THE ANNELIDS (Annulata) give comparatively large ova, and each has a double gelatinous envelope and contains food-yolk ; some cast them abroad; by many they are inclosed in packets or cocoons (as is the case in the earthworm), or stick together or to the parent or to some out side object. The egg of the flatworms has a pro tective cbitinoid integument, contains food-yolk, and sometimes (as among mollusks. insects. etc.) has an operculum facilitating the escape of the matured embryo; also external cement for at tachment to each other or to some fixed object. The same is true of nematodes, which oviposit vast numbers of eggs—I5.000 a day, it is stated. The Trochelminthes (rotifers) lay eggs of three kinds, those produced in the autumn having thick shells to enable them to survive the winter and develop in the spring. Polyzoans and brachiopods produce very few eggs at a time, and these are developed into larva. in 'brood-pouches' within the body. The eggs of echinoderms are minute globules. consisting of germ-cell, food-yolk, and a glassy exterior layer. They are generally set free in the water, where they are fertilized and devel oped. Sometimes they stick to the surface upon which they are laid. and develop there. The eggs of eadenterates and sponges are hardly worthy of the name, being merely microscopic germ-cells, set free in vast quantities as one of several means of reproduction.
BIBLIOGRAPHY. ,standard :Vat ural History (11Bibliography. ,standard :Vat ural History (11 Boston, 1885) ; Parker and Haswell, Text Book of ZoOlogy (London and New York, 1897) ; Flower and Lydekker, Mammals, Liring and Ex tinct (London, 1891) : Newton, article "Eggs," with many bibliographieal references, in Diction ary of Birds (London, 1893-96) : Wallace. Darwin ism ( London, 1889 ) ; Poulton, Co/ors of Animals (London and New York, 1 stm) : Inger soll. "A Bird's Egg." in Harper's Magazine (New York, I)eeesnher, 1897) ; Gunther, Study of Fishes (London, 1SS0) Publications Vnited Statics Fish ( Washing' on, 1875 to the present) Kirby and Spence, introduction to En/Nino/0T/ ( London. 1828 ; Carpenter. inserts: Their Structure and Life (London, 1399) Pack ard, _t Text-book of Entomology (New York, 18981.
EGG. In plants, the more technical term for he egg is „Among 1 he lowest plants in which sexuality is developed. such as the
lower alga. and fungi, the two sex-cells (gametes) do not (lifter in appearance, a condition which is called 'isogamy.' :Most plants, however, are not isogamons, but their pairing gametes are very dissimilar. The male gamete, called the sperm. is small and usually motile by means of cilia; while the female gamete. called the egg.
is relatively large and passive. Plants with such dissimilar gametes are said to be 'heter ogamous.' Both sperms and eggs are single, naked cells, but the former arc characterized by their activity. the latter by their food-supply. As ill all living cells, the egg consists of a nucleus invested by more or less cytoplasm, in this case the cytoplasm being conspicuous in amount and containing an abundance of reserved food. Usually a special organ is set apart to develop within itself a single egg. Among algae and fungi this female organ is generally a single spherical cell, and is called the ; among the bryophytes (mosses and their allies), pteri dophytes (ferns and their allies), and gymno sperms (conifers and their allies), the female organ is a many-celled. flask-shaped structure called the 'archegonium,' in whose -eater (the bulbous part) the egg is organized; while among spermatophytes (seed-plants) there is no female organ, the egg appearing as a free cell in the embryo-sae which is imbedded in the ovule. In most cases a solitary egg is formed by a sex organ, in which it passively remains till fertil ized by the spore, but there are some interest ing exceptions. For example. in the common rockweed ( Fucus), a brown alga of the sea shore. some of the species have til;gonia which produce eight eggs. that are discharged into the water for fertilization. Under ordinary eirenmstances, an egg must unite with a sperm, that is, it inn,t, be fertilized before it can ac•om plish anything. The new cell thus formed by the in--ion of two cells is known as the 'or.spore' or 'fertilized egg.' Occasionally, however. an Un fertilized egg may produce a new plant, the phenomenon being known as 'parthenogenesis.' Parthenogenesis is rather eorrimon among the towel• '111(1 it 111 110t irely unknown I'Vell seed-plants, although its occurrenee there has been reported in very few forms. See EM