 strata of the cutis, and consisting of a number 

 of fibres, running in different directions, 

 variously interlacing, anastomosing, and 

 crossing, so as to form numerous plates or 

 secondary layers, many of which present a 

 fenestrated appearance ; as many as sixteen 

 of these plates have been separated. 



The veins or nerves of the elytra .either 

 traverse the intermediate thick layer of the 

 elytra, or run between its under surface and 

 the inner epidennic layer, to which they 

 sonir'times remain adherent. See Insects. 



The structure of the elytra can only be 

 made out by macerating them for a long 

 time in solution of caustic potash, or water. 



BiBL. Schmidt, Tai/lor's Sc. Memoirs, 

 V. IG; Meyer, MilUer's Archiv, 1842, 12; 

 Nicolet, Ann. Sc. Nat. 3 s^r. vii. ; and the 



BiBL. OF IXSECTS. 



EMBRY'O, OF Plaxts.— This is the 

 name applied to the rudimentary plant con- 

 tained in all true seeds. Seeds cnnt'\ining 

 embryos are borne exclusively by Flowering 

 Plants ; and while the external conditions 

 under wliich the seeds are produced afford 

 the character for the first subdivision of this 

 province of the Vegetable Kingdom (Ax- 

 GIOSPKRMS and Gymxosperms), the struc- 

 tui'e of the embryo is taken as the most 



striking character in further subdividing the 

 Angiospermous Flowering Plants into their 

 two great natural groups, viz. Monocotyle- 

 dons and Dicotyledons, in which, respect- 

 ively, the embryo bears one or two cotyle- 

 dons or seed-leaves. Cases occur both 

 among the Dicotyledons and the Monocot}-- 

 ledons where the typical structure is de- 

 parted from. ThusinOrobanchaceJB (Dico- 

 tyledons) the embiyo is a mere globular 

 mass of cellular tissue, the result of an arrest 

 of development, the cotyledons and radicle 

 never becoming distinct ; the same is the 

 case in the Orchidaceae among the Monoco- 

 tyledons, the embryo not advancing beyond 

 the state of a globular mass of parenchyma. 

 The relation of such embryos to the perfect 

 forms is well illustrated by comparing the 

 stages of gi'owth of embryos which acquire 

 fully-developed cotyledons and radicle (fig. 

 192). In Cuscuta, a leafless plant, the 



Fig. 192. 



A young Dicotyledonous embryo in successive stages 

 of (ievelopment. All exhibit the suspensor; and 4 has 

 the cotyledons appearing, separated by a notch. Mag- 

 nified 50 diameters. 



embryo has no distinct cotyledons. Other 

 anomalies of another kind also occur. Some 

 Monocotyledons, such as those of Grasses, 

 have the rudiment of a second cotyledon ; 

 but this is above and not opposite the other 

 larger one. In Dicotyledons the cot^dedons 

 are not unfrequently unequal, and some- 

 times soldered together. In the Coniferae 

 the embryos appear to have four, eight, or 

 more cotyledons in different cases ; but it is 

 stated that there exist only two, divided or 

 compound, cotyledons (see Seeds), 



Occasionally more than one embryo occurs 

 in a seed (see Polyembeyoxy) ; and in the 

 Coniferae a number of embryos are at first 

 produced, of which one only becomes per- 

 fectly developed (see Gyjixospermia). 



The enibrvo sometimes constitutes the 

 whole mass of the seed, merely enclosed in 

 the coats ; in other cases it is embedded in 

 a mass of albumem. In the former case the 

 tissue of the cotyledons often assumes cha- 

 racters similar to those of the albumex, 

 serving as a receptacle for stored nutriment 

 for the germinating plant, in the form of 



