WEYSSE. — BLASTODERMIC VESICLE OF SUS SCROFA. 293 



large for a bridge which has developed to so small an extent, and 

 furthermore the bridge is more widely separated from the underlying 

 ectoderm than it is in many embryos of this stage. 



Third Stage. (Plate I. Fig. 2 , Plate IV. Figs. 21 and 26 ) 



In Fig. 2, Plate I., is shown a surface view of a germinal disk from 

 an embryo taken from still another sow, which was killed ten days 

 after copulation. The blastodermic vesicle was almost completely 

 distended, there being but one marked fold, and measured 1.95 mm. 

 in diameter. The germinal disk was about 0.15 mm. in diameter 

 and circular in outline. It was cut into fourteen sections ; these were 

 all drawn with the aid of the camera lucida to an enlargement of 275 

 diameters, and from these drawings a reconstruction of the surface 

 was made and then reduced to an enlargement of 100 diameters, as it 

 appears in P^ig. 2. A reconstruction in wax was also made as a surer 

 means of control to guard against any error in the graphic reconstruc- 

 tion. Two sections of this embryo are shown on Plate IV., Figs. 21 

 and 26, made through the region of the germinal disk and the extra- 

 germinal area respectively. The sections are oblique to the antero- 

 posterior axis of the disk, cutting it at an angle of about 45°, and run- 

 ning on the reconstruction drawing from the lower right-hand corner 

 to the upper left, Fig. 2, Plate I. Of the fourteen sections into which 

 the disk was cut. Fig. 21, Plate IV., represents the seventh, and 

 passes through the deeper portion of the depression which lies just 

 beneath the central point of the free margin of the bridge, as seen in 

 Fig. 2. In this specimen the two lateral elevations mentioned in the 

 embryos already described are seen to have increased in size until 

 they have come into contact with the posterior overgrowth, with which 

 they have fused at two points, thus giving the bridge three points of 

 attachment to the ectoderm proper and leaving three openings from 

 the depression or cavity beneath it to the outside. This bridge, then, 

 has manifestly three points of origin, one posterior and two lateral. 

 Further evidence in corroboration of a similar method of formation 

 will appear in the course of the description of the other embryos. 



Before leaving the ectoderm of the germinal disk, it should be added 

 that it is relatively very thick in all these younger embryos, consisting 

 of two or even three interlocking layers of cells, m addition to the 

 bridge cells, which are usually two layers deep. This great thickening 

 of the ectoderm of the germinal disk is not, however, the earliest con- 

 dition ontogeneiically, for in younger stages of development, as, for 

 example, those represented by Figs. 7-10, Plate II., it is much thinner 



