A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. 125 



A little anterior to the sections just mentioned is found the first ventral opening 

 of the archenteric canal. It is very small, located entirely in one section, and appear- 

 as a narrow passage connecting the canal with the cavity of the yolk-sac. Just 

 beyond this is the second ventral opening (plate 2, fig. 4; plate 3, fig. 4), very large, 

 mainly on the left side, and limited dorsally by the beautifully epithelial notochordal 

 plate. The epithelial cells of the plate are here columnar, their nuclei are slightly 

 nearer the base than the free surface, and their cytoplasm stains rather intensely. 

 In spite of their character these cells are not to be separated laterally from the 

 adjoining mesoderm and entoderm. In the sections which follow, the notochordal 

 plate varies considerably both in breadth and distinctness. The canal likewise is 

 not always well defined; its floor varies in thickness or may even appear deficient. 

 It can be traced forward, however, to the third and last definite ventral opening, 

 where again the notochordal plate is very conspicuous, while in the floor appear 

 a few cells, some of which may be free. This last portion of the head process 

 hardly projects above the level of the neighboring mesoderm. 



Anterior to the head process and continuous with it lies the so-called completion 

 plate, the Erganzungsplatte (des Urdarmstranges) of Bonnet. Its posterior limit 

 may be placed just in front of the third and most anterior opening of archenteric 

 canal, while at its opposite extremity it is gradually lost as the two lower layers of 

 the blastoderm becomes distinct. Its length, with the limits just noted, may be 

 taken as about 0.4 mm.; its width, averaging about 0. 06 mm., is greater than that 

 of the head process proper. In structure it differs very markedly from the typical 

 head process just described. The transition between the two appears to be gradual, 

 at least so far as can be determined on a transverse series. On following the sections 

 forward it is seen that the conspicuous dorsal cells (notochordal plate) rapidly lose 

 their epithelial character, and the lumen (which was such a prominent feature before) 

 becomes very doubtful if not actually wanting (indicated by the dotted outlines in 

 figs. 3, 4, and 5, solid line in fig. 2) . At the same time there is a gradual but not uni- 

 form increase in the breadth and thickness of the plate until it reaches nearly twice 

 the dimensions of the head process, bulging below into the yolk-sac and above into 

 the space between the ectoderm and mesoderm. Along its lateral borders, which 

 are never sharply marked, it is directly continuous with the mesoderm, as this layer 

 is with the head process farther back. At its anterior end it is gradually lost in an 

 ill-defined, axial condensation of mesoderm, and very soon this also disappears. 

 Structurally the completion plate is made up of a rather closely packed mass of cells 

 in which no details can be made out. The entoderm beneath does not lose its iden- 

 tity to quite the extent which it does in the head-process region, but still can hardly 

 be recognized as a distinct layer. Toward the anterior limits of the plate the 

 entoderm appears as a definite layer of large, thick, almost cuboidal cells. At 

 certain points there are indications of a sort of doubling in the plate, due to the 

 presence of a shallow furrow on its dorsal surface. Here, and also where this feature 

 is not apparent, the faintly defined cavity lies distinctly on the right side. Far 

 forward, near the extremity of the plate, there is again a faint indication of a small 

 cavity. One peculiar feature of the plate is the presence in or between the cells 



