THIRD PERIOD OF DEVELOPMENT. 87 



tliis arching, so that on the optical longitudinal 

 section, if one follows the epiblast at the posterior 

 end from the ventral side towards the dorsal side, 

 it is seen not to end at the gastrula-mouth, but 

 can. be followed over the same dorsal-wards some 

 considerable distance, covering the neural plate 

 (Fig. 37). 



AVe can observe this advance of the arching from 

 back to front in the series of transverse sections also. 

 We already see in section, Fig. 72, which is drawn 

 somewhat obliquely, the arching further advanced 

 upon its left than upon its right side, which answers 

 to a region situated further forward. 



If we further observe a series of sections from a 

 somewhat older embryo, in which the first mesoblastic 

 somite is already formed, we shall in just the same 

 way see the arching further advanced in the sec- 

 tion (Figs. 75, 76) taken through the posterior third 

 of the embryo than in the neighbourhood of the first 

 mesoblastic somite (Fig. 74). In the region immedi- 

 ately in front of the mesoblastic somite we find 

 too the neural plate separated laterally by sharp 

 boundaries. Nevertheless the arching has as yet not 

 begun at all here (Fig. 73). This is the place in 



