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, :Ind often nearly conieal; while sonic are oblong, having a decided length but no excess of thickness at either end. Eggs laid in holes or deep, cuplike 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 eonoid, and likely to he large fur the size of the bird. This shape gives them two advantages: first it enables them to lie more closely together, the 'mints toward a common centre. so that they are easily covered by the brooding bird; and. second. in the ease of eggs. like those of guillemots, which are laid upon bare ledges of sea-eliffs with no encircling nest. the conical shape prevents 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 performs the foremost is the protection of the embryo from iitinrions changes of temperature ;mil from too such light. See ROM T Corot. Birds' eggs may be said to be nor mally chalky white. hut in a great number of groups they are colored. The color may he a uniform tint over the whole surface., or a tint with markings; or simply dots. spots. streaks. or blotches of one or more hues on an otherwise colorless shell. These colors exude from uterine glands in the form of pig,m•nt-corpuseles depos ited with the forming shell. and stain its deep er as well as its more external layers. :Many of the in tint of the spots are thus due simply to their being overlaid by more or less transparent material, but certain real colors exist. This matter was studied by II. C. Sorby by the methods of spectrum analysis described by him in the Proceedings of the Zoiilogical fY of London for IS75. Be learned that the
cohering matters in egg-shells are connected with hermaglobin and the bile-pigments, and are def inite physiological products. He reported seven of these products as follows: 11 I Oiirhoficinc. The most important of the color.. present in almost all eggs. which gives an effect when alone of brownish red. The spec troscope shows so close a similiarity between this and the product of the decomposition of the red corpuscles of the blood as to make it probable that they are physiologically identical. and de rived from the same source. Such reel-brown eggs as those of the grouse are mainly tinted by this material.
(2) Oberon. A fine blue.
(3) Banded (Megan. Also a fine blue, but giv ing a different spectrum. A chemical similarity is apparent between the two oiicyans and the pig ments of the bile, and they must lee regarded as derived by separate physiological processes from the same source.
(4) Yr/tow 06,ronthine. Clear yellow; unsta ble and liable to fade.
(5) llufous Ooranthine. Reddish yellow.
(6) .4 substance giving narrow absorption hands in the red. Probably brown.
(7) Lichenosanthinc. A brick-red substance, apparently identical with a coloring matter com mon in plants, especially in lichens and fungi.
The mixture of these pigments in various way; gives the varied colors observed in egg shells. Thus the nightingale's egg has been shown to he colored with a mixture of o;irhodeine and o6cyan; while all the varying greens, and letups see common as ground-tints are due to vary ing mixtures of &'ecyan with ooxanthine, and such are likely to fade or to change color when long exposed to the light.
An interesting feature in this connection is that not only the state of health, but the state of mind of a bird, may effect the colors of its as Hewitson long ago recorded.