THE STRUCTURE OF THE EGG.

Dominion, Volume 4, Issue 1031, 21 January 1911, Page 11

THE STRUCTURE OF THE EGG.

Thd Poultry Expert of South Australia "(Mr-' T>. Laurie) at tho "kito- 1 conference hold at Adelaide gave the ifollowing address, which was illustiated by lantern slides. ' .' • ' . If a hen bo; dissected-, there will be found in each side of :'tho..backboiie4wp little buuehes of immature eggs. These are the oraries; one remains rudimentary and the other (usually that on the left side) .will • in time become active and produce ripened yolks, as shown on the screen. Various calculations have been made as to the number of ovules in a hell's ovary, and although the usual estimate is 800, cases arc on record 'of hens which ha.vo' laid 1000 eggs. Roughly, the immature egg consists of the germ, the'yolk material, and the delicate membrano enclosing it; ..these are, in the ovary, ■ included m- the ovisac, which is connected with - the ovary by. a. thin tube termed the pedicle. If the hen under examination is laying, these bvjiles. will vary in cize from a maturo yolk of ordinary size down to small ones whoso size cannot bo gauged by the unaided eye. As. soon as tho yolk is ripe, or matured, and ready to be shed, or cast off by .the ovary, tho ovisac, which has during distension become very thin at its greatest diameter, bursts at a line called tho stigma, and so releases the yolk. At- < tached to the hen's intestines by vanou3 | processes will bo found a pinkish tubo which is the oviduct. One end, that nearest tho ovary, is free, and is funnel-1 shaped. Tho' other end communicates with the cloaca at the. vent. The oviduct varies from 18in. in a small fowl to 24in. in' a largo one such, as a ISlack Orpington. Coincident with tho rupture >of tho ovisac and the shedding of its enclosed yolk, the free funnel-shaped end of tho oviduct rises and grasps tho 'yolk as its leaves its sac. When first 'received, in the oviduct, and until it has, travelled about 4in., there is. no addition of albumin or white, but at about that point secretion takes ijlace, and seven or eight layers of white or albumin, of different thicknesses and differing degrees of density, are secreted by a set of Elands and cucloso tho yolk. When this is finished, the inner skin— a very delicate, semi-transparent membrane—is secreted and envelopes botli the white and, its included yolk. A fresh set of glands now secretes a much stronger membrane, which is tho outer skin lining the shell of the egg. Between these skins, and, as a rule, at the large end of tho egg, these membranes separate and form the air sac. Further down tho oviduct the egg receives -its shell, which is secreted by other glands. The egg shell is composed of lime, salts, etc., and is deposited in tho form

of prisms, the. formation allowing , for numerous-"pores;' 1 or minute openings, through tho walls of tho shell. In liens which lay eggs with tinted shells, there are other glands which discharge' a pigment which colours, the shells more or less. Shortly after this the egg is ex; traded. The time occupied in the formation aiid passage of the egg down tho oviduct is, under normal conditions, about 18 hours. Abnormal Eggs. Double or Treble-Yolked ' Eggs—lf the ovaries arc unduly excited it liiay happen that two or- even three yolks mature. and aro shed: simultaneously.. -These become enveloped .in albumin, then in. the membranes,, and finally in the shell, which is of size proportionate to the contents. ■ .' Egg Within.an Egg—After the formation of an egg there may bo a temporary cessation tjf peristaltic action, and tly?n, apparently, the egg may retrogress and receive a furtlir coating of albumin • (enclosing the shell), and again an outer enclosing shell is deposited. Small eggs containing •no yolk and only a little albumin are due to false stimulation of the'oviduct-. No yolk is shed, by , the ovary, but. the glands in the duct secrete a quantity, generally small, of albumin, which is enclosed, in the'-'membranes and a small rounded shell. -" Shell-less or soft-shelled eggs are laid by -liens which , are ovcrfat, have been harried, have access to insufficient shellforming material,'or in cases wl\ere-in-flammation of the ovidlict affects tho : shell-making glands. . Rough-shelled eggs are due' to in-' flamed conditions of the oviduct. '. - Yolks only • aro sometimes laid by a hen, and indicate that tho whole of tho . oviduct is inflamed. The case is theii serious. _ Soft-shelled eggs of various sizes joined together are due to inflammation; as aro similar abnormalities 'of different appearaiicfe':"' Most cases are due. 1 to over-stimulation, arid the lien.should' bo fed sparingly with" a view t-o stopping egg production. 1 • • Structure of An, Egg.. ; -ilf.'WO-'break all egg/carefully■ ilito a cup so that the yolk is suspended or floating in the whitoVor'albumin', wo can lioi-o the following circumstances: —Tho yolk is rounded ill appearance, and on iti upper surface we' can seo a small •spot or ring about an eighth of an inch iiij diameter. .This is. the blastoderm or germ skin containing tho germ vesicle. When it is known that the egg is laid by an unmated hen it will of course be infertile. 'If, however, such is not known definitely, wo _ can . determine • whether it be fertilo or'not by examination with the aid of a-.strong lens. In an infertile, egg this little disc or ring is whitish with a few irregular clear spots. _ Iu the. fertilo egg, however, there is a clear centre, circular in Eha_po, iu which are very small, white spots, Now let us examine the albumin, and .to.are'.at..onco>struck vyitli.the.presence at tach side of tho yolk of two whitish, thickened; 'twisted," cor'd-liko "bodies. These aro the 'clialazo' which" apparently attach the.yplk, to, the. white..- They-are often referred to as the "troaddlo" or "tread," and have 'nothing to do: with , the question of fertility in any way. It was at nno time thought tliat they.acted as balances and assisted to keep the blastoderm side of tho ,yolk always uppermost. Some physiologists, however, declare that this .is duo to the fact that thg.,.lower. part of the yollc is of greater specific gravity than the upper portion. It may be noted-that 011 examining the ovary of a-lien, when tliero are yolks of. different sizes, the .blastoderm or germ skin can be plainly seen through the membranes of .tho ovisac.

