62 



THE EARLY DEVELOPMENT OF MAMMALS. 



* The Embryonic Shield. 



Sooner or later in the early history of every blastodermic vesicle, and always 

 as the first indication of the development of the embryo proper, there appears a 

 thickening of a small oval area of the outer layer of the blastodermic vesicle. 

 This thickening is known as the embryonic shield. In the fresh specimen it 

 marks itself by the greater opacity which it causes in the walls of the ovum 

 where it lies. It is produced always at the point where the inner cell mass was 

 originally attached to the subzonal layer. In those cases where the thickening 

 of the ectoderm to form the trophoblast extends over the entire blastodermic 

 vesicle, it is very difficult to follow the early history of the embryonic shield. In 

 other cases, however, where the trophoblast occupies a special restricted area, 

 the history of the embryonic shield may be more readily followed. The animals 

 in which it has hitherto been chiefly studied are the rabbit, dog, cat, and sheep. 

 In all of these the embryonic shield is simply a thickening of the outer layer (Fig. 



FIG. 18. TRANSVERSE SECTION THROUGH THE EMBRYONIC SHIELD JDF THE BLASTODERMIC VESICLE OK 

 A DOG OF ELEVEN OR FIFTEEN DAYS (PRECISE AGE UNKNOWN). 

 O.L, Outer layer. Ent, Entoderm. X 2O diams. (After Sonnet.} 



1 8). 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. Soon 

 a linear shadow becomes visible extending from the primitive knot toward a 

 point at the periphery of the embryonic shield Fig. 19, p. s, which represents 

 the embryonic shield of a dog at about two weeks. The shadow from the primi- 

 tive knot is termed the primitive streak, and it very soon becomes further charac- 

 terized 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 ascer- 

 tained, and its interpretation is still a matter of scientific discussion. A trans- 

 verse section through the primitive streak of a vesicle of a common European 



