THE EGG AND DEVELOPMENT OF THE YOUNG BIRD 95 



The Yolk. 



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." [The eggs should 

 be turned by the breeder at least once a 

 day imtil they are returned to the birds 

 to incubate.] 



" Turning now to the yolk, this is con- 

 tained within a very delicate vitelline 

 membrane, v. It is composed 

 of both white and yellow 

 cells, and if an egg [fowl's] be boiled hard 

 and cut across, it can be seen that there 

 is a flask-shaped nucleus or centre of white 

 yolk, w Y, rovmd which are several con- 

 centric 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 yolk rests the blastoderm [germ-skin], 

 a small disk shown at b l. The difference 

 between a fertilised and 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, except for small clear spots very 

 irregularly 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 devel- 

 oped. 



" It should be clearh' 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 subjected to the 

 heat of the hen's body whilst traversing the 

 oviduct. As it entered the oviduct the 

 germinal disk consisted of only a single 

 cell. During its passage this cell becomes 

 traversed by successive furrows or divisions, 

 dividing and subdividing it into many 



cells — the lirst 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 egg 

 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 

 Weakly j.^^. ^ ^^^^ ^j. ^^^,^ bevond the 

 Embryos. • . " 



natural time ; m that case the 



development or hatching will be con- 

 tinued, and the new-laid egg may contain 

 a visible embryo. Again, since even the 

 new-laid egg is already an organism, which 

 has attained a certain stage of growth, 

 it is subject to disease, or weakness, or 

 accident, like other organisms. Thus an 

 egg may be fertile, and the germ may begin 

 to develop, but may i)erish at any stage 

 from sheer lack of strength, precisely as a 

 weakly baby may die at any age. Quite 

 apart from accidents or injuries whilst 

 hatching, there is no doubt that in many 

 eggs the embryo is not strong enough in 

 itself ever to come to maturity. Such 

 deaths at various stages, within the shell, 

 are in no essential respect different from 

 deaths of weakly chicks at various early 

 stages after leaving the shell ; the necessary 

 vigour may fail the infant creature at any 

 particular time, so also the embryo can 

 be injured within the shell in various ways ; 

 and while it might be fanciful to say it 

 can be ' frightened,' there is much evidence 

 to show that it may suffer from some kind 

 of nervous shock, as in a severe thunder- 

 storm." 



Such shocks, however, would be more 

 likely to affect the eggs of birds which 

 build their nests upon the ground, such as 



