30 K. MITSUKURI; 



to be present, there is visible instead a median white streak, bounded 

 on either side by a dark streak and on the right, a second fainter 

 white one. It appears therefore, as tliough there were two distinct 

 ridges here, but I believe that the right hand light streak is due to 

 some secondary circumstance, and that the one in the median line 

 alone indicates proliferation of cells in the epiblast and is comparable 

 to the streak visible below the yolk-plug in Fig. 23. 



Fig. 25 is important as showing how the tail-swelling arises. 

 In this embryo, three concentric zones are visible in the pjirt of the 

 embryo exposed behind the posterior horse-shoe shaped edge of the 

 amnion. These are simply the surface expressions of various thick- 

 nesses of tissues in different parts. The yolk-plug and a median 

 streak under it are seen faintly on the outermost of these zones. 

 The line that marks the outermost from the middle zone is somewhat 

 faint. The innermost zone, however, is sharply marked olf from the 

 middle one and is to be looked on as showini»' the extent of the " End- 

 wulst." It is in fact this zone which is now rising up as the tail- 

 swelling, as will become evident from the sections. Here, again, it 

 is evident that the yolk-plug lies clearly behind the tail-swelling. 

 Fig. 25a (PI. II.) is the ventral view of the same embryo, and 

 shows well the ventral opening of the neurenteric canal. 



Flu:. 26 is the last stag-e in Avhich I was able to detect the volk- 

 plug in a surface-view. The tail-swelling is now distinctly bounded 

 by a deep groove all around. At the posterior edge of this groove, 

 partly covered by the amniotic fold is the yolk-plug with its charac- 

 teristic shape in the median line. Fig. 26a (PI. II.) is the ventral 

 view of the posterior portion of the same embryo. The semilunar 

 notch behind the ventral opening of the neurenteric canal is the 

 commencing allantois. 



After this stage, the part where the yolk-plug ought to be visible, 



