WEYSSE. — BLASTODERMIC VESICLE OF SUS SCROPA. 295 



bridge, where the lateral overgrowth has not as yet entirely fused with 

 the overgrowth from the posterior end to form a continuous bridge 

 such as exists in Fig. 3. The left-hand side, however, is complete, 

 exactly as in Fig. 3. This germiual disk might be considered as illus- 

 tratinor a stajre intermediate between Figs. 2 and 3, so far as the 

 fusion of the various parts of the bridge is concerned ; it is, however, 

 clearly more advanced, not alone on account of the greater size of the 

 germinal disk, — for this may vary greatly, as we have already seen, 

 — but because of the greater extent and degree of development of 

 the bridge. The embryonic vesicle in this case was only slightly 

 folded, nearly circular in outline, and about 2.75 mm. in diameter; 

 the germinal disk measured 0.2 mm. in diameter. 



Figures 11, 12, and 13, Plate II., represent sections through a 

 germinal disk in which the bridge has reached about the same stage of 

 development as in the two cases last described. The embryo from 

 which these three sections were taken came from the same uterus as 

 that of Figs. 7, 8, and 9, and the sow was killed therefore on the tenth 

 day after coitus. This vesicle measured about 1.-55 mm. in diameter, 

 while the disk, which was slightly elliptical, measured 0.145 mm. in 

 its longer axis (the sections shown in Figs. 11, 12, and 13 are parallel 

 to this axis), and 0.11 mm. in its shorter axis. This shows, then, a 

 still greater elongation than the preceding germinal disk in the direc- 

 tion of the chief axis of the future embryo. The disk was cut into 

 eleven sections, of which Fig. 11 represents the fifth; it is, therefore, 

 a little to one side of the median plane. This section shows well a 

 phenomenon which is present in all sections of bridges that have devel- 

 oped to some extent, and seems to point to a double origin of this 

 structure. It will be noticed that the extra-germinal ectoderm appears 

 to extend as a continuous layer over the right-hand portion of the disk 

 and to constitute the upper layer of bridge cells, while the lower layer 

 is clearly derived from the true ectoderm of the germinal disk itself, as 

 we can see from the position of the nuclei at the region of contact of 

 the bridge with the underlying ectoderm. It should be observed, 

 however, that this apparent extension of the extra-germinal ectoderm 

 over the ectoderm of the germinal disk occurs in the region of the 

 bridge only. The whole appearance suggests an upfolding of the mar- 

 gin of the disk, which carries both the extra-germinal and the germi- 

 nal ectoderm with it. Figs. 12 and 13 represent the eighth and ninth 

 sections respectively through the same disk. Fig. 12 is a section at 

 the extreme lateral margin of the opening of the bridge, the cavity 

 beneath it appearing as a somewhat triangular space in the section. 



