298 PROCEEDINGS OF THE AMERICAN ACADEMY. 



normally in close contact with the ectoderm, but in the region of this 

 membrane it seems to be more loosely attached than elsewhere, or else 

 the cells are less resistant, for we often find that here they are some- 

 what torn away, as though by some mechanical injury, possibly due to 

 the effect of the fixing or the hardening reagents, which necessarily 

 produce a slight shrinking of the vesicle. This membrane will be 

 more fully discussed farther on. 



I have not introduced the figure of the next section in the series, 

 since it merely shows a condition intermediate between the preceding 

 and following sections. Fig. 17 passes near the lateral margin of 

 the free edge of the bridge, and shows that the cavity beneath the 

 bridge extends much farther towards the posterior end of the germinal 

 disk, and also that the bridge itself is thinner just at this point than in 

 the preceding and the succeeding sections. By comparing this with 

 both Figs. 18 and 19, it will be readily seen that this diminution in 

 thickness results in a ring or crescent very near the inner margin of 

 the under side of the bridge, which in Fig. 17 is cut nearly longitu- 

 dinally, and in Figs. 18 and 19 transversely near the posterior point 

 of attachment of the bridge to the underlying ectoderm. Fig. 18 

 shows the first section which passes through the free anterior margin 

 of the bridge. 



Figures 19 and 20 may best be considered together. Fig. 19 repre- 

 sents the section which lies in the median plane of the embryo, and con- 

 sequently here the free edge of the bridge is farthest removed from the 

 anterior end of the disk. In Fig. 20, Plate IV. the free margin of the 

 bridge has begun to advance towards the anterior end of the disk again. 

 The phenomenon of greatest significance in these sections, however, is 

 found at the point where the bridge comes in contact with the ectoderm 

 of the germinal disk near the posterior pole of the chief axis. In Fig. 

 19 it will be noticed that the cavity beneath the bridge appears to 

 extend between the cells posteriorly as a narrow opening for a short 

 distance. In Fig. 20 we find a continuous canal passing from the 

 cavity beneath the bridge into the space between the ectoderm and the 

 entoderm of the extra-germinal area just at the margin of the germinal 

 disk. This canal appears to arise in the median plane of the embryo, 

 and to pass between the ectodermal cells into the cavity just mentioned 

 in a direction slightly oblique to that plane. At least that must be the 

 conclusion, provided the sections are exactly parallel to the median 

 plane of the disk; but if the sections were only very slightly oblique, 

 they would make a canal as small as this appear to have an oblique 

 direction, even though it were actually parallel to the chief axis of the 



