PLATE III. 



All figures magnified 400 diameters, and all drawn from longitudinal sections 

 through tlie germinal disk of Fig. 6. Tiie anterior end is at the right. 



Figure 14. First section of the disk ; the disk cells first appear near the anterior 

 end on account of the ovate outline of the disk. 



15. Second section in the series ; merely shows an extension of the 

 germinal-disk ectoderm. 



16. Fourth section. First appearance of the bridge, marked off by a 

 distinct cavity. 



17. Sixth section through the disk. Extension of the cavity and decrease 

 in thickness of the bridge. 



18. Seventh section, showing free margin of the bridge at the anterior 

 end, with posterior attachment. 



19. Eighth or median section ; here we have the most posterior point of 

 the free margin of the bridge. The cavity beneath extends a short 

 distance towards the posterior pole, between the cells. 



