254 NATURAL THEOLOGY. 



IX. 



MECHANISM OF THE EGG, OR REVOLVING OF THE 

 YOLK. 



The illustration used in the Preliminary Dis- 

 course, of the manner in which the cicatricula, 

 containing the chick within the egg, is presented 

 to the breast of the hen, requires the following 

 diagrams in explanation. 



When we hold an egg steady, and chip it at 

 the upper part, we find the yolk close to the shell ; 

 and on its upper surface a pale vesicle, the cica- 

 tricula, which contains the embryo chick. When 

 the hen sits, the heat of her body developes the 

 action of the living principle in the embryo, and 

 on the second and third day a little zone of blood- 

 vessels appears ; these vessel:3 run towards the 

 embryo, and carry nourishment to it ; and day by 

 day we mry watch its sensible growth. From 

 the delicacy of this action, we may perceive how 

 necessary it is that the embryo at this early pe- 

 riod should be close to the breast of the hen, and 

 not at the cold bottom of the nest. We shall now 

 see how it is accomplished. The yolk is a globe 

 of nutritious matter, and the little vesicle with the 

 embryo is involved in the surrounding membrane, 

 and consequently, as we have said, is at the sur- 



