WEYSSE. — BLASTODERMIC VESICLE OF SUS SCROFA. 297 



the bridge, and this thickening appears on the surface as a slight ele- 

 vation, which I have tried to show by the shading in the drawing. 

 This germinal disk illustrates the form and structure of the bridge so 

 clearly and so typically, and represents almost, if not quite, its maxi- 

 mum development as a free and independent structure, that I have 

 thought best to illustrate it more fully by sections than I have done in 

 the case of the preceding embryos. 



The sections were made in a direction very nearly parallel to the 

 long axis of the germinal disk, which at this stage corresponds to 

 the anterp-posterior axis of the embryo. The disk was divided into fif- 

 teen sections, and of these, Figs. 14-19, Plate III., and Fig. 20, Plate 

 IV., represent respectively the first, second, fourth, sixth, seventh, 

 eighth, and ninth ; the sections beyond these simply present the same 

 phenomena in reversed order. Little need be said in explanation of 

 the first two sections. Figs. 14 and 15, Plate III. Figure 14 is taken at 

 the extreme lateral margin of the disk and is consequently nearer the 

 anterior than the posterior end, because of the ovate outline of the disk. 

 The section immediately preceding this consisted merely of a layer of 

 typical ectodermal cells on the outside with a layer of characteristic 

 entodermal cells a short distance beneath. Figs. 14 and 15 show the 

 great rapidity with which the ectoderm increases in thickness in 

 passing from the lateral margin of the disk towards its centre. The 

 third section in the series has not been represented here, since it shows 

 no new features beyond those represented in Fig. 15, except ah ex- 

 tension of the ectodermal cells in a posterior direction. The next 

 section is represented by Fig. 16, in which occurs the first appear- 

 ance of the cavity which marks off the bridge from the rest of the 

 germinal disk. The bridge itself consists of several irregular layers of 

 cells, the outei'most of which is made up of cells considerably flattened 

 in the plane of the germinal disk. As to the general characteristics of 

 the disk in section, it should be noted that the anterior (in the figure, 

 the right-hand) end is thicker and more rounded than the posterior, 

 which is rather attenuated. It will also be seen that the lower or inner 

 boundary of the ectodermal cells of the disk is marked by a relatively 

 sharp line due to the presence of a very delicate membrane. At 

 the margin of the germinal ectoderm, — where, between the extra- 

 embryonic ectoderm and entoderm, a space (triangular in section) 

 occurs which completely surrounds the disk, — this membrane loses 

 its connection with the ectoderm of the germinal disk and stretches 

 across the space to meet the extra-embryonic portion of the outer layer 

 at some distance from the margin of the disk. The whole entoderm is 



