48 THE EARLY DEVELOPMENT OF MAMMALS. 



the rabbit, dog, cat, and sheep. In all of these the embryonic shield is simply a 

 thickening of the outer layer (Fig. 13). The embryonic shield is at first small, but 

 it rapidly expands and assumes a rounded or oval form. There next appears, in 

 a more or less central position in the shield, a small, darker spot, which marks 

 what is known as the primitive knot, a peculiarity of which is that it corresponds 

 to an intimate union of the cells of the inner with those of the outer layer of the 

 blastodermic vesicle. (Compare Fig. 126, B, page 171.) Soon a linear shadow be- 

 comes visible extending from the primitive knot toward a point at the periphery 

 of the embryonic shield (Fig. 14) which represents the embryonic shield of a dog 



O o 



Kn. 



O O o 



FIG. 14. SURFACE VIEW OF THE EMBRYONIC SHIELD OF THE BLASTODERMIC VESICLE OF A DOG OF THIRTEEN 



TO FIFTEEN DAYS (PRECISE AGE UNKNOWN). 



The specimen had been preserved with sublimate and stained with borax-carmin. Sh, Embryonic shield. Kn, 

 Hensen's knot, p.s, Primitive streak. X 100 diams. (After Bonnet.) 



at about two weeks. The shadow, p.s, from the primitive knot is termed the 

 primitive streak, and it very soon becomes further characterized by the formation 

 of a fine groove caused by a depression in the outer layer of cells. This is known 

 as the primitive groove, and has been observed in all amniote embryos. Its exact 

 significance has never been satisfactorily ascertained, and its interpretation is still a 

 matter of scientific discussion. A transverse section through the primitive streak of 

 a vesicle of a common European mole is shown in figure 15. At about the time the 

 primitive streak appears the embryonic shield becomes oval in form. In those 

 animals, such as the carnivora and ungulates, which have a large elongated blasto- 

 dermic vesicle, we find that the long axis of the embryonic shield is nearly at 



