A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. 



123 



is continued some distance farther forward, but distinctly to the right of the head 

 process, where it gradually fades out. Here, at the anterior end of the primitive 

 streak, is the primitive or Hensen's node. There is, strictly speaking, no real node, 

 knot, or distinguishable enlargement at this point, and nothing to indicate any 

 separation between the groove and the archenteric canal. Immediately caudal 

 to the beginning of the canal the ectoderm becomes thinner and there appears a 

 wide interval between the entoderm and the last of the primitive streak dorsal 

 to it. 



Lateral to the node, or better in it, i. e., in the walls of the first part of the canal, 

 the ectoderm and mesoderm are in broad connection. Just anterior to the node the 

 head process has freed itself from the overlying ectoderm, is continuous with the 

 mesoderm on either side, and fused with the entoderm below. The posterior ecto- 

 dermic opening of the archenteric canal is very minute, being only about 0.005 mm. 

 in diameter (plate 2, fig. 2; plate 3, fig. 2). The actual opening on the surface can 

 hardly be made out, since it is bounded only by the slightly staining cytoplasm of the 

 surrounding cells, the nuclei of which, in contrast to other regions, are here at a 



P 399. 



CD )9.) 



P HO 



Flo. 5. Head process and completion plate. Right lateral view. The primitive streak, on the left, and the posterior 

 end of the areheuteric canal are represented us being in line with the more anterior structures. Other explana- 

 tions under fig. 4. 



greater distance from the free surface. From this point the canal passes directly 

 ventrad through the substance of the primitive node, turns forward and to the left, 

 and again forward in the line of the head process (figs. 2, 3, and 4). It is at the node, 

 and here only, that the three germ-layers are fused with each other (plate 3, fig. 5). 



