EXPLANATION OF FIGURES ON PLATES. 169 



Fig. 30. The same in optical front section, as seen from the 

 dorsal surface. 



Fig. 31. Further stage in optical longitudinal section. On 

 the upper surface minute cilia make their appearance. 



Fig. 32. The same stage in optical front section, as seen from 

 the dorsal surface. 



Fig. 33. Further stage of elongated body form: shortly 

 before the formation of the neural plate and the mesoblast 

 folds ; optical longitudinal section. In this, as in most of the 

 succeeding figures, the cilia are not drawn upon the upper 

 surface. 



Fig. 34. The same stage in optical front section, as seen from 

 the dorsal surface. 



Fig. 35. Stage with dorsal furrow and first primitive seg- 

 ment in optical longitudinal section. In all the optical sec- 

 tions hitherto drawn, the cell mosaic has been represented in 

 the background of the cavities. In the succeeding figures this 

 is omitted. 



Fig. 36. The same stage, as seen from the dorsal surface. 

 Before everything else is drawn the optical front section. The 

 first mesoblastic somite (I. US.), as well as the mesoblast folds, 

 are indicated by shading. Further, the distinct posterior bor- 

 der of the first mesoblastic somite is drawn. We have, more- 

 over, the borders of the layer (V.) which grows forward to the 

 middle line over the neural plate and the gastrula - mouth 



(GM.). 



Fig. 37. The second mesoblastic somite is by this time in 

 formation ; optical longitudinal section, mesoblastio somites, 

 and mesoblast fold are indicated. 



Fig. 38. Stage with the first mesoblastic somite, as seen from 

 the dorsal surface, from the living object. 



Fig. 39. Stage with two mesoblastic somites, as seen from 

 the dorsal surface, from the living object. 



