DEVELOPMENT OF THE MEDULLARY TUBE, ETC. 



543 



contact in the median line dove-tail into one another. The rudiment 

 of the chorda is then a solid strand-like thickening of the dorsal 

 entoderm- wall, from which it soon hecomes severed as an independent 

 strand (Fig. 2S4, ch). In the meantime the cells of this rudiment 

 press between each other in such a way that each cell finally extends 

 transversely across the entire rudiment. 



The chorda develops, on the whole, from before backward. It 

 commences to develop in the region of the primitive segments. The 

 anterior part of the chorda, which extends above the first primitive 

 segment towards the anterior end of the body, only develops later, 



V 



.1 



b mi 



u 



Fig. 285. — Embryo of Ampkioxus with live pairs of primitive segments (after II \tschek. 

 from 0. Bbrtwig's Text-book). I, lateral aspect ; B, viewed from the dorsal side. 

 ill:, ectoderm; en, aeurenteric canal; dh, enteric cavity; ik, ectoderm; mk, 

 mesoderm-fold ; //, neural tube; ud , archenteron ; as', first primitive segment ; ush, 

 cavity of primitive segment ; I', anterior, //, posterior. 



and then no1 through the independent growing out of the already 

 developed rudiment, but through a separation of cells at the most 

 anterior part of the archenteron from which no primitive segment is 

 abstricted. 



The whole of the entoderm-plate which lies between the mesoderm- 

 folds is not, however, used up for the formation of the chorda, but only 

 the median part of it, the lateral parts, according to Hatschek, 

 beinii' utilised for the completion of the dorsal wall of the alimentary 



