SUMNER: KUPFFER'S VESICLE 



49 



stages of development. Figure 1 represents the early segmented germ-disc. In the 

 stage shown in figure 2 the blastoderm has increased in area but diminished in 

 thickness, and a thin superficial layer of cells (" Deckschicht," GOTTE) has become 

 differentiated. In the stage shown in figure 3 an important change has taken place 

 in the relative thickness of the parts. The central region has become thinner, both 

 relatively and absolutely, while the marginal region is now thicker. The natural 



FIGURE 2. 



inference is that a part of the central cells have migrated peripherad, and this is the 

 inference drawn by GOTTE. This thickened marginal region is called by GOTTE 

 "Randwulst" (von Baer). It is seen in the figure that the superficial layer 

 (" Deckschicht ") extends a trifle beyond the cells of the marginal wall and that 

 its border rests directly on the yolk, leaving a small space, triangular in section, 

 around the margin. Finally in figure 4, gastrulation is shown in progress. From 

 the marginal wall a projecting ledge of cells is growing inward over the yolk. This 

 process begins on one side of the blastoderm (the future posterior end of the embryo) 



FlOUEE 3. 



but soon extends to the whole circumference, although remaining most conspicuous 

 on the embryonic side. Figure 4, A, represents a blastoderm of this stage as seen 

 in surface view. The invaginate layer (" Secundiire Keimschicht," GOTTE) is seen to 

 take the form of a diaphragm with an excentric and not quite circular aperture. The 

 " Deckschicht," as before, ends freely on the yolk. Figure 4, B, is a section cut in 

 the plane of the dotted line in A. 



