PLATE IV. 



Figures 20 and 27 magnified 400 diameters, all others 275 diameters. Ecto- 

 derm in every case uppermost. 



Figure 20. The ninth section in the same series as those of Plate III. The free' 

 margin of the bridge is again nearer the anterior pole, while the 

 cavity beneath extends by a narrow canal completely through the 

 ectoderm of the disk into the extra-germinal space between ecto- 

 derm and entoderm. 



" 21. Somewhat oblique longitudinal section nearly through the centre of 

 Fig. 2, Plate I. The ectoderm of the disk is greatly thickened, the 

 cavity beneath the bridge is rather shallow, but there is a marked 

 depression just in front of tiie free edge of tiie bridge. Tlie ento- 

 derm is thickened beyond the area of the disk in the extra-embryonic 

 region. 



" 22. A nearly transverse section through a much older germinal disk, 

 which shows the decrease in thickness of the ectoderm attending 

 an extension in area and the fusion of the bridge along the median 

 portion. 



" 23. A slightly more anterior section of the same disk as Fig. 22, where 

 we see at the left lateral margin the fusion of the bridge not quite 

 completed. 



" 24. Transverse section, the fourth from the anterior end of the germinal 

 disk of Fig. 6, showing the bridge cells in contact with the germi- 

 nal-disk ectoderm. 



" 25. Seventh section of same disk as the preceding, showing near the 

 median line the last trace of the fusion of two bridge cells with the 

 ectoderm. 



" 26. Section through the extra-germinal region of the blastodermic vesi- 

 cle of Fig. 2, Plate I. On the upper side are a few " Deckschicht " 

 cells, flattened and disintegrating; then a layer of typical ecto- 

 dermal cells, and, below, the normally distributed entodermal cells 

 of this stage. 



" 27o Section through the extra-germinal region of Fig. 3, Plate I. The 

 " Deckschicht " cells are more numerous than in the preceding case, 

 and at one point several are in contact with one another, — an un- 

 usual condition. The entodermal cells are more numerous tlian 

 in the preceding, since this is a later stage of development. 



