WEYSSE. — BLASTODERMIC VESICLE OP SUS SCROFA. 291 



there is a slight elevation of the ectodermal cells at the margin of the 

 disk on either side. The significance of this fact will appear in my 

 further description of the bridge. Moreover, in sections seven, eight, 

 and nine of the disk there is a distinct groove or furrow in the upper 

 surface of the ectoderm ; this runs along the median line, and is there- 

 fore in a line continuous with that of the bridge cells described above. 

 And now a word as to similar stages in embryos which I have not 

 figured here. 



The ten other embryos which came from the same uterus as the one 

 I have just described vary in size from 1 mm. to 1.9 mm. in diameter, 

 and represent very diverse stages of development. Only two of these 

 are in about the same stage as the one mentioned above, the others 

 being clearly much more advanced, and of these two I consider one 

 further developed than the other, since it has many more bridge cells 

 on the disk. The younger embryo measured 1.4 mm. in diameter, 

 while its germinal disk was 0.11 mm. in diameter and very nearly 

 circular in outline. An enumeration of the nuclei shows approxi- 

 mately 117 cells in the ectoderm proper, 33 in the entoderm, and 6 

 in the bridge. It is somewhat difficult to determine with absolute cer- 

 tainty the number of cells in the bridge in this case, for some cells are 

 just in process of passing over from the true ectoderm of the disk to 

 the bridge ; the direction of the spindle in the karyokinetic figures 

 makes this point indisputable. The mass of bridge cells appears, as in 

 the specimen already described, nearer one margin of the germinal disk, 

 and for that reason I term the margin near the bridge cells the 

 posterior end of the embryo. Anterior to this point there is the same 

 lateral or marginal uprising which I mentioned in the first case. The 

 entoderm consists of greatly flattened cells with widely separated 

 nuclei, but so far as I can determine they are all connected by delicate 

 protoplasmic masses, which often appear like a thin membrane lying 

 across the rounded inner boundaries of the ectodermal cells. One may 

 therefore pass through several sections without finding an entodermal 

 nucleus on a certain area of the embryonic vesicle, but I should not 

 feel justified in assuming that the entoderm fails to cover any portion 

 of the interior surface of the ectoderm of this vesicle. 



The second embryo to which I referred in connection with this was 

 but 1 mm. in diameter, and therefore the smallest in my collection. 

 The germinal disk is, however, clearly in a more advanced stage of 

 development, for the reason which I have already given, and the vesicle 

 also has a larger number of entodermal cells, so that there can be no 

 doubt in this case that they form a complete layer on the inside of the 