If an egg bo hard-boiled, say cookcd for ten minutes, and when cold cut across its long diameter the different layers of albumin c<fn be separately peeled off. Examination will show that the centre of the yolk is ' pear-shaped, and' consists-of 1 whito yolk nearly., half ;an .inch across.'' This white yolk'-'' is granulaiv : in..form.,and ' the'; blastoderm "by .'a. i( jiahw canal "or tube. Tlio surrounding' yolk is of sevbroad layers of yello# yolk alternated with verynarrow' bands of white yolk. Fertilisation, Etc. The spermatozoa of tlie male ibird consists of a sharp head, a short neck,, and a long tail. Owing to attracting forces these bodies mako a rapid movement along the oviduct. Only one spermatozoon can compete or is necessary for fertilisation, and immediately the fusion of a male germ with the female germ takes place the ingress of all others is barred. . Doublo fertilisation cannot take place. Thus, if a fertilised yolk, that is a'yolk bearing a fertilised germ, bo met in. the oviduct by a male germ, -no effect, is produced, but if. .an infertile,-germ-celled .y01k.., be-.met ■ fertilisation tirih of somo of the eggs in the ovaries willffrsullf.'^ "all the eggs which-aro, laid in one batch •are fertilised during several immediately consecutive scrvicps. . 'As..regards the sex- of.'the egg,-; maiiy well-known scientific . men have Been ■ at .work for • ypars making the most exhaustive iuquiries into the 'relationship arid numbers of the chromosomes.: '-At any-rate, it is certain that the sex of the egg of domestic fowls is decided immediately 'upon' fusion. ' It .must bo. remembered that the germ itself is minute: and the .question of sex even beyond-our best microscopos; . Immediately after the fusion .of the-.two cells, male and felriaie; the . process of incubation, or rather the growth of. the embryo, takes place. l After the fusion of the male and female cell,into onei.division.in to two takes place, .and cach in turn divides; so that by the "time the egg is laid considerable ..structural, alterations .will-,be 7 revealed on microscopic examinatifm. When t!ie egg 'is' laid, multiplicatiou .of ..tlie cells ceases' initil the process of incubation is resumed, either under tlio hen or in the incubator;;but the sex does not alter —how can it ?

: Referring again to the structure of the shell, with its • numerous .pores, .1. must point out that tile watery contents of the egg pass through the pores by evaporation. It is thus that-stale eggs become jglack, and in time tlio infertile egg, which will keep quito fresh for an extended period, gradually dries out until only the solids .remain. The analysis of an egg.is as follows: — '

• 4. k'S v 11' '-J-- • . 3-S- . "8 a«'' >' < o p=i PTho white .:. 16 . 30,: ' 1 . 53 = 100 The yolk ... .12, : 2 1.2 84.8; 100 Tho shell'constitutes about 11 percent., .tlio yolk 32 per cent., and the \vhitc 57 per cent, of the total weight of the egg. As previously "shown, the process of incubation is actually in' progress at the time the egg is laid.- It is known that high temperatures such as aro common in summer will cause partial incubation of an. cga. This practically means that tho fertile egg is unreliable as nn article of commerce. The infertile egg contains no livin;; germ and is reliable, and poultry-keepers aro urged to secure the production of such eggs only for market purposes. They keep almost indefinitely if stored in a clean, wholesome condition. Contact with strong-smelling substances affects tho flavour of prejudicially.. Contact with filth may "result in organisms _ of decay passing through tho pores into the substanoe of the egg.