300 PROCEEDINGS OF THE AMERICAN ACADEMY. 



right as well in the sections immediately following. The stage of de- 

 velopment represented by these sections can, I think, be readily shown 

 to be only a more advanced condition of the phenomena presented by 

 the germinal disk of Fig. 5. The change is brought about by a simple 

 obliteration of the cavity between the disk and the bridge, produced by 

 the siukins down of the bridge until it comes in contact with the sur- 

 face of the disk, with which it fuses. The cavity which appears at the 

 left in Fig. 23, and on the right in succeeding sections, is in all 

 probability the last trace of the marginal groove which I mentioned in 

 the description of the preceding embryo as running around the under 

 surface of t^e bridge. 



Another embryo shows a second and later phase in the disappearance 

 of the bridge. Fig. 6, Plate I., represents the germinal disk of an em- 

 bryo taken from the same uterus as that of Fig. 5. The blastodermic 

 vesicle was nearly circular in outline, slightly folded, and measured 

 3.9 mm. in diameter. The germinal disk was distinctly ovate in outline, 

 as shown in Fig. 6, and measured in its long axis 0,265 mm. and in 

 its greatest breadth 0.23 mm., thus exceeding in size the disk of Fig. 5 

 by just 0.03 mm. in each diameter. Though somewhat smaller than 

 the disk just described, it is clearly older, as the description of tlie sec- 

 tions will show. It was cut into thirty sections in a transverse direction, 

 i. e. at right angles to the long axis of the embryo. Starting from the 

 broader end of the disk, Fig. 24 represents the fourth section in the 

 series, and Fig. 25 the seventh. Here the only evidence we have 

 that a bridge has been present is in the shape and position of some of 

 tlie more superficial cells and their nuclei. In Fig. 24 several of the 

 surface cells show the characteristic elongated outline with flattened 

 nuclei which the more superficial cells of the bridge present in its 

 greatest development, and in Fig. 25 two cells near the centre show, 

 for the same reason, an undoubted origin from the bridge. 



3. Summary of Observations on the Blastodermic Vesicle of the Pig. 



I have given above in some detail the principal jihenomena which 

 my material presents, and now I wish to give in a more compact form 

 what I take to be the typical changes which occur in the period of 

 development which these embryos cover, and to point out one or two 

 variations from the type which serve to throw some light upon its 

 meaning. 



The earliest sta^e which I have, shows a blastodermic vesicle con- 

 sisting of a sharply defined inner layer of flattened cells, — the ento- 

 derm, — which forms a closed sac. In contact with the outside of this 



