THE OVUM. 653 



allow it to rotate, so that the cicatricula is always the uppermost 

 part of the yelk when the egg is on its side. 



The main structural differences between the human ovum and 

 that of a fowl are apparent from the above description ; the 

 essential peculiarity of the development of the hen's egg is that 

 only a portion of the yelk is engaged in the formation of the first 

 signs of the chick and its membranes, by far the greater part 



FIG. 249. 



ut. 



Diagram of a section of an unimpregnated fowl's egg. (From Foster and Balfour, 

 after Allen Thomson.) W. Blastoderm or cicatricula. w.y. White yelk. y.y. Yellow 

 yelk. ch.l. Chalaza. i*.m. Inner layer of shell membrane, s.m. Outer layer of shell 

 membrane. *. Shell. a.ch. Air space, tr. The white of the egg. vt. Vitelline mem- 

 brane, x. The denser albuminous layer which lies next to the vitelline membrane. 



of the egg, both yelk and albumin, being utilized in supplying 

 the nourishment during the subsequent stages of development. 



After the egg has been laid, it obtains no help from the out- 

 side world, except the oxygen of the air and the heat of the 

 mother's body ; it is, as it were, fenced in with a protecting 

 membrane, garrisoned with the quantity of provisions required, 

 and by the warmth of the hen's body stimulated to growth and 

 activity. 



