WEYSSE. BLASTODERMIC VESICLE OP SUS SCROPA. 287 



could affect the histological coudition of my specimens. I will now 

 give a description of the embryos themselves, and then proceed to a 

 consideration of the investigations of other authors, and to the theo- 

 retical interpretation of the phenomena described. 



II. Description of the Embryos. 

 1. General Oharacteristics of the Blastodermic Vesicle. 



The embryos are all in more or less advanced stages of the so-called 

 blastodermic vesicle or didermic blastocyst, as it has been described 

 by Balfour ('81), Van Beneden ('80), Hubrecht ('90), Bonnet ('91), 

 and others. They consist in general of at least two well defined 

 layers, — an outer of more or less isodiametric cells, and an inner, in 

 contact with the outer, of greatly flattened cells, which are very much 

 larger than thos3 of the outer layer. There is no trace of cells lying 

 between these two layers at any point. At one region of the embryo, 

 the outer layer of cells is thickened, forming the germinal disk. For 

 the sake of convenience this region may be spoken of as the germinal 

 or embryonic area, and the rest of the vesicle as the extra-germinal or 

 extra-embryonic region. In most cases there is evidence of a third 

 layer of cells, which is outside the two layers just mentioned, and is in 

 a more or less disintegrated condition. I shall later refer to this more 

 at length. 



It has not seemed to me necessary to give figures of the whole 

 blastocyst. In each case it consists of a hollow vesicle with a double 

 wall, which if fully distended would be about spherical. I have never 

 found the vesicles completely distended, and often they are greatly 

 folded, so that it is not always possible to make sections in just the 

 plane one wishes ; for if the germinal disk lies on the edge of a fold, it 

 is usually necessary to cut at right angles to the fold in order to avoid 

 getting sections oblique to the surface of the disk. Something of an 

 idea of the general appearance of the vesicle can be gained from Bon- 

 net's ('84, Taf. IX. Fig. 2) figure of a sheep embryo of the thirteenth 

 day, which has the same general characters as the pig embryos which 

 I have studied. 



The germinal disk of this stage can be detected by the naked eye, 

 even before staining, as a very small opaque white spot on the surface 

 of the vesicle. The smallest vesicle which I have examined is about 

 1 mm. in diameter, while the largest is about 4.5 mm. In the latter 

 there is no trace whatever of the beginning of the excessive elongation 



