THIRD PERIOD OF DEVELOPMENT. 107 



neighbouring cells of the latter (stage with eight 

 primitive segments), and that in such a way that 

 this marked-off strand, which may now be described 

 as notochord, still takes direct part in the bounding 

 of it (Figs. 109, 110, 111). 



The notochord foundation becomes, for the first 

 time in the succeeding stages, with nine and ten 

 mesoblastic somites, gradually shut off from the 

 bounding of the archenteron. It remains, however, 

 at first regularly wedged into its upper wall (Fig. 

 116). 



The alteration in the arrangement of the cells of 

 the notochord makes progress simultaneously. Indeed, 

 the Union of the originally right and left row of cells 

 progresses so far that all the cells at last pass over the 

 whole transverse diameter of the notochord (Figs. 113- 

 117). And with that a point is reached which is of 

 importance for the later structure. 



On a consideration of the whole embryo we notice 

 on a dorsal view more than before the arrangement 

 of the series of cells which extend along the whole 

 transverse diameter of the notochord (Fig. 52), this 

 arrangement having previously concerned only the 

 dorsal cells of the notochord. On the side view 



