20 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



which the embryo is formed is a small, white disk lying directly beneath 

 the vitelline membrane and termed the tread,the blastoderm or cicatricula. 

 In the hen's egg this disk is about four millimeters in diameter, and is 

 always found in the upper surface of the yelk. If a hen's egg is hard- 

 ened by boiling, and then cut in two by a vertical section so as to bisect 

 •the yelk, the latter will be found not to be perfectly homogeneous. The 

 yelk is clothed externally by a thin layer of differ&nt material, which at 

 the edge of the blastoderm passes beneath it and becomes thicker so as 

 to form a bed on which the blastoderm rests, to become connected by a 

 narrow neck with a mass of similar matter occupying the centre of the 



N.P. 



w.v. W -. Y - \ 

 ™- i \ \ 



CH.L 



Fig. 9.— Diagrammatic Section of an Unincubated Fowl's Egg, after 

 Allen Thompson. (Fostei- and Balfour.) 



BL, blastoderm ; WY, white yelk— this consists of a central, flask-shaped mass and a number of 

 layers arranged concentrically around this ; YY, yellow yelk ; VT, vitelline membrane ; X, layer of 

 more fluid albumen immediately surrounding the yelk ; W, albumen, consisting of alternate denser and 

 more fluid layers ; CHL, ehalaz03 ; ACH, air-chamber at the broad end of the egg — this chamber is 

 merely a space left between the two layers of the shell-membrane ; ISM, internal layer of sheli-membrane ; 

 SM, external layer of shell-membrane ; S, shell ; NP, nucleus of Pander. 



yelk, which nearly always remains partially fluid in the hard-boiled egg. 

 Within the yelk again are several concentric layers of this white yelk, 

 separated from each other by layers of yellow yelk. The yellow yelk is 

 composed of comparatively large, unnucleated cells filled with highly 

 refractive granules, and containing vitellin, lecithin, and various fatty 

 bodies. The cells which form the white yelk are much smaller, are 

 nucleated, and often a large cell will be seen to contain numerous similar 

 but smaller cells. 



When the egg is laid by the hen it has already undergone changes 

 which result from fertilization. We will- first describe the characters of 



