124 A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. 



Extending cephalad from the primitive node in the axis of the blastoderm 

 is the head process of the primitive streak (figs. 1 to 5). This structure, including 

 the completion plate in front, is slightly longer than the primitive streak, measuring 

 about 0.75 mm. in length; its diameter is variable, but in general gradually increases 

 from behind forward. The posterior half, or head process proper, varies in width 

 from 0.03 to 0.05 mm., its lumen from 0.006 to 0.01 mm., while the length of its 

 lumen, the archenteric or canal of Lieberkuhn, is 0.34 mm. The average breadth 

 of the completion plate is about 0.06 mm. 



The head process is an axially placed, hollow, cylindrical mass which, at its origin 

 in the primitive node, is directly continuous with the superficial ectoderm and the 

 substance of the primitive streak, as well as with the mesoderm on either side. It 

 very soon becomes free from the ectoderm above and fuses with entoderm below; 

 its lumen, which is at first nearer the dorsal surface of the process, takes up a central 

 position, while at the same time the dorso-ventral diameter diminishes somewhat. 

 In considering this structure we shall begin at its posterior end, at the point where it 

 has just disengaged itself from the surface ectoderm. It appears here in section as a 

 roughly pyramidal or wedge-shaped mass projecting well into the space below the 

 ectoderm. Sharply limited above, this mass is fused at its base with the entoderm 

 and mesoderm. The lumen is yet eccentrically placed; the cells dorsal to the lumen 

 are much fewer in number, more deeply staining in their cytoplasm, and have a more 

 epithelial arrangement than those between the lumen and the yolk-sac. These 

 latter cells are much more numerous, more irregularly massed together, and are quite 

 indistinguishable from the neighboring mesodermic and entodermic elements. 

 A few sections in advance (plate 2, fig. 3; plate 3, fig. 3) the head process is rather 

 lower and distinctly broader, its free outlines more curved, while its cavity has 

 increased in size and lies about the center. The cells which bound the archenteric 

 canal dorsally are frankly epithelial; their nuclei are nearer the base of the cells, 

 while the cytoplasm is deeply stained. 



The cells ventral to the lumen show no definite arrangement; they stain only 

 faintly and no layer of entoderm can be made out beneath them. The mesoderm 

 is directly continuous with both these groups of cells, but rather more definitely, 

 on account of their staining reactions, with the cell-mass below the canal. The 

 ventral surface of the head process is, near its posterior end, concave from side to 

 side and at about its margins the entoderm can be recognized as a separate cell- 

 layer. The thinning out and eventual loss of the floor of the canal is apparently 

 due to the rearrangement of the cells here (cf. fig. 5) , there being no evidence of a 

 corresponding loss or destruction of cells. We have in these two distinct cell- 

 groups, dorsal and ventral to the lumen of the head process, the plaque notochordale 

 and plaque lecithoenterique respectively of van Beneden (1899). To these we shall 

 take occasion to recur later. Near its posterior end, where the lumen is more 

 dorsally placed, the cells of the plaque notochordale are only about half as numerous 

 as those of the plaque lecithoenterique; at the point shown in the illustrations 

 they are approximately equal in number. 



