io6 THE AMPHIOXUS. 



The notocliord fold consists, according to its develop- 

 ment, of two rows of cells, which are inclined in a 

 somewhat oblique direction (Figs. 98, 101) towards 

 the central line which has resulted from the former 

 lumen, of the channel. A cell situated in the dorsal 

 middle line forms the transition from the rio-ht to 



o 



the left row of cells upon the transverse section. 

 These dorsal cells of the notocliord foundation lie 

 behind one another in a series, which on a considera- 

 tion of such an embryo from the dorsal surface 

 strikes our notice, owing to the regular arrange- 

 ment of its cells (Fig. 49). 



The straight slit of the notocliord foundation, which 

 may be referred to the process of folding, vanishes 

 directly, and that, owing to the fact that the cells 

 from both sides begin to grow in between one another 

 (stage with six mesoblastic somites, Figs. 99, 100). 

 The cells, as well of the right as of the left side, grow 

 out over the middle line. The character of a fold is 

 thereby lost ? and the notocliord foundation has now 

 the appearance, with reference to the arrangement of 

 its cell position, of a many layered dorsal thickening 

 of the archenteron. This thickened dorsal part 

 now becomes by degrees sharply marked off from the 



