294 Vertebrate Embryology 



layers of the vesicle are closely united. Extend- 

 ing from the primitive knot towards the cir- 



*SP 



**; 



v* _O n 



^^>.* 'y/^^v *%.! 



^ki;:5t3:>^X 



##!&& 



^jftft* 



>tjik 



FIG. 91. SURFACE VIEW OF THE EMBRYONIC SHIELD 

 OF A DOG. MAGNIFIED 120 DIAMETERS. (From Koll- 

 mann, after Bonnet.) 



<5, blastoderm, e, embryonic area. /, border furrow, k, notch. 



cumference of the shield is soon seen an opaque 

 line, the primitive streak (Fig. 92), along the 

 axis of which extends a shallow groove, the 

 primitive groove. The embryonic shield at 



