Page:The New International Encyclopædia 1st ed. v. 06.djvu/776

EGG. 676 EGG. The Egg-shell. The deposits of albumen are overlaid, when the egg arrives at the constricted neck of the uterus, by a membrane secreted by the lining of the oviduct, and formed of albumen mixed with felted fibres. This is the 'egs-pod' lining the shell. It is double, and by the separa- tion of its two layers at one end the 'air-chamber" is formed. The egg, now five or six hours old. in the case of the common fowl, passes on into the uterine dilatation of the oviduct, where it rests for about 20 hours, and receies from numerous villi in the lining of the walls an even coating of a viscid fluid thickly imbued with lime, which rapidly crystallizes and compacts into a calca- reous somewhat felted shell around the whole egg. This is attached at many se])arate points to the shell-membrane, leaving inimerous minute spaces between them tilled with air through pijjc- like interstices in the shell. "■Besides thi> mam- millary and porous layer, the shell of most birds," according to Xewton, '"possesses a cutic- ular layer. This outermost layer is the most variable part of the shell; it is apjiarently struc- tureless, either very poor in calcine salts, and in this case smooth and shiny, or considerably in- tillratcd with calcareous niatter, and then ex- liibiting the' well-known chalky and often rough appearance of the eggs of the ani, cormorants, grebes, and flamingoes. Even when well devel- 0|K'd this cuticular layer is always extremely thin. In the ostrich and in rhea it is hard and brittle, like the glaze of pottery; in the common fowl and turkey it is parchment-like; in auks and apparently in gulls it is absent. The cuticle is spread over the whole surface of the egg. and into the pits or surface-ends of the air-canals, which are therefore closed when such a cuticle is present. The latter, however, readily admits the passage of air when dry, but when wet or r.-.oist is impermeable to air." The vei'y great ililTerences obsjrvable in the surface of eggs, from rough and pitted to enameled, or polished, or translucently brilliant, is due to the varying crystalline structure of the shell. This has been very thoroughly worked out by German micros- copists, whose extensive investigations arc re- corded in the Journal fiir Oniitholoriie for 1871 and subsequently, and who have shown that a well-defined type of shell-structure belongs to each of various groups. Further information is given in the Zoologixchir Amciijer ( 1885-87). The porosity of the shell is needful, of course, in order to admit air to the embryo; and the pores are large enough to admit also some bac- teria, so that after a short time eggs not ener- gized by vital processes become corrujit and rot- ten. The various preservative processes that have been attenijited to prevent decay dei)end for their success upon the com])lete exclusion of bacteria, and only succeed when they are begun before any infection has been received, and when they absolutely close all the pores of the shell. This can best be dime bj' soaking in lime-water, in effect reinforcing and completing the natural shell. XiMBEB AND SiZE OF Egg.s. The number of eggs that constitute a 'clutch' or 'set' is fairly uniform with each species, but differs greatly among different species. Pelagic waterfowl, and the larger birds of prey usually lay only one egg, rarely more than two; many game birds and lake and river fowl lay a dozen ; wood- land and field birds average five, though in some species two or three are prevailing num- bers, and in others, for example titmice, the clutch may consist of ten eggs. The luunber of eggs laid, except in cases correlated with peculiar circumstances in life history are so nearly constant among s]M'cies of similar habit and bear so evident a relation to the average comparative danger to which the species is ex- posed, that naturalists recognize evidences of a seeming general princi]ile which governs the size of the brood, and if any ])eculiarity should appear in the nesting or the rearing habits that might accrue to the welfare of the individual species, it is offset by a reduced reproductive jjower. Another noteworthy fact is that great diversity exists in the size and weight of eggs as compared with the size and weight of the mother bird, llewitson states, for example, that the raven and guillemot are of about equal size, but their eggs vary as four to one, the latter"s being as big as those of an eagle. JStill more remarkable for disproportionate bigness are the eggs of the .us- tralian mound-birds, which measure 3V4 by 2':i inches, though the hen is only about the size of a common fowl : and of the extinct moas, epi- ornis and the existing kiwis (Apteryx), that of one of the kiwis weighing nearly one-fourth as much as the hen whii'h produces it. Much indi- vidual variation exists, and several factors seem to enter into the explanatinn ; but in general eggs much larger than ordinary, relatively to the size of the mother, belong to birds whose precocial young will receive little or no help and care from their p;irents after birth, and hence nuist remain within the protection of the egg until they are considerably advanced toward maturity. This requires a very much larger egg than is needed for the early-born, ]iarcnt-supported. altricial birds, since a much greater quantity of food (yolk) must be stored up for them, and they must have room to grow. It is ])lain that few such eggs can be jiroduced from the resouri'cs of the mother's organization, and as a matter of fact these huge eggs are laid singly: as a rule, relatively large eggs mean few in a brood. Shape. In shape birds' eggs are rarely spheri- cal, though the contained yolk is always so, but ordinarily is 'ovoid,' being much larger at one end than the other, and often nearly conical; while some are oblong, having a decided length but no excess of thickness at either end. Kggs laid in holes or deej), cup-like nests, where they cannot fall out, are likely to be spherical or ovoid; while those laid upon the ground, with little or no nest, are usually conoid, and likely to be large for the size of the bird. This shape gives them two advantages; first it enal)lcs them to lie more closely together, the points towai'd a common centre, so that they are easily covered by the brodding bird; and, sccoikI. in the case of eggs, like those of guillemots, which are laid upon bare ledges of sea-clilfs with no encircling nest, the conical shape jircvents their being easily rolled or blown away when uncovered, because when moved they will tend to gyrate upon their points and not roll away. Among the services which the egg-shell ])erfonns the foremost is the protection of the embryo from injurious changes of temperature and from too nuuji light. See (Growth.