THE EARLY DEVELOPMENT OF THE MAMMAL 395 



embryonic shield forms a well-marked oval thickening about 

 1.5X1.0 mm., the entire vesicle at this time measuring about 

 4.5X3.5 mm. 



Sections through the embryonic shield show that it is formed 

 largely by a circumscribed thickening of the embryonic ecto- 

 derm, three or four cells deep, beneath which the endoderm 

 remains only one cell in thickness (Fig. 154). Over the surface 

 of the shield the original trophoblast cells (Rauber's layer) are 

 no longer distinguishable. Peripherally the shield passes into 

 the thinner extra-embryonic ectoderm, or trophoblast of these 

 forms. 



The next phase of development is indicated (rabbit, about the 

 end of the seventh day, dog, about thirteen to fifteen days) by 

 the appearance of a slight opacity toward the middle of the em- 

 bryonic shield. This is known as the primitive knot or Hensen's 

 node. Usually the primitive knot is eccentric in position, 

 toward what proves to be the anterior margin of the shield. 

 At about this same time, or even before Hensen's node is dis- 

 tinctly visible, an opaque line appears across the shield, extend- 

 ing from the node to the posterior margin of the shield, where 

 it joins an opaque crescent-shaped region (Fig. 158, A). This 

 line is the primitive streak, and usually, along its middle can be 

 seen the clearer primitive groove (Figs. 157, F; 159). A little 

 later a less distinct opaque line may be seen extending a short 

 distance forward from Hensen's node; this is the head process. 



These superficial appearances in the embryonic shield may be 

 understood only by the examination of sections. Sections 

 across the primitive streak of the rabbit or mole, for instance, 

 show that the opacity of the region is due to a very marked 

 thickening of the ectoderm, with which is associated the forma- 

 tion of the middle layer or mesoderm. A section like that 

 illustrated in Fig. 159, A, shows the primitive streak to be a 

 region of rapid cell proliferation in the ectoderm. From the 

 sides and inner surface of the primitive streak cells are given 

 off which gradually take on the arrangement of a definite layer 

 quite distinct from the ectoderm, and between the ectoderm 

 and endoderm. This is the mesoderm which here, therefore, 



