THE OVUM. 



655 



which a portion of the yolk called the cicatricula rests. This 

 cicatricula, which lies between the vitelline membrane and the 

 white yolk, is the active growing part of the egg, and out of it are 

 developed the chick and the embryonic membranes. 



Extending through the albumin from the vitelline membrane 

 to the ends of the egg are two twisted membranous cords the 



FIG. 249. 



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

 Balfour, after Allen Thomson.) bl. Blastoderm or cicatricula. w.y. White 

 yolk, y.y. Yellow yolk. ch.l. Chalaza. i.s.m. Inner layer of shell mem- 

 brane, s.m. Outer layer of shell membrane, s. Shell, a.ch. Air space. 

 w. The white of the egg. vt. Vitelline membrane, x. The denser albu- 

 minous layer which lies next to the vitelline membrane. 



chalazce, which fix and protect the delicate yolk from shocks, but 

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

 part of the yolk 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 essen- 

 tial peculiarity of the development of the hen's egg is that only 

 a portion of the yolk is engaged in the formation of the first signs 



