Al ie NEW ZEALAND NATURE-STUDY BOOK 
al 
The yolk is a yellow fluid contained in a thin elastic 
membrane which gives to this body a spherical shape. 
Like the white it becomes solid when heated. Under the 
yolk-membrane on its upper surface is a small circular 
patch somewhat paler in colour than the yolk, known as 
the germinal disc. This is the living germ from which 
the chick is developed; the rest of the yolk and the 
white are simply a store of food for the nourishment of 
the growing bird. 
The germinal dise seen in the yolk is really the upper 
and expanded portion of a flask-shaped mass extending for 
some distance into the yellow fluid. Make a mark on the 
shell, place the egg in a vessel with the mark uppermost, and 
boil until it is hard. Remove the shell carefully and make a 
clean cut through the egg along the mark and the direc- 
tion of the long axis. If the experiment be successful, the 
flask-shaped mass will be clearly seen with the open end of 
the flask uppermost, that is, directly under the mark pre- 
viously made on the shell. 
Function of the Balancers.—Make a hole in a 
fresh egg sufficiently large to enable the contents to be 
examined ; the germinal-dise will be seen on the top of the 
yolk. Coverup the hole with a piece of gum-paper, turn 
the egg over and make another hole on the opposite side— 
the disc will again be seen on the top. Clearly since there 
is only one germinal-disc the yolk must have turned round 
inside the egg. 
In order that the germ may grow into a chick it must be 
kept warm. The heat is supplied by the sitting bird, 
and that portion of the egg which is nearest to the bird’s 
breast is warmer than the rest; hence the necessity of 
keeping the germ uppermost. If the egg be turned slowly, 
the yolk keeps pace with the movement and remains “ right 
side up ”’—if, on the other hand, it be rotated quickly, the 
