296 PEOCEEDINGS OP THE AMERICAN ACADEMY. 



Fig. 13 lies beyond the region of the cavity, and the bridge is here in 

 contact with the underlying ectoderm. 



I am so fortunate as to have another embryo from the same uterus 

 from which this came, which is in almost precisely the same stage of 

 development, and is cut very nearly at right angles to the chief axis of 

 the germinal disk. This gives a series of sections transverse to those 

 which I have just described. The general relation of the overgrowth, 

 or bridge, to the underlying ectoderm will be plain, if I describe 

 briefly two or three of the sections. Beginning with the second sec- 

 tion in the series, which lies at what I term the posterior end of the 

 embryo, a condition is found which is very like that represented in 

 Fig. 13, Plate II,, consisting of a layer of true, somewhat columnar 

 ectodermal cells, with the long axis perpendicular to the surface of the 

 disk, and overlaid by a layer of bridge cells, slightly elongated in a 

 direction parallel to the surface of the disk. The next section anterior 

 to this shows the bridge passing across the germinal disk from one 

 side to the other, and separated from it in the central region by a cavity ; 

 the structure here resembles an arch spanning the disk from side td 

 side. Taking next a still more anterior section, the bridge is repre- 

 sented by an upfolded region at either side of the germinal disk, 

 overhanging the true ectoderm, and presenting very much the appear- 

 ance that Fig. 10, Plate II., would present if there were also an 

 upfolding at the right-hand side of the drawing similar to that at the 

 left. The succeeding sections simply show these lateral upfoldings 

 diminishing in size until they disappear near the anterior margin of 

 the germinal disk. Thus this series of sections, together with the series 

 described just before it, gives the basis for a very complete conception 

 of the bridge as it appears at this stage. 



Returning now to the figures, we find a condition somewhat more 

 advanced than the preceding, represented by Fig. 5, Plate I. The 

 vesicle from which this germinal disk came was taken from the ute- 

 rus on the eleventh day after coitus, together with three others, which 

 were all manifestly older. The vesicle was somewhat wrinkled, and 

 measured 3.4 mm. in its longest diameter. Tlie disk lay near one 

 margin in an unwrinkled area, and was ovate in outline, the long axis 

 being 0.23 mm. and the greatest transverse axis 0.2 mm. in length. 

 At the broad or anterior end the crescent-shaped free margin of the 

 bridge appears sharply marked off from the underlying ectoderm by a 

 cavity which extends to within a short distance of the margin of the 

 disk on all sides except at the anterior end, where the bridge is wanting. 

 At this point the ectoderm is slightly thicker than in the region beneath 



