STRUCTURE OF THE EGG. 



material, and thus the prismatic bodies are slowly 

 softened and disintegrated. The shell thus be- 

 comes far softer and more brittle as hatching 

 approaches ; and so great is the difference, that 

 if the edge of a fracture made across a fresh 

 egg-shell, and another of one hatched or hatch- 

 ing, be examined under a microscope, it will 

 be instantly seen that the two are in a quite 

 different molecular condition. Were it not for 

 this beautiful provision of Nature, the chick 

 could never break the shell. 



The outer and inner shell-membranes M and 

 m', separating at the air-chamber A, need no 

 further explanation. Proceeding inwards, we 

 come ne.xt to the white or albumen W. This is 

 composed of a denser, and a more fluid kind, 

 arranged in layers, which can be peeled off in a 

 hard-boiled egg, like the layers of an onion. A 

 layer of the more fluid kind is always next the 

 shell, and another thin one, F, next the yolk, but 

 enveloped by another layer, D, of the dense kind. 

 If an egg be broken into a basin, there will 

 further be observed attached to two opposite 

 sides of the yolk, two slightly opaque and rather 

 twisted thick cords, C H, of still denser albumen, 

 termed the clialazce. They are not attached to 

 the shell, but to opposite sides of the dense layer 

 of albumen, D, which envelops the inner fluid 

 layer and the yolk. They are so attached at 

 opposite sides, rather below the centre ; thus 

 they act as balancing weights, keeping the side 

 of the yolk which carries the germ always 

 uppermost, and very nearly in floating equili- 

 brium. If the egg be turned round, therefore, 

 the yolk itself does not turn with it, but retains 

 its position with the germ on the upper side. 



It will be seen how elaborately and beauti- 

 fully the yolk, bearing upon its upper surface 

 the tender germ, is protected within the egg. 

 Itself rather lighter at the upper part, it is 

 further balanced by the chalazcz, so as to float 

 germ uppermost in the albumen. It is usually 

 very slightly lighter than the albumen, but 

 scarcely perceptibly so; thus it floats near the 

 upper side of the shell, but always separated 

 from it by a layer of albumen of more or less 

 thickness, and oscillating gently away from the 

 shell on the least motion. In a few cases it 

 probably floats more strongly up against the 

 shell, and these are generally the cases in which 

 adherence takes place, or the yolk is ruptured 

 during hatching ; but an exquisitely delicate 

 floating balance is the rule. Nevertheless, it 

 will be readily understood why it is inadvisable 

 to leave an egg, and above all a hatching egg, 

 lying on the same side for any length of time. 

 The shell being porous, and permitting of 

 evaporation, such a course keeps the germ close 



to the portion of albumen which is slowly drying 

 up, and may cause a tendency to adhesion. 



Turning now to the yolk, this is contained 

 within a very delicate vitelline membrane, V. 

 It is composed of both white and yellow cells, 

 and if an egg be boiled hard and cut across, it 

 can be seen that there is a flask-shaped nucleus 

 or centre of white yolk, W Y, round which are 

 several concentric layers of yellow yolk, Y Y. 

 Under the microscope additional thin layers of 

 white yolk cells can be distinguished amongst 

 the yellow layers. On the top of the white 

 yolk rests the blastoderm (germ-skin), a small 

 disk about one-eighth of an inch across, shown 

 at B L. The difference between a fertilised and 

 an unfertilised egg is solely to be found in this 

 small disk, and much of its detail can only be 

 distinguished under the microscope ; but with 

 a pocket lens it can be discerned that whilst 

 in an unfertilised egg the little disk is whitish, 

 all over, e.xcept for small clear spots very irre- 

 gularly distributed over its surface, in the 

 fertilised egg an outer ring or margin is whitish 

 while in the centre is a smaller clear circle, in 

 which are very small white spots. This central 

 clear space is the germ from which the chick 

 will be developed. 



It should be clearly understood that, at the 

 stage when thus examined, after the egg has 

 been laid, development or "hatching" has already 

 been carried on to a certain extent, due to 

 the eighteen or twenty hours it has been sub- 

 jected to the heat of the hen's body whilst 

 traversing the oviduct. As it entered the ovi- 

 duct, the germinal disk consisted of only a single 

 cell. During its passage this cell 



Early becomes traversed by successive 



Development ^ i- • • i- • ,• , 



of the Egg. lurrows or divisions, dividing and 



sub-dividing it into many cells — 



the first stage in developing a real organism 



out of the single cell. This process goes on not 



only on the surface, but beneath, so that by the 



time the &^^ is laid, the blastoderm consists of 



two sheets or layers of cells. At about this 



stage the egg should be laid, and with the 



cessation of warmth the process ceases, or nearly 



so, but not exactly at the same point in every 



case. Perhaps the most wonderful thing about 



an egg is the power it has of keeping the 



development, already commenced, suspended 



for a time when warmth is withdrawn ; to be 



resumed and carried on whenever the necessary 



warmth is restored. 



Several points which puzzle many people 



will now be understood. It may happen that 



an egg is retained for a day or two beyond the 



natural time ; in that case the development of 



hatching will be continued, and the new-laid 



