WEYSSE. — BLASTODERMIC VESICLE OF SUS SCROFA. 299 



embryo. This phenomenon is not confined to this embryo. A pre- 

 cisely similar canal can be traced in the embryo figured ou Plate I. 

 Fio-. 3, and in two or three other cases there are suggestions of a 

 similar condition, but not sufficiently well marked for me to put much 

 stress upon them. It will be readily seen that unless the section 

 should pass in exactly the right direction, i. e. very nearly through the 

 lono- axis of the canal, it would be impossible to establish its presence 

 except in extremely thin sections. Its occurrence in Fig. 20 is be- 

 yond question ; I shall discuss its possible morphological significance 

 later. 



The fact that the free surface of the germinal ectoderm shows no 

 trace of cells resembling the ectoderm of the extra-germinal area, such 

 as are present on the upper surface of the bridge, may be mentioned 

 here again in passing. 



Fifth Stage. (Plate I. Fig. 6 ; Plate IV. Figs. 22-25.) 



"We now come to the last stage in the history of the bridge. This 

 can best be shown in two phases, the first of which is represented by 

 Figs. 22 and 23, Plate IV. These are from sections of an embryo 

 taken from the same uterus as that represented by Fig. 2, Plate I. 

 The blastodermic vesicle was small in comparison with the size of the 

 germinal disk, being but 3.1 mm. in diameter, while the disk, which 

 was ovate in outline, measured about 0.3 mm. in its long diameter, and 

 in its greatest width 0.29 mm. It was cut into thirty sections in a 

 direction nearly perpendicular to the long (chief) axis. Fig. 22 

 represents the thirteenth section in the series, which begins at the 

 narrower or posterior end, and Fig. 23 represents the seventeenth, 

 which is therefore somewhat more anterior. Fig. 22 shows that the 

 germinal disk consists of cells whose nuclei lie at varying distances 

 from the surfoce of the disk, and that it has a rather broad median region 

 pretty clearly marked off from a marginal region on either side, by the 

 fact that it contains more nuclei, and also because the general surface 

 of the median region is here slightly elevated above the lateral portions 

 of the disk. This median elevation is more or less marked in the sec- 

 tions which precede this in the series, maintaining about the same 

 relative extent. In the succeeding sections, however, it is no longer 

 marked, so that the surface of the ectoderm is pretty uniformly flat. 

 At the same time there is to be noticed at the margin a layer of cells 

 cut off from the underlying ectoderm by a narrow cavity. This occurs 

 on each side of the disk, but owing to the obliquity of the sections it 

 appears in Fig. 23 at the left-hand side only, while it is seen at the 



