Development of the Mammal 293 



accomplished by the destruction of the tissues 

 of the uterus. The first function is to aid in 

 the implantation of the ovum ; this may con- 

 sist simply in the formation of a placental 

 attachment, to be described later, or in the 

 practical burial of the ovum in the lining of 

 the uterus, much as a hot bullet would bury 

 itself in a cake of wax. The second function 

 is to supply nourishment to the embryo by the 

 destruction of the uterine tissue. 



FIG. 90. SECTION THROUGH THE EMBRYONIC SHIELD OF A DOG. 

 (From Kollmann, after Bonnet.) 



ec, ectoderm of embryonic area, ecj ectoderm of blastoderm, en, yolk 

 entoderm. 



The first indication of the formation of the 

 embryo proper is the embryonic shield. This 

 is formed by a thickening of the outer layer of 

 cells in the region where the inner mass is at- 

 tached (Fig. 90). Its distinctness varies in 

 different animals, but it soon assumes a circular 

 or oval outline (Fig. 91), and develops, often 

 near its centre, a small opacity known as the 

 primitive knot. At the primitive knot the two 



